Python Beginner

Python is an implicit type language, which means you don't need to declare a type for the variable, only you need to do is giving a variable a value, the python knows what's type it is.

1. Accept user input.

name = raw_input(str hint)  # User can enter some STRING to the 'name' according to the 'hint'

Example:

name = raw_input("Enter your name")print "Hello " + name

2. Logical condition

In the 'if - else' or 'for' statement, python will use indentation to indicate logical block, instead of bracket.

Example:

if x<2:   # use colon to indicate the beginning    print 'Small'   # use indentation to indicate the logical blockelif x<10:    print 'Medium'else:    print 'Large'print 'All done'

# some logical operators:  'or' ,  'and'  ,  'not'   (using these words rather than | & !)

3. try - except for dealing with the error

try:

    statement

except:

    statement (dealing with the error)

Example:

astr  = 'Hello Bob'try:    istr = int(astr)   # there will be an error

    print 'There will be an error'  # this line will not be executedexcept:    istr = -1

======= An example based on 1,2,3===============

score = raw_input('input your score')try:    point = float(score)    if point<0 | point > 1.0:        print 'error message'        quit()except:    print 'error message'    quit()if point >= 0.9:    print 'A'elif point >= 0.8:    print 'B'elif point >= 0.7:    print 'C'elif point >= 0.6:    print 'D'else:    print 'F'

=============================================

4. Functions

The format of defining a function

def FuncName([Argument]):

    statement   # the function content after a indentation

                       # here, we can return some function result

Example: use function

def computepay(h,r):    if h>40:        total = 40*r + (h-40)*r*1.5    else:        total = h*r    return totalhrs = raw_input("Enter Hours:")h = float(hrs);rate = raw_input("Enter Rate:")r = float(rate);p = computepay(h,r)print p

5. Loops and Iteration

5.1 while

while condition

    statement       # this statement will be executed over and over again until condition being false

Example:

n=5while n>0:   # here have a colon    print n    n = n-1  # we can also use 'break' to jump out of the loops or use 'continue' to stop current iterationprint 'Blastoff'

5.2 For

for i in [array]

    statement      # using an array here

Example:

for i in [5,4,3,2,1]    print iprint 'Blastoff'

Note: "is" and "is not" are logical operator, which is similar to ==, but stronger than ==.  Mostly, used in "is None"  or "is false"

======= An example based on Loop===============

largest = Nonesmallest = Nonewhile True:    num = raw_input("Enter a number: ")    if num == "done" : break    try:        x = int(num)        if smallest is None:            smallest = x        if largest is None:            largest = x        if x < smallest:            smallest = x        if x > largest:            largest = x    except:        print 'Invalid input'    print "Maximum is", largestprint "Minimum is", smallest

=============================================

6. String

6.1 Concatenate two strings

Here, we can concatenate two strings together with '+' . However, this operation couldn't do type conversion automatically.

So if do this:

    x = 1

    y = '124'

    z = x + y   # this statement will have an error

6.2 Get the char in the String

We can seen String as a char Array. So we can get the 'letter' in the String by using index (from 0):

    Ex:

        X = 'banana'

        letter = X[2]   # letter = 'n'

        num = len(X)  # num=6; get the length of a string using len() function

note: we can't attempt to index beyond the end of a String (out of range)

6.3 Loop through String

X = 'banana'

index = 0

while index<len(X)

    letter = X[index]

    index = index + 1

More General way:   (The iteration variable moves each value in the sequence)

X = 'banana'for letter in X:   # this method is much more elegant    print letter

6.4 Retrieving continuous section of a String

We can also look at any continuous section of a string using acolon operator:    X[startIndex : endIndex]

Note: the second number indicates the end of the substring, but not including

So:

    S = 'Monty Python'

    X = S[0:4]   # X = 'Mont' , S[4] will not be included

Note: The endIndex can be out of the boundary.

If startIndex is none, it refers to the beginning of the String (0); if endIndex is none, it refers to the end of String (len-1)

So S[:] could be 'Month Python'  - the same to 'S'

6.5 Using 'in' as an Operator

Here, 'in' operator is similar to the SQL. (estimate whether the String have some char(s))

Example:

X = 'banana''n' in X  # true'nan' in X #true'm' in X #false

6.6 String Library

greet = 'Banana'

zap = greet.lower()  # lower() function gives us lower case of a String

type(greet)               # define what's the type of greet, actual any variable can use this function

dir(greet)                  # is equal to 'help', means what can I do to this 'greet'

str.capitalize()

str.replace(old,new[, count])

str.center(width[, fillchar])      #Return centered in a string of lengthwidth. Padding is done using the specifiedfillchar (default is a space).

str.endswith(suffix[, start[, end]])

str.startswith(suffix[, start[, end]])

str.find(sub[, start[, end]])       # return the firstindex of the sub containing in str ; if nothing found, return -1 [we can find chars start from the 'start' to 'end']

str.strip([chars])    #also have rstrip() and lstrip() ,, strip space like trim() function

Example:

text = "X-DSPAM-Confidence:    0.8475";index = text.find(':')print float(text[index+1:])   # will print 0.8475

7 File Processing

A text file can be seen as a sequence of lines

7.1 First: open()  function  -  handle = open(filename, mode)

      Ex:  handle = open('mbox.txt','r')   # the data is still in disk driver

      if print handle,   <open file 'mbox.txt', mode 'r' at 0x1005088b0>

The newline (\n) Character to indicate when a line ends,Newline is still one character not two. So a text file hasnewlines at the end of each line

A file handle open for read can be treated as a sequence of strings where each line in the file is a string in the sequence. So we can use the 'for' loop to iterate this file, like:

 

xfile = open('mbox.txt')for 'line' in xfile:    print line

7.2 Reading the entire file

fhand = open('mbox.txt')

inp = fhand.read()     # read the whole file, make sure you file not larger than your memory

print len(inp)

Practice1:

# Use words.txt as the file namefname = raw_input("Enter file name: ")fh = open(fname)for line in fh:    print line.rstrip().upper()

Practice2:

# Use the file name mbox-short.txt as the file namefname = raw_input("Enter file name: ")fh = open(fname)count = 0sum1 = 0for line in fh:    if not line.startswith("X-DSPAM-Confidence:") : continue    count = count + 1    index = line.find(':')    sum1 = sum1+float(line[index+1:])    print "Average spam confidence:", sum1/count

8. List (similar to array)

Example:  X = ['red', 'yellow', 'blue']   (collection)

NOTE:  A list element can be any Python object - even another list. So the list can be nested

We can also use index to access the element in the list like the string

Ex:

      friends = ['Joseph','Glenn']   # so friends[0] = 'Joseph'

However, Strings are 'immutable', mean we can't use str[0]='c' to change the letter in 'str'; But Lists are 'mutable', we can do this to a list

At the same time, we can still use len() function to get length of a list

range() function returns a list of numbers that range from 0 to one less than the parameter, here is a example:

range(4)  # return a list [0,1,2,3], So we can use range(len(list)) to construct a 'for' loop

We can concatenate lists using '+'   # a = [1,2]   b = [3,4]   c = a+b # which is [1,2,3,4]

List can be sliced using ':', the same to String  # the second number is "up to but not including"

dir(list)  # we can get some function about list object:

1. list.append()  # we can add any kind of type at the end of list

2. Is Something in a list?   using 'in' operator

3. List is an Ordered Sequence..  So we can use  list.sort() function to sort the list

4. Use some math functions, like: max(list), min(list), sum(list) ............

5. We can use str.split() function to construct a list. # split() uses space to separate a string by default ( many spaces are seen as one space, we can also specify the delimiter)

Practice:

fname = raw_input("Enter file name: ")fh = open(fname)lst = list()for line in fh:    templist=line.rstrip().split()    for item in templist:        if item in lst: continue        lst.append(item)            lst.sort()print lst     # can't combine these two code, like print lst.sort(), it will print nothing

9. Materials - Dictionaries

There are two kinds of Collections, List and Dictionary

List: a linear collection of values that stay in order

Dictionary: a 'bag' of values, each with its own label. (key - value)

Dictionaries are python's most powerful data collection, which have different names in other languages:

Associative Arrays - Perl/php

Properties or Ma or HashMap - java

Property Bag - C# / .Net

Example of Dictionaries:

    purse = dict()

    purse['money'] = 12

    purse['candy'] = 3

    purse['tissues'] = 75

      #{'money':12, 'tissues':75, 'candy':3}

    we can access item in the 'bag' like: purse['money'] = purse['money'] + 2

Dictionaries are like Lists except that they use keys instead of positions to look up values

The 'get' method for dictionary:  dict.get('key',0)  # if key doesn't exist, it returns 0 by default

So here is an easy way to construct a dictionary:

counts = dict()names = ['csev','cwen','csev','zqian']for name in names:    counts[name] = counts.get(name,0)+1print counts

Traverse the Dictionaries, using definite loops (very efficient)

       counts = {..............}   # a dictionary

       for key in counts

             print key, counts[key]

list(dict) and dict.keys()  functions will both return the keylist of the dictionary

dict.values() function will return the valuelist of the dictionary

dict.items() function will return 'tuple' of the dictionary  # tuple is also kind of list, but the item is a bag

So another way of traversing the dictionary:

      counts = {..............}   # a dictionary

       for key, value in counts.items()

             print key, value     # which is more straightforward

10. Tuples

Tuples are much like List, but tuple is immutable. and its format is (1,2,3)  not [1,2,3]

We can do assignment to the tuple like this: (x,y) = (4,'fred')

Compared to list, tuple is much more efficient because it is immutable.

Tuples are Comparable. Python goes on to the next element from the first item until it finds elements that differ.

list.sort(reverse=True)  # sort the list in descent order

Some Built-in function ,If you are still interested in more about the Python.

  Built-in Functions  abs()divmod()input()open()staticmethod()all()enumerate()int()ord()str()any()eval()isinstance()pow()sum()basestring()execfile()issubclass()print()super()bin()file()iter()property()tuple()bool()filter()len()range()type()bytearray()float()list()raw_input()unichr()callable()format()locals()reduce()unicode()chr()frozenset()long()reload()vars()classmethod()getattr()map()repr()xrange()cmp()globals()max()reversed()zip()compile()hasattr()memoryview()round()__import__()complex()hash()min()set() delattr()help()next()setattr() dict()hex()object()slice() dir()id()oct()sorted()

In addition, there are other four built-in functions that are no longer considered essential:apply(),buffer(),coerce(), andintern(). They are documented in theNon-essential Built-in Functions section.

abs(x)¶

Return the absolute value of a number. The argument may be a plain or long integer or a floating point number. If the argument is a complex number, its magnitude is returned.

all(iterable)¶

Return True if all elements of theiterable are true (or if the iterable is empty). Equivalent to:

def all(iterable):    for element in iterable:        if not element:            return False    return True

New in version 2.5.

any(iterable)¶

Return True if any element of theiterable is true. If the iterable is empty, returnFalse. Equivalent to:

def any(iterable):    for element in iterable:        if element:            return True    return False

New in version 2.5.

basestring()¶

This abstract type is the superclass for str andunicode. It cannot be called or instantiated, but it can be used to test whether an object is an instance ofstr orunicode.isinstance(obj,basestring) is equivalent to isinstance(obj,(str,unicode)).

New in version 2.3.

bin(x)¶

Convert an integer number to a binary string. The result is a valid Python expression. Ifx is not a Pythonint object, it has to define an__index__() method that returns an integer.

New in version 2.6.

classbool([x])¶

Return a Boolean value, i.e. one of True orFalse.x is converted using the standard truth testing procedure. Ifx is false or omitted, this returnsFalse; otherwise it returnsTrue.bool is also a class, which is a subclass ofint. Classbool cannot be subclassed further. Its only instances areFalse andTrue.

New in version 2.2.1.

Changed in version 2.3: If no argument is given, this function returnsFalse.

classbytearray([source[,encoding[,errors]]])¶

Return a new array of bytes. The bytearray class is a mutable sequence of integers in the range 0 <= x < 256. It has most of the usual methods of mutable sequences, described inMutable Sequence Types, as well as most methods that the str type has, seeString Methods.

The optional source parameter can be used to initialize the array in a few different ways:

• If it is unicode, you must also give the encoding (and optionally,errors) parameters;bytearray() then converts the unicode to bytes usingunicode.encode().
• If it is an integer, the array will have that size and will be initialized with null bytes.
• If it is an object conforming to the buffer interface, a read-only buffer of the object will be used to initialize the bytes array.
• If it is an iterable, it must be an iterable of integers in the range0<=x< 256, which are used as the initial contents of the array.

Without an argument, an array of size 0 is created.

New in version 2.6.

callable(object)¶

Return True if theobject argument appears callable,False if not. If this returns true, it is still possible that a call fails, but if it is false, callingobject will never succeed. Note that classes are callable (calling a class returns a new instance); class instances are callable if they have a__call__() method.

chr(i)¶

Return a string of one character whose ASCII code is the integer i. For example,chr(97) returns the string'a'. This is the inverse oford(). The argument must be in the range [0..255], inclusive;ValueError will be raised ifi is outside that range. See also unichr().

classmethod(function)¶

Return a class method for function.

A class method receives the class as implicit first argument, just like an instance method receives the instance. To declare a class method, use this idiom:

class C(object):    @classmethod    def f(cls, arg1, arg2, ...):        ...

The @classmethod form is a functiondecorator – see the description of function definitions inFunction definitions for details.

It can be called either on the class (such as C.f()) or on an instance (such asC().f()). The instance is ignored except for its class. If a class method is called for a derived class, the derived class object is passed as the implied first argument.

Class methods are different than C++ or Java static methods. If you want those, seestaticmethod() in this section.

For more information on class methods, consult the documentation on the standard type hierarchy inThe standard type hierarchy.

New in version 2.2.

Changed in version 2.4: Function decorator syntax added.

cmp(x,y)¶

Compare the two objects x and y and return an integer according to the outcome. The return value is negative ifx<y, zero ifx==y and strictly positive ifx>y.

compile(source,filename,mode[,flags[,dont_inherit]])¶

Compile the source into a code or AST object. Code objects can be executed by anexec statement or evaluated by a call toeval().source can either be a Unicode string, a Latin-1 encoded string or an AST object. Refer to theast module documentation for information on how to work with AST objects.

The filename argument should give the file from which the code was read; pass some recognizable value if it wasn’t read from a file ('<string>' is commonly used).

The mode argument specifies what kind of code must be compiled; it can be'exec' ifsource consists of a sequence of statements,'eval' if it consists of a single expression, or'single' if it consists of a single interactive statement (in the latter case, expression statements that evaluate to something other thanNone will be printed).

The optional arguments flags and dont_inherit control which future statements (seePEP 236) affect the compilation ofsource. If neither is present (or both are zero) the code is compiled with those future statements that are in effect in the code that is callingcompile(). If theflags argument is given and dont_inherit is not (or is zero) then the future statements specified by theflags argument are used in addition to those that would be used anyway. Ifdont_inherit is a non-zero integer then the flags argument is it – the future statements in effect around the call to compile are ignored.

Future statements are specified by bits which can be bitwise ORed together to specify multiple statements. The bitfield required to specify a given feature can be found as thecompiler_flag attribute on the_Feature instance in the__future__ module.

This function raises SyntaxError if the compiled source is invalid, andTypeError if the source contains null bytes.

If you want to parse Python code into its AST representation, see ast.parse().

Note

When compiling a string with multi-line code in 'single' or 'eval' mode, input must be terminated by at least one newline character. This is to facilitate detection of incomplete and complete statements in thecode module.

Changed in version 2.3: The flags anddont_inherit arguments were added.

Changed in version 2.6: Support for compiling AST objects.

Changed in version 2.7: Allowed use of Windows and Mac newlines. Also input in'exec' mode does not have to end in a newline anymore.

classcomplex([real[,imag]])¶

Return a complex number with the value real + imag*1j or convert a string or number to a complex number. If the first parameter is a string, it will be interpreted as a complex number and the function must be called without a second parameter. The second parameter can never be a string. Each argument may be any numeric type (including complex). Ifimag is omitted, it defaults to zero and the function serves as a numeric conversion function likeint(),long() andfloat(). If both arguments are omitted, returns0j.

Note

When converting from a string, the string must not contain whitespace around the central+ or- operator. For example,complex('1+2j') is fine, butcomplex('1 + 2j') raises ValueError.

The complex type is described in Numeric Types — int, float, long, complex.

delattr(object,name)¶

This is a relative of setattr(). The arguments are an object and a string. The string must be the name of one of the object’s attributes. The function deletes the named attribute, provided the object allows it. For example, delattr(x,'foobar') is equivalent todelx.foobar.

class dict(**kwarg)

class dict(mapping,**kwarg)

class dict(iterable,**kwarg)

Create a new dictionary. The dict object is the dictionary class. Seedict andMapping Types — dict for documentation about this class.

For other containers see the built-in list,set, andtuple classes, as well as thecollections module.

dir([object])¶

Without arguments, return the list of names in the current local scope. With an argument, attempt to return a list of valid attributes for that object.

If the object has a method named __dir__(), this method will be called and must return the list of attributes. This allows objects that implement a custom__getattr__() or__getattribute__() function to customize the waydir() reports their attributes.

If the object does not provide __dir__(), the function tries its best to gather information from the object’s__dict__ attribute, if defined, and from its type object. The resulting list is not necessarily complete, and may be inaccurate when the object has a custom__getattr__().

The default dir() mechanism behaves differently with different types of objects, as it attempts to produce the most relevant, rather than complete, information:

• If the object is a module object, the list contains the names of the module’s attributes.
• If the object is a type or class object, the list contains the names of its attributes, and recursively of the attributes of its bases.
• Otherwise, the list contains the object’s attributes’ names, the names of its class’s attributes, and recursively of the attributes of its class’s base classes.

The resulting list is sorted alphabetically. For example:

>>>

>>> import struct>>> dir()   # show the names in the module namespace['__builtins__', '__doc__', '__name__', 'struct']>>> dir(struct)   # show the names in the struct module['Struct', '__builtins__', '__doc__', '__file__', '__name__', '__package__', '_clearcache', 'calcsize', 'error', 'pack', 'pack_into', 'unpack', 'unpack_from']>>> class Shape(object):        def __dir__(self):            return ['area', 'perimeter', 'location']>>> s = Shape()>>> dir(s)['area', 'perimeter', 'location']

Note

Because dir() is supplied primarily as a convenience for use at an interactive prompt, it tries to supply an interesting set of names more than it tries to supply a rigorously or consistently defined set of names, and its detailed behavior may change across releases. For example, metaclass attributes are not in the result list when the argument is a class.

divmod(a,b)¶

Take two (non complex) numbers as arguments and return a pair of numbers consisting of their quotient and remainder when using long division. With mixed operand types, the rules for binary arithmetic operators apply. For plain and long integers, the result is the same as (a // b, a % b). For floating point numbers the result is(q,a%b), whereq is usuallymath.floor(a/b) but may be 1 less than that. In any caseq*b+ a % b is very close toa, if a% b is non-zero it has the same sign asb, and0<=abs(a %b)< abs(b).

Changed in version 2.3: Using divmod() with complex numbers is deprecated.

enumerate(sequence,start=0)¶

Return an enumerate object. sequence must be a sequence, an iterator, or some other object which supports iteration. Thenext() method of the iterator returned byenumerate() returns a tuple containing a count (fromstart which defaults to 0) and the values obtained from iterating oversequence:

>>>

>>> seasons = ['Spring', 'Summer', 'Fall', 'Winter']>>> list(enumerate(seasons))[(0, 'Spring'), (1, 'Summer'), (2, 'Fall'), (3, 'Winter')]>>> list(enumerate(seasons, start=1))[(1, 'Spring'), (2, 'Summer'), (3, 'Fall'), (4, 'Winter')]

Equivalent to:

def enumerate(sequence, start=0):    n = start    for elem in sequence:        yield n, elem        n += 1

New in version 2.3.

Changed in version 2.6: The start parameter was added.

eval(expression[,globals[,locals]])¶

The arguments are a Unicode or Latin-1 encoded string and optional globals and locals. If provided,globals must be a dictionary. If provided,locals can be any mapping object.

Changed in version 2.4: formerly locals was required to be a dictionary.

The expression argument is parsed and evaluated as a Python expression (technically speaking, a condition list) using theglobals andlocals dictionaries as global and local namespace. If theglobals dictionary is present and lacks ‘__builtins__’, the current globals are copied intoglobals beforeexpression is parsed. This means thatexpression normally has full access to the standard__builtin__ module and restricted environments are propagated. If thelocals dictionary is omitted it defaults to theglobals dictionary. If both dictionaries are omitted, the expression is executed in the environment whereeval() is called. The return value is the result of the evaluated expression. Syntax errors are reported as exceptions. Example:

>>>

>>> x = 1>>> print eval('x+1')2

This function can also be used to execute arbitrary code objects (such as those created bycompile()). In this case pass a code object instead of a string. If the code object has been compiled with'exec' as themode argument,eval()‘s return value will beNone.

Hints: dynamic execution of statements is supported by the exec statement. Execution of statements from a file is supported by theexecfile() function. Theglobals() andlocals() functions returns the current global and local dictionary, respectively, which may be useful to pass around for use byeval() orexecfile().

See ast.literal_eval() for a function that can safely evaluate strings with expressions containing only literals.

execfile(filename[,globals[,locals]])¶

This function is similar to the exec statement, but parses a file instead of a string. It is different from theimport statement in that it does not use the module administration — it reads the file unconditionally and does not create a new module.[1]

The arguments are a file name and two optional dictionaries. The file is parsed and evaluated as a sequence of Python statements (similarly to a module) using theglobals andlocals dictionaries as global and local namespace. If provided,locals can be any mapping object. Remember that at module level, globals and locals are the same dictionary. If two separate objects are passed asglobals andlocals, the code will be executed as if it were embedded in a class definition.

Changed in version 2.4: formerly locals was required to be a dictionary.

If the locals dictionary is omitted it defaults to the globals dictionary. If both dictionaries are omitted, the expression is executed in the environment whereexecfile() is called. The return value isNone.

Note

The default locals act as described for function locals() below: modifications to the defaultlocals dictionary should not be attempted. Pass an explicitlocals dictionary if you need to see effects of the code onlocals after functionexecfile() returns.execfile() cannot be used reliably to modify a function’s locals.

file(name[,mode[,buffering]])¶

Constructor function for the file type, described further in sectionFile Objects. The constructor’s arguments are the same as those of theopen() built-in function described below.

When opening a file, it’s preferable to use open() instead of invoking this constructor directly.file is more suited to type testing (for example, writingisinstance(f,file)).

New in version 2.2.

filter(function,iterable)¶

Construct a list from those elements of iterable for which function returns true.iterable may be either a sequence, a container which supports iteration, or an iterator. Ifiterable is a string or a tuple, the result also has that type; otherwise it is always a list. Iffunction is None, the identity function is assumed, that is, all elements ofiterable that are false are removed.

Note that filter(function,iterable) is equivalent to[itemforiteminiterableiffunction(item)] if function is notNone and[itemforiteminiterableifitem] if function isNone.

See itertools.ifilter() anditertools.ifilterfalse() for iterator versions of this function, including a variation that filters for elements where thefunction returns false.

classfloat([x])¶

Return a floating point number constructed from a number or string x.

If the argument is a string, it must contain a possibly signed decimal or floating point number, possibly embedded in whitespace. The argument may also be [+|-]nan or [+|-]inf. Otherwise, the argument may be a plain or long integer or a floating point number, and a floating point number with the same value (within Python’s floating point precision) is returned. If no argument is given, returns0.0.

Note

When passing in a string, values for NaN and Infinity may be returned, depending on the underlying C library. Float accepts the strings nan, inf and -inf for NaN and positive or negative infinity. The case and a leading + are ignored as well as a leading - is ignored for NaN. Float always represents NaN and infinity as nan, inf or -inf.

The float type is described in Numeric Types — int, float, long, complex.

format(value[,format_spec])¶

Convert a value to a “formatted” representation, as controlled byformat_spec. The interpretation offormat_spec will depend on the type of thevalue argument, however there is a standard formatting syntax that is used by most built-in types:Format Specification Mini-Language.

Note

format(value,format_spec) merely callsvalue.__format__(format_spec).

New in version 2.6.

class frozenset([iterable])

Return a new frozenset object, optionally with elements taken fromiterable.frozenset is a built-in class. Seefrozenset andSet Types — set, frozenset for documentation about this class.

For other containers see the built-in set,list,tuple, anddict classes, as well as thecollections module.

New in version 2.4.

getattr(object,name[,default])¶

Return the value of the named attribute of object. name must be a string. If the string is the name of one of the object’s attributes, the result is the value of that attribute. For example,getattr(x,'foobar') is equivalent to x.foobar. If the named attribute does not exist,default is returned if provided, otherwiseAttributeError is raised.

globals()¶

Return a dictionary representing the current global symbol table. This is always the dictionary of the current module (inside a function or method, this is the module where it is defined, not the module from which it is called).

hasattr(object,name)¶

The arguments are an object and a string. The result is True if the string is the name of one of the object’s attributes,False if not. (This is implemented by callinggetattr(object,name) and seeing whether it raises an exception or not.)

hash(object)¶

Return the hash value of the object (if it has one). Hash values are integers. They are used to quickly compare dictionary keys during a dictionary lookup. Numeric values that compare equal have the same hash value (even if they are of different types, as is the case for 1 and 1.0).

help([object])¶

Invoke the built-in help system. (This function is intended for interactive use.) If no argument is given, the interactive help system starts on the interpreter console. If the argument is a string, then the string is looked up as the name of a module, function, class, method, keyword, or documentation topic, and a help page is printed on the console. If the argument is any other kind of object, a help page on the object is generated.

This function is added to the built-in namespace by the site module.

New in version 2.2.

hex(x)¶

Convert an integer number (of any size) to a lowercase hexadecimal string prefixed with “0x”, for example:

>>>

>>> hex(255)'0xff'>>> hex(-42)'-0x2a'>>> hex(1L)'0x1L'

If x is not a Python int orlong object, it has to define an __index__() method that returns an integer.

See also int() for converting a hexadecimal string to an integer using a base of 16.

Note

To obtain a hexadecimal string representation for a float, use thefloat.hex() method.

Changed in version 2.4: Formerly only returned an unsigned literal.

id(object)¶

Return the “identity” of an object. This is an integer (or long integer) which is guaranteed to be unique and constant for this object during its lifetime. Two objects with non-overlapping lifetimes may have the sameid() value.

CPython implementation detail: This is the address of the object in memory.

input([prompt])¶

Equivalent to eval(raw_input(prompt)).

This function does not catch user errors. If the input is not syntactically valid, aSyntaxError will be raised. Other exceptions may be raised if there is an error during evaluation.

If the readline module was loaded, theninput() will use it to provide elaborate line editing and history features.

Consider using the raw_input() function for general input from users.

classint(x=0)¶

class int(x,base=10)

Return an integer object constructed from a number or string x, or return0 if no arguments are given. Ifx is a number, it can be a plain integer, a long integer, or a floating point number. Ifx is floating point, the conversion truncates towards zero. If the argument is outside the integer range, the function returns a long object instead.

If x is not a number or if base is given, then x must be a string or Unicode object representing aninteger literal in radix base. Optionally, the literal can be preceded by+ or- (with no space in between) and surrounded by whitespace. A base-n literal consists of the digits 0 to n-1, witha toz (orA toZ) having values 10 to 35. The defaultbase is 10. The allowed values are 0 and 2-36. Base-2, -8, and -16 literals can be optionally prefixed with0b/0B,0o/0O/0, or0x/0X, as with integer literals in code. Base 0 means to interpret the string exactly as an integer literal, so that the actual base is 2, 8, 10, or 16.

The integer type is described in Numeric Types — int, float, long, complex.

isinstance(object,classinfo)¶

Return true if the object argument is an instance of the classinfo argument, or of a (direct, indirect orvirtual) subclass thereof. Also return true ifclassinfo is a type object (new-style class) andobject is an object of that type or of a (direct, indirect orvirtual) subclass thereof. If object is not a class instance or an object of the given type, the function always returns false. Ifclassinfo is a tuple of class or type objects (or recursively, other such tuples), return true ifobject is an instance of any of the classes or types. If classinfo is not a class, type, or tuple of classes, types, and such tuples, aTypeError exception is raised.

Changed in version 2.2: Support for a tuple of type information was added.

issubclass(class,classinfo)¶

Return true if class is a subclass (direct, indirect or virtual) of classinfo. A class is considered a subclass of itself.classinfo may be a tuple of class objects, in which case every entry inclassinfo will be checked. In any other case, aTypeError exception is raised.

Changed in version 2.3: Support for a tuple of type information was added.

iter(o[,sentinel])¶

Return an iterator object. The first argument is interpreted very differently depending on the presence of the second argument. Without a second argument,o must be a collection object which supports the iteration protocol (the__iter__() method), or it must support the sequence protocol (the__getitem__() method with integer arguments starting at0). If it does not support either of those protocols,TypeError is raised. If the second argument,sentinel, is given, then o must be a callable object. The iterator created in this case will callo with no arguments for each call to itsnext() method; if the value returned is equal tosentinel,StopIteration will be raised, otherwise the value will be returned.

One useful application of the second form of iter() is to read lines of a file until a certain line is reached. The following example reads a file until thereadline() method returns an empty string:

with open('mydata.txt') as fp:    for line in iter(fp.readline, ''):        process_line(line)

New in version 2.2.

len(s)¶

Return the length (the number of items) of an object. The argument may be a sequence (such as a string, bytes, tuple, list, or range) or a collection (such as a dictionary, set, or frozen set).

classlist([iterable])¶

Return a list whose items are the same and in the same order as iterable‘s items.iterable may be either a sequence, a container that supports iteration, or an iterator object. Ifiterable is already a list, a copy is made and returned, similar to iterable[:]. For instance,list('abc') returns['a','b','c'] andlist((1,2,3)) returns[1,2,3]. If no argument is given, returns a new empty list,[].

list is a mutable sequence type, as documented inSequence Types — str, unicode, list, tuple, bytearray, buffer, xrange. For other containers see the built indict,set, andtuple classes, and thecollections module.

locals()¶

Update and return a dictionary representing the current local symbol table. Free variables are returned bylocals() when it is called in function blocks, but not in class blocks.

Note

The contents of this dictionary should not be modified; changes may not affect the values of local and free variables used by the interpreter.

classlong(x=0)¶

class long(x,base=10)

Return a long integer object constructed from a string or number x. If the argument is a string, it must contain a possibly signed number of arbitrary size, possibly embedded in whitespace. Thebase argument is interpreted in the same way as for int(), and may only be given whenx is a string. Otherwise, the argument may be a plain or long integer or a floating point number, and a long integer with the same value is returned. Conversion of floating point numbers to integers truncates (towards zero). If no arguments are given, returns0L.

The long type is described in Numeric Types — int, float, long, complex.

map(function,iterable,...)¶

Apply function to every item of iterable and return a list of the results. If additionaliterable arguments are passed,function must take that many arguments and is applied to the items from all iterables in parallel. If one iterable is shorter than another it is assumed to be extended withNone items. Iffunction isNone, the identity function is assumed; if there are multiple arguments,map() returns a list consisting of tuples containing the corresponding items from all iterables (a kind of transpose operation). Theiterable arguments may be a sequence or any iterable object; the result is always a list.

max(iterable[,key])¶

max(arg1,arg2,*args[,key])

Return the largest item in an iterable or the largest of two or more arguments.

If one positional argument is provided, iterable must be a non-empty iterable (such as a non-empty string, tuple or list). The largest item in the iterable is returned. If two or more positional arguments are provided, the largest of the positional arguments is returned.

The optional key argument specifies a one-argument ordering function like that used forlist.sort(). Thekey argument, if supplied, must be in keyword form (for example, max(a,b,c,key=func)).

Changed in version 2.5: Added support for the optionalkey argument.

memoryview(obj)

Return a “memory view” object created from the given argument. See memoryview type for more information.

min(iterable[,key])¶

min(arg1,arg2,*args[,key])

Return the smallest item in an iterable or the smallest of two or more arguments.

If one positional argument is provided, iterable must be a non-empty iterable (such as a non-empty string, tuple or list). The smallest item in the iterable is returned. If two or more positional arguments are provided, the smallest of the positional arguments is returned.

The optional key argument specifies a one-argument ordering function like that used forlist.sort(). Thekey argument, if supplied, must be in keyword form (for example, min(a,b,c,key=func)).

Changed in version 2.5: Added support for the optionalkey argument.

next(iterator[,default])¶

Retrieve the next item from the iterator by calling its next() method. Ifdefault is given, it is returned if the iterator is exhausted, otherwiseStopIteration is raised.

New in version 2.6.

classobject¶

Return a new featureless object. object is a base for all new style classes. It has the methods that are common to all instances of new style classes.

New in version 2.2.

Changed in version 2.3: This function does not accept any arguments. Formerly, it accepted arguments but ignored them.

oct(x)¶

Convert an integer number (of any size) to an octal string. The result is a valid Python expression.

Changed in version 2.4: Formerly only returned an unsigned literal.

open(name[,mode[,buffering]])¶

Open a file, returning an object of the file type described in sectionFile Objects. If the file cannot be opened, IOError is raised. When opening a file, it’s preferable to useopen() instead of invoking thefile constructor directly.

The first two arguments are the same as for stdio‘sfopen():name is the file name to be opened, andmode is a string indicating how the file is to be opened.

The most commonly-used values of mode are 'r' for reading, 'w' for writing (truncating the file if it already exists), and'a' for appending (which onsome Unix systems means that all writes append to the end of the file regardless of the current seek position). Ifmode is omitted, it defaults to'r'. The default is to use text mode, which may convert'\n' characters to a platform-specific representation on writing and back on reading. Thus, when opening a binary file, you should append'b' to themode value to open the file in binary mode, which will improve portability. (Appending'b' is useful even on systems that don’t treat binary and text files differently, where it serves as documentation.) See below for more possible values ofmode.

The optional buffering argument specifies the file’s desired buffer size: 0 means unbuffered, 1 means line buffered, any other positive value means use a buffer of (approximately) that size (in bytes). A negativebuffering means to use the system default, which is usually line buffered for tty devices and fully buffered for other files. If omitted, the system default is used.[2]

Modes 'r+', 'w+' and 'a+' open the file for updating (reading and writing); note that'w+' truncates the file. Append'b' to the mode to open the file in binary mode, on systems that differentiate between binary and text files; on systems that don’t have this distinction, adding the'b' has no effect.

In addition to the standard fopen() values mode may be 'U' or 'rU'. Python is usually built withuniversal newlines support; supplying 'U' opens the file as a text file, but lines may be terminated by any of the following: the Unix end-of-line convention'\n', the Macintosh convention'\r', or the Windows convention'\r\n'. All of these external representations are seen as'\n' by the Python program. If Python is built without universal newlines support amode with'U' is the same as normal text mode. Note that file objects so opened also have an attribute callednewlines which has a value ofNone (if no newlines have yet been seen),'\n','\r','\r\n', or a tuple containing all the newline types seen.

Python enforces that the mode, after stripping 'U', begins with 'r','w' or'a'.

Python provides many file handling modules including fileinput,os,os.path,tempfile, andshutil.

Changed in version 2.5: Restriction on first letter of mode string introduced.

ord(c)¶

Given a string of length one, return an integer representing the Unicode code point of the character when the argument is a unicode object, or the value of the byte when the argument is an 8-bit string. For example,ord('a') returns the integer97,ord(u'\u2020') returns8224. This is the inverse ofchr() for 8-bit strings and ofunichr() for unicode objects. If a unicode argument is given and Python was built with UCS2 Unicode, then the character’s code point must be in the range [0..65535] inclusive; otherwise the string length is two, and aTypeError will be raised.

pow(x,y[,z])¶

Return x to the power y; if z is present, return x to the power y, modulo z (computed more efficiently thanpow(x,y)% z). The two-argument formpow(x,y) is equivalent to using the power operator:x**y.

The arguments must have numeric types. With mixed operand types, the coercion rules for binary arithmetic operators apply. For int and long int operands, the result has the same type as the operands (after coercion) unless the second argument is negative; in that case, all arguments are converted to float and a float result is delivered. For example,10**2 returns100, but10**-2 returns0.01. (This last feature was added in Python 2.2. In Python 2.1 and before, if both arguments were of integer types and the second argument was negative, an exception was raised.) If the second argument is negative, the third argument must be omitted. Ifz is present,x andy must be of integer types, and y must be non-negative. (This restriction was added in Python 2.2. In Python 2.1 and before, floating 3-argumentpow() returned platform-dependent results depending on floating-point rounding accidents.)

print(*objects,sep=' ',end='\n',file=sys.stdout)¶

Print objects to the stream file, separated by sep and followed byend.sep,end and file, if present, must be given as keyword arguments.

All non-keyword arguments are converted to strings like str() does and written to the stream, separated bysep and followed byend. Bothsep and end must be strings; they can also beNone, which means to use the default values. If noobjects are given,print() will just writeend.

The file argument must be an object with a write(string) method; if it is not present or None, sys.stdout will be used. Output buffering is determined byfile. Usefile.flush() to ensure, for instance, immediate appearance on a screen.

Note

This function is not normally available as a built-in since the name print is recognized as the print statement. To disable the statement and use theprint() function, use this future statement at the top of your module:

from __future__ import print_function

New in version 2.6.

classproperty([fget[,fset[,fdel[,doc]]]])¶

Return a property attribute for new-style classes (classes that derive fromobject).

fget is a function for getting an attribute value. fset is a function for setting an attribute value.fdel is a function for deleting an attribute value. Anddoc creates a docstring for the attribute.

A typical use is to define a managed attribute x:

class C(object):    def __init__(self):        self._x = None    def getx(self):        return self._x    def setx(self, value):        self._x = value    def delx(self):        del self._x    x = property(getx, setx, delx, "I'm the 'x' property.")

If c is an instance of C, c.x will invoke the getter,c.x=value will invoke the setter anddelc.x the deleter.

If given, doc will be the docstring of the property attribute. Otherwise, the property will copyfget‘s docstring (if it exists). This makes it possible to create read-only properties easily usingproperty() as adecorator:

class Parrot(object):    def __init__(self):        self._voltage = 100000    @property    def voltage(self):        """Get the current voltage."""        return self._voltage

The @property decorator turns thevoltage() method into a “getter” for a read-only attribute with the same name, and it sets the docstring forvoltage to “Get the current voltage.”

A property object has getter,setter, anddeleter methods usable as decorators that create a copy of the property with the corresponding accessor function set to the decorated function. This is best explained with an example:

class C(object):    def __init__(self):        self._x = None    @property    def x(self):        """I'm the 'x' property."""        return self._x    @x.setter    def x(self, value):        self._x = value    @x.deleter    def x(self):        del self._x

This code is exactly equivalent to the first example. Be sure to give the additional functions the same name as the original property (x in this case.)

The returned property object also has the attributes fget, fset, andfdel corresponding to the constructor arguments.

New in version 2.2.

Changed in version 2.5: Use fget‘s docstring if nodoc given.

Changed in version 2.6: The getter, setter, anddeleter attributes were added.

range(stop)¶

range(start,stop[,step])

This is a versatile function to create lists containing arithmetic progressions. It is most often used infor loops. The arguments must be plain integers. If thestep argument is omitted, it defaults to1. If thestart argument is omitted, it defaults to0. The full form returns a list of plain integers[start,start+ step,start+ 2* step, ...]. Ifstep is positive, the last element is the largeststart +i*step less thanstop; ifstep is negative, the last element is the smalleststart+i* step greater thanstop. step must not be zero (or elseValueError is raised). Example:

>>>

>>> range(10)[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]>>> range(1, 11)[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]>>> range(0, 30, 5)[0, 5, 10, 15, 20, 25]>>> range(0, 10, 3)[0, 3, 6, 9]>>> range(0, -10, -1)[0, -1, -2, -3, -4, -5, -6, -7, -8, -9]>>> range(0)[]>>> range(1, 0)[]

raw_input([prompt])¶

If the prompt argument is present, it is written to standard output without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that. When EOF is read,EOFError is raised. Example:

>>>

>>> s = raw_input('--> ')--> Monty Python's Flying Circus>>> s"Monty Python's Flying Circus"

If the readline module was loaded, thenraw_input() will use it to provide elaborate line editing and history features.

reduce(function,iterable[,initializer])¶

Apply function of two arguments cumulatively to the items of iterable, from left to right, so as to reduce the iterable to a single value. For example,reduce(lambdax,y: x+y,[1,2, 3,4,5]) calculates ((((1+2)+3)+4)+5). The left argument,x, is the accumulated value and the right argument,y, is the update value from theiterable. If the optionalinitializer is present, it is placed before the items of the iterable in the calculation, and serves as a default when the iterable is empty. Ifinitializer is not given anditerable contains only one item, the first item is returned. Roughly equivalent to:

def reduce(function, iterable, initializer=None):    it = iter(iterable)    if initializer is None:        try:            initializer = next(it)        except StopIteration:            raise TypeError('reduce() of empty sequence with no initial value')    accum_value = initializer    for x in it:        accum_value = function(accum_value, x)    return accum_value

reload(module)¶

Reload a previously imported module. The argument must be a module object, so it must have been successfully imported before. This is useful if you have edited the module source file using an external editor and want to try out the new version without leaving the Python interpreter. The return value is the module object (the same as themodule argument).

When reload(module) is executed:

• Python modules’ code is recompiled and the module-level code reexecuted, defining a new set of objects which are bound to names in the module’s dictionary. Theinit function of extension modules is not called a second time.
• As with all other objects in Python the old objects are only reclaimed after their reference counts drop to zero.
• The names in the module namespace are updated to point to any new or changed objects.
• Other references to the old objects (such as names external to the module) are not rebound to refer to the new objects and must be updated in each namespace where they occur if that is desired.

There are a number of other caveats:

When a module is reloaded, its dictionary (containing the module’s global variables) is retained. Redefinitions of names will override the old definitions, so this is generally not a problem. If the new version of a module does not define a name that was defined by the old version, the old definition remains. This feature can be used to the module’s advantage if it maintains a global table or cache of objects — with atry statement it can test for the table’s presence and skip its initialization if desired:

try:    cacheexcept NameError:    cache = {}

It is generally not very useful to reload built-in or dynamically loaded modules. Reloadingsys,__main__,builtins and other key modules is not recommended. In many cases extension modules are not designed to be initialized more than once, and may fail in arbitrary ways when reloaded.

If a module imports objects from another module using from ...import ..., callingreload() for the other module does not redefine the objects imported from it — one way around this is to re-execute thefrom statement, another is to useimport and qualified names (module.*name*) instead.

If a module instantiates instances of a class, reloading the module that defines the class does not affect the method definitions of the instances — they continue to use the old class definition. The same is true for derived classes.

repr(object)¶

Return a string containing a printable representation of an object. This is the same value yielded by conversions (reverse quotes). It is sometimes useful to be able to access this operation as an ordinary function. For many types, this function makes an attempt to return a string that would yield an object with the same value when passed toeval(), otherwise the representation is a string enclosed in angle brackets that contains the name of the type of the object together with additional information often including the name and address of the object. A class can control what this function returns for its instances by defining a__repr__() method.

reversed(seq)¶

Return a reverse iterator. seq must be an object which has a__reversed__() method or supports the sequence protocol (the__len__() method and the__getitem__() method with integer arguments starting at0).

New in version 2.4.

Changed in version 2.6: Added the possibility to write a custom__reversed__() method.

round(number[,ndigits])¶

Return the floating point value number rounded to ndigits digits after the decimal point. Ifndigits is omitted, it defaults to zero. The result is a floating point number. Values are rounded to the closest multiple of 10 to the power minusndigits; if two multiples are equally close, rounding is done away from 0 (so, for example,round(0.5) is1.0 andround(-0.5) is-1.0).

Note

The behavior of round() for floats can be surprising: for example,round(2.675,2) gives2.67 instead of the expected2.68. This is not a bug: it’s a result of the fact that most decimal fractions can’t be represented exactly as a float. SeeFloating Point Arithmetic: Issues and Limitations for more information.

class set([iterable])

Return a new set object, optionally with elements taken fromiterable.set is a built-in class. Seeset andSet Types — set, frozenset for documentation about this class.

For other containers see the built-in frozenset,list,tuple, anddict classes, as well as thecollections module.

New in version 2.4.

setattr(object,name,value)¶

This is the counterpart of getattr(). The arguments are an object, a string and an arbitrary value. The string may name an existing attribute or a new attribute. The function assigns the value to the attribute, provided the object allows it. For example, setattr(x,'foobar',123) is equivalent tox.foobar=123.

classslice(stop)¶

class slice(start,stop[,step])

Return a slice object representing the set of indices specified byrange(start,stop,step). The start andstep arguments default toNone. Slice objects have read-only data attributesstart,stop andstep which merely return the argument values (or their default). They have no other explicit functionality; however they are used by Numerical Python and other third party extensions. Slice objects are also generated when extended indexing syntax is used. For example:a[start:stop:step] ora[start:stop,i]. See itertools.islice() for an alternate version that returns an iterator.

sorted(iterable[,cmp[,key[,reverse]]])¶

Return a new sorted list from the items in iterable.

The optional arguments cmp, key, and reverse have the same meaning as those for thelist.sort() method (described in sectionMutable Sequence Types).

cmp specifies a custom comparison function of two arguments (iterable elements) which should return a negative, zero or positive number depending on whether the first argument is considered smaller than, equal to, or larger than the second argument:cmp=lambdax,y:cmp(x.lower(), y.lower()). The default value isNone.

key specifies a function of one argument that is used to extract a comparison key from each list element:key=str.lower. The default value isNone (compare the elements directly).

reverse is a boolean value. If set to True, then the list elements are sorted as if each comparison were reversed.

In general, the key and reverse conversion processes are much faster than specifying an equivalentcmp function. This is becausecmp is called multiple times for each list element whilekey andreverse touch each element only once. Use functools.cmp_to_key() to convert an old-stylecmp function to akey function.

The built-in sorted() function is guaranteed to be stable. A sort is stable if it guarantees not to change the relative order of elements that compare equal — this is helpful for sorting in multiple passes (for example, sort by department, then by salary grade).

For sorting examples and a brief sorting tutorial, see Sorting HOW TO.

New in version 2.4.

staticmethod(function)¶

Return a static method for function.

A static method does not receive an implicit first argument. To declare a static method, use this idiom:

class C(object):    @staticmethod    def f(arg1, arg2, ...):        ...

The @staticmethod form is a functiondecorator – see the description of function definitions inFunction definitions for details.

It can be called either on the class (such as C.f()) or on an instance (such asC().f()). The instance is ignored except for its class.

Static methods in Python are similar to those found in Java or C++. Also see classmethod() for a variant that is useful for creating alternate class constructors.

For more information on static methods, consult the documentation on the standard type hierarchy inThe standard type hierarchy.

New in version 2.2.

Changed in version 2.4: Function decorator syntax added.

classstr(object='')¶

Return a string containing a nicely printable representation of an object. For strings, this returns the string itself. The difference withrepr(object) is thatstr(object) does not always attempt to return a string that is acceptable toeval(); its goal is to return a printable string. If no argument is given, returns the empty string,''.

For more information on strings see Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange which describes sequence functionality (strings are sequences), and also the string-specific methods described in theString Methods section. To output formatted strings use template strings or the% operator described in theString Formatting Operations section. In addition see the String Services section. See also unicode().

sum(iterable[,start])¶

Sums start and the items of an iterable from left to right and returns the total.start defaults to0. Theiterable‘s items are normally numbers, and the start value is not allowed to be a string.

For some use cases, there are good alternatives to sum(). The preferred, fast way to concatenate a sequence of strings is by calling''.join(sequence). To add floating point values with extended precision, seemath.fsum(). To concatenate a series of iterables, consider usingitertools.chain().

New in version 2.3.

super(type[,object-or-type])¶

Return a proxy object that delegates method calls to a parent or sibling class oftype. This is useful for accessing inherited methods that have been overridden in a class. The search order is same as that used bygetattr() except that thetype itself is skipped.

The __mro__ attribute of thetype lists the method resolution search order used by bothgetattr() andsuper(). The attribute is dynamic and can change whenever the inheritance hierarchy is updated.

If the second argument is omitted, the super object returned is unbound. If the second argument is an object,isinstance(obj,type) must be true. If the second argument is a type,issubclass(type2,type) must be true (this is useful for classmethods).

Note

super() only works fornew-style classes.

There are two typical use cases for super. In a class hierarchy with single inheritance,super can be used to refer to parent classes without naming them explicitly, thus making the code more maintainable. This use closely parallels the use ofsuper in other programming languages.

The second use case is to support cooperative multiple inheritance in a dynamic execution environment. This use case is unique to Python and is not found in statically compiled languages or languages that only support single inheritance. This makes it possible to implement “diamond diagrams” where multiple base classes implement the same method. Good design dictates that this method have the same calling signature in every case (because the order of calls is determined at runtime, because that order adapts to changes in the class hierarchy, and because that order can include sibling classes that are unknown prior to runtime).

For both use cases, a typical superclass call looks like this:

class C(B):    def method(self, arg):        super(C, self).method(arg)

Note that super() is implemented as part of the binding process for explicit dotted attribute lookups such assuper().__getitem__(name). It does so by implementing its own__getattribute__() method for searching classes in a predictable order that supports cooperative multiple inheritance. Accordingly,super() is undefined for implicit lookups using statements or operators such assuper()[name].

Also note that super() is not limited to use inside methods. The two argument form specifies the arguments exactly and makes the appropriate references.

For practical suggestions on how to design cooperative classes using super(), seeguide to using super().

New in version 2.2.

tuple([iterable])¶

Return a tuple whose items are the same and in the same order as iterable‘s items.iterable may be a sequence, a container that supports iteration, or an iterator object. Ifiterable is already a tuple, it is returned unchanged. For instance, tuple('abc') returns('a','b','c') andtuple([1,2,3]) returns (1,2, 3). If no argument is given, returns a new empty tuple,().

tuple is an immutable sequence type, as documented in Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange. For other containers see the built indict,list, andset classes, and thecollections module.

classtype(object)¶

class type(name,bases,dict)

With one argument, return the type of an object. The return value is a type object. Theisinstance() built-in function is recommended for testing the type of an object.

With three arguments, return a new type object. This is essentially a dynamic form of theclass statement. Thename string is the class name and becomes the __name__ attribute; thebases tuple itemizes the base classes and becomes the__bases__ attribute; and thedict dictionary is the namespace containing definitions for class body and becomes the__dict__ attribute. For example, the following two statements create identicaltype objects:

>>>

>>> class X(object):...     a = 1...>>> X = type('X', (object,), dict(a=1))

New in version 2.2.

unichr(i)¶

Return the Unicode string of one character whose Unicode code is the integer i. For example, unichr(97) returns the stringu'a'. This is the inverse oford() for Unicode strings. The valid range for the argument depends how Python was configured – it may be either UCS2 [0..0xFFFF] or UCS4 [0..0x10FFFF].ValueError is raised otherwise. For ASCII and 8-bit strings seechr().

New in version 2.0.

unicode(object='')¶

unicode(object[,encoding[,errors]])

Return the Unicode string version of object using one of the following modes:

If encoding and/or errors are given, unicode() will decode the object which can either be an 8-bit string or a character buffer using the codec forencoding. Theencoding parameter is a string giving the name of an encoding; if the encoding is not known,LookupError is raised. Error handling is done according toerrors; this specifies the treatment of characters which are invalid in the input encoding. Iferrors is'strict' (the default), aValueError is raised on errors, while a value of'ignore' causes errors to be silently ignored, and a value of'replace' causes the official Unicode replacement character,U+FFFD, to be used to replace input characters which cannot be decoded. See also thecodecs module.

If no optional parameters are given, unicode() will mimic the behaviour ofstr() except that it returns Unicode strings instead of 8-bit strings. More precisely, ifobject is a Unicode string or subclass it will return that Unicode string without any additional decoding applied.

For objects which provide a __unicode__() method, it will call this method without arguments to create a Unicode string. For all other objects, the 8-bit string version or representation is requested and then converted to a Unicode string using the codec for the default encoding in 'strict' mode.

For more information on Unicode strings see Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange which describes sequence functionality (Unicode strings are sequences), and also the string-specific methods described in theString Methods section. To output formatted strings use template strings or the% operator described in theString Formatting Operations section. In addition see the String Services section. See also str().

New in version 2.0.

Changed in version 2.2: Support for __unicode__() added.

vars([object])¶

Return the __dict__ attribute for a module, class, instance, or any other object with a__dict__ attribute.

Objects such as modules and instances have an updateable __dict__ attribute; however, other objects may have write restrictions on their__dict__ attributes (for example, new-style classes use a dictproxy to prevent direct dictionary updates).

Without an argument, vars() acts likelocals(). Note, the locals dictionary is only useful for reads since updates to the locals dictionary are ignored.

xrange(stop)¶

xrange(start,stop[,step])

This function is very similar to range(), but returns anxrange object instead of a list. This is an opaque sequence type which yields the same values as the corresponding list, without actually storing them all simultaneously. The advantage ofxrange() overrange() is minimal (sincexrange() still has to create the values when asked for them) except when a very large range is used on a memory-starved machine or when all of the range’s elements are never used (such as when the loop is usually terminated withbreak). For more information on xrange objects, seeXRange Type andSequence Types — str, unicode, list, tuple, bytearray, buffer, xrange.

CPython implementation detail: xrange() is intended to be simple and fast. Implementations may impose restrictions to achieve this. The C implementation of Python restricts all arguments to native C longs (“short” Python integers), and also requires that the number of elements fit in a native C long. If a larger range is needed, an alternate version can be crafted using theitertools module:islice(count(start,step),(stop-start+step-1+2*(step<0))//step).

zip([iterable,...])¶

This function returns a list of tuples, where the i-th tuple contains thei-th element from each of the argument sequences or iterables. The returned list is truncated in length to the length of the shortest argument sequence. When there are multiple arguments which are all of the same length,zip() is similar tomap() with an initial argument ofNone. With a single sequence argument, it returns a list of 1-tuples. With no arguments, it returns an empty list.

The left-to-right evaluation order of the iterables is guaranteed. This makes possible an idiom for clustering a data series into n-length groups usingzip(*[iter(s)]*n).

zip() in conjunction with the* operator can be used to unzip a list:

>>>

>>> x = [1, 2, 3]>>> y = [4, 5, 6]>>> zipped = zip(x, y)>>> zipped[(1, 4), (2, 5), (3, 6)]>>> x2, y2 = zip(*zipped)>>> x == list(x2) and y == list(y2)True

New in version 2.0.

Changed in version 2.4: Formerly, zip() required at least one argument andzip() raised aTypeError instead of returning an empty list.

__import__(name[,globals[,locals[,fromlist[,level]]]])¶

Note

This is an advanced function that is not needed in everyday Python programming, unlikeimportlib.import_module().

This function is invoked by the import statement. It can be replaced (by importing the__builtin__ module and assigning to__builtin__.__import__) in order to change semantics of theimport statement, but nowadays it is usually simpler to use import hooks (seePEP 302). Direct use of__import__() is rare, except in cases where you want to import a module whose name is only known at runtime.

The function imports the module name, potentially using the given globals and locals to determine how to interpret the name in a package context. Thefromlist gives the names of objects or submodules that should be imported from the module given byname. The standard implementation does not use its locals argument at all, and uses itsglobals only to determine the package context of theimport statement.

level specifies whether to use absolute or relative imports. The default is-1 which indicates both absolute and relative imports will be attempted.0 means only perform absolute imports. Positive values forlevel indicate the number of parent directories to search relative to the directory of the module calling__import__().

When the name variable is of the form package.module, normally, the top-level package (the name up till the first dot) is returned,not the module named byname. However, when a non-emptyfromlist argument is given, the module named byname is returned.

For example, the statement importspam results in bytecode resembling the following code:

spam = __import__('spam', globals(), locals(), [], -1)

The statement import spam.ham results in this call:

spam = __import__('spam.ham', globals(), locals(), [], -1)

Note how __import__() returns the toplevel module here because this is the object that is bound to a name by theimport statement.

On the other hand, the statement fromspam.hamimporteggs, sausage as saus results in

_temp = __import__('spam.ham', globals(), locals(), ['eggs', 'sausage'], -1)eggs = _temp.eggssaus = _temp.sausage

Here, the spam.ham module is returned from__import__(). From this object, the names to import are retrieved and assigned to their respective names.

If you simply want to import a module (potentially within a package) by name, useimportlib.import_module().

Changed in version 2.5: The level parameter was added.

Changed in version 2.5: Keyword support for parameters was added.

3. Non-essential Built-in Functions¶

There are several built-in functions that are no longer essential to learn, know or use in modern Python programming. They have been kept here to maintain backwards compatibility with programs written for older versions of Python.

Python programmers, trainers, students and book writers should feel free to bypass these functions without concerns about missing something important.

apply(function,args[,keywords])¶

The function argument must be a callable object (a user-defined or built-in function or method, or a class object) and theargs argument must be a sequence. Thefunction is called withargs as the argument list; the number of arguments is the length of the tuple. If the optionalkeywords argument is present, it must be a dictionary whose keys are strings. It specifies keyword arguments to be added to the end of the argument list. Callingapply() is different from just callingfunction(args), since in that case there is always exactly one argument. The use ofapply() is equivalent tofunction(*args,**keywords).

Deprecated since version 2.3: Use function(*args, **keywords) instead ofapply(function,args,keywords) (see Unpacking Argument Lists).

buffer(object[,offset[,size]])¶

The object argument must be an object that supports the buffer call interface (such as strings, arrays, and buffers). A new buffer object will be created which references theobject argument. The buffer object will be a slice from the beginning ofobject (or from the specified offset). The slice will extend to the end ofobject (or will have a length given by thesize argument).

coerce(x,y)¶

Return a tuple consisting of the two numeric arguments converted to a common type, using the same rules as used by arithmetic operations. If coercion is not possible, raiseTypeError.

intern(string)¶

Enter string in the table of “interned” strings and return the interned string – which isstring itself or a copy. Interning strings is useful to gain a little performance on dictionary lookup – if the keys in a dictionary are interned, and the lookup key is interned, the key comparisons (after hashing) can be done by a pointer compare instead of a string compare. Normally, the names used in Python programs are automatically interned, and the dictionaries used to hold module, class or instance attributes have interned keys.

Changed in version 2.3: Interned strings are not immortal (like they used to be in Python 2.2 and before); you must keep a reference to the return value ofintern() around to benefit from it.