python笔记-list and dict

List Methods

Here are some other common list methods.

• list.append(elem) – adds a single element to the end of the list. Common error: does not return the new list, just modifies the original.
• list.insert(index, elem) – inserts the element at the given index, shifting elements to the right.
• list.extend(list2) adds the elements in list2 to the end of the list. Using + or += on a list is similar to using extend().
• list.index(elem) – searches for the given element from the start of the list and returns its index. Throws a ValueError if the element does not appear (use “in” to check without a ValueError).
• list.remove(elem) – searches for the first instance of the given element and removes it (throws ValueError if not present)
• list.sort() – sorts the list in place (does not return it). (The sorted() function shown below is preferred.)
• list.reverse() – reverses the list in place (does not return it)
• list.pop(index) – removes and returns the element at the given index. Returns the rightmost element if index is omitted (roughly the opposite of append()).

Notice that these are methods on a list object, while len() is a function that takes the list (or string or whatever) as an argument.

  list = ['larry', 'curly', 'moe']  list.append('shemp')         ## append elem at end  list.insert(0, 'xxx')        ## insert elem at index 0  list.extend(['yyy', 'zzz'])  ## add list of elems at end  print list  ## ['xxx', 'larry', 'curly', 'moe', 'shemp', 'yyy', 'zzz']  print list.index('curly')    ## 2  list.remove('curly')         ## search and remove that element  list.pop(1)                  ## removes and returns 'larry'  print list  ## ['xxx', 'moe', 'shemp', 'yyy', 'zzz']

Common error: note that the above methods do not return the modified list, they just modify the original list.

  list = [1, 2, 3]  print list.append(4)   ## NO, does not work, append() returns None  ## Correct pattern:  list.append(4)  print list  ## [1, 2, 3, 4]

Python Sorting

As another example, specifying “str.lower” as the key function is a way to force the sorting to treat uppercase and lowercase the same:

  ## "key" argument specifying str.lower function to use for sorting  print sorted(strs, key=str.lower)  ## ['aa', 'BB', 'CC', 'zz']You can also pass in your own MyFn as the key function, like this:  ## Say we have a list of strings we want to sort by the last letter of the string.  strs = ['xc', 'zb', 'yd' ,'wa']  ## Write a little function that takes a string, and returns its last letter.  ## This will be the key function (takes in 1 value, returns 1 value).  def MyFn(s):    return s[-1]  ## Now pass key=MyFn to sorted() to sort by the last letter:  print sorted(strs, key=MyFn)  ## ['wa', 'zb', 'xc', 'yd']

To use key= custom sorting, remember that you provide a function that takes one value and returns the proxy value to guide the sorting. There is also an optional argument “cmp=cmpFn” to sorted() that specifies a traditional two-argument comparison function that takes two values from the list and returns negative/0/positive to indicate their ordering. The built in comparison function for strings, ints, … is cmp(a, b), so often you want to call cmp() in your custom comparator. The newer one argument key= sorting is generally preferable.

Dict Formatting

The % operator works conveniently to substitute values from a dict into a string by name:

  hash = {}  hash['word'] = 'garfield'  hash['count'] = 42  s = 'I want %(count)d copies of %(word)s' % hash  # %d for int, %s for string  # 'I want 42 copies of garfield'

Del

The “del” operator does deletions. In the simplest case, it can remove the definition of a variable, as if that variable had not been defined. Del can also be used on list elements or slices to delete that part of the list and to delete entries from a dictionary.

  var = 6  del var  # var no more!  list = ['a', 'b', 'c', 'd']  del list[0]     ## Delete first element  del list[-2:]   ## Delete last two elements  print list      ## ['b']  dict = {'a':1, 'b':2, 'c':3}  del dict['b']   ## Delete 'b' entry  print dict      ## {'a':1, 'c':3}

Files

The open() function opens and returns a file handle that can be used to read or write a file in the usual way. The code f = open(‘name’, ‘r’) opens the file into the variable f, ready for reading operations, and use f.close() when finished. Instead of ‘r’, use ‘w’ for writing, and ‘a’ for append. The special mode ‘rU’ is the “Universal” option for text files where it’s smart about converting different line-endings so they always come through as a simple ‘\n’. The standard for-loop works for text files, iterating through the lines of the file (this works only for text files, not binary files). The for-loop technique is a simple and efficient way to look at all the lines in a text file:

  # Echo the contents of a file  f = open('foo.txt', 'rU')  for line in f:   ## iterates over the lines of the file    print line,    ## trailing , so print does not add an end-of-line char                   ## since 'line' already includes the end-of line.  f.close()

Reading one line at a time has the nice quality that not all the file needs to fit in memory at one time – handy if you want to look at every line in a 10 gigabyte file without using 10 gigabytes of memory. The f.readlines() method reads the whole file into memory and returns its contents as a list of its lines. The f.read() method reads the whole file into a single string, which can be a handy way to deal with the text all at once, such as with regular expressions we’ll see later.

For writing, f.write(string) method is the easiest way to write data to an open output file. Or you can use “print” with an open file, but the syntax is nasty: “print >> f, string”. In python 3000, the print syntax will be fixed to be a regular function call with a file= optional argument: “print(string, file=f)”.

Files Unicode

The “codecs” module provides support for reading a unicode file.

import codecs

f = codecs.open('foo.txt', 'rU', 'utf-8')for line in f:  # here line is a *unicode* string

For writing, use f.write() since print does not fully support unicode.