Python 文档阅读02

if语句

else是可以选择的，elif（else if缩写）也是可以0个或多个。

>>> x = int(input("Please enter an integer: "))Please enter an integer: 42>>> if x < 0:...     x = 0...     print('Negative changed to zero')... elif x == 0:...     print('Zero')... elif x == 1:...     print('Single')... else:...     print('More')...More

for语句

python的for语句与常见的C语言中的for语句是不同的。for语句的作用是遍历序列类型中的每一个项目。如果你想要在迭代中更改某些值，建议通过切片拷贝来操作。如下

>>> # Measure some strings:... words = ['cat', 'window', 'defenestrate']>>> for w in words:...     print(w, len(w))...cat 3window 6defenestrate 12>>>>>> for w in words[:]:  # Loop over a slice copy of the entire list....     if len(w) > 6:...         words.insert(0, w)...>>> words['defenestrate', 'cat', 'window', 'defenestrate']

range()函数

如果想要一个只包含数字的序列，可通过内置函数range()来操作；该函数共有3个参数，当只填写一个时，默认改值为结束位置，起始为0，步数为1；当填写两个时，默认为起始与结束位置，步数为1；很显然，第三个参数为步数；注意，结束位置永远不包含在内。如下

>>> for i in range(5):...     print(i)...01234>>>range(5, 10)   5 through 9range(0, 10, 3)   0, 3, 6, 9range(-10, -100, -30)  -10, -40, -70

为了在迭代中显示索引，可通过range()与len()实现；第二种更常用的方法是通过enumerate()函数。

>>> a = ['Mary', 'had', 'a', 'little', 'lamb']>>> for i in range(len(a)):...     print(i, a[i])...0 Mary1 had2 a3 little4 lamb>>>>>>li = ['a', 'b', 'c', 'd']>>>for i in enumerate(li, 0):...    print(i)...    (0, 'a')(1, 'b')(2, 'c')(3, 'd')

为了节省空间，range()返回的是一个迭代器对象，我们只有通过迭代才能将值取出来，for语句可用于迭代，list()也可用于迭代。

>>> print(range(5))range(0, 5)>>> list(range(5))[0, 1, 2, 3, 4]

break, continue语句以及循环中else子句

break用于跳出最近的for或while循环；循环中的else子句是循环结束或条件为假并且不由break终止时执行的语句；try语句中的else当没有异常发生时执行，循环中的else当没有break时执行；continue用于跳出当前循环，进行下次循环，借用自C语言。如下

>>># 素数，只能被自己整除的数>>> for n in range(2, 10):...     for x in range(2, n):...         if n % x == 0:...             print(n, 'equals', x, '*', n//x)...             break...     else:...         # loop fell through without finding a factor...         print(n, 'is a prime number')...2 is a prime number3 is a prime number4 equals 2 * 25 is a prime number6 equals 2 * 37 is a prime number8 equals 2 * 49 equals 3 * 3>>>>>> for num in range(2, 10):...     if num % 2 == 0:...         print("Found an even number", num)# even number 偶数...         continue...     print("Found a number", num)Found an even number 2Found a number 3Found an even number 4Found a number 5Found an even number 6Found a number 7Found an even number 8Found a number 9

pass语句

pass语句用于占位符，不执行任何操作，但符合语法结构，以示解释器不会报错。

>>> while True:...     pass  # Busy-wait for keyboard interrupt (Ctrl+C)...>>> class MyEmptyClass:...     pass...

定义函数

def是定义函数的关键字，后面还必须要有函数名与括号，参数可选；函数整体的语句是缩进的；函数缩进语句块中的第一行建议写函数注释；函数括号中的参数通常称之为形参，函数调用时的参数称之为实参；函数内部是无法修改外部的变量，除非调用global语句。如下产生斐波拉契数列

>>> def fib(n):    # write Fibonacci series up to n...     """Print a Fibonacci series up to n."""...     a, b = 0, 1...     while a < n:...         print(a, end=' ')...         a, b = b, a+b...     print()...>>> # Now call the function we just defined:... fib(2000)0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597>>>>>>>>> f = fib>>> f(100)0 1 1 2 3 5 8 13 21 34 55 89

当函数没有return语句时默认返回None；函数调用是函数对象的调用，如下；result.append(a)也是result对象调用appen方法。

>>> fib<function fib at 10042ed0>>>>>>> print(fib(0))None>>>>>> def fib2(n):  # return Fibonacci series up to n...     """Return a list containing the Fibonacci series up to n."""...     result = []...     a, b = 0, 1...     while a < n:...         result.append(a)    # see below...         a, b = b, a+b...     return result...>>> f100 = fib2(100)    # call it>>> f100                # write the result[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

函数定义的更多方法

默认参数值

最常用的方式是对一个或多个参数指定默认值，这样当函数被调用时可以传入更少的值。

def ask_ok(prompt, retries=4, reminder='Please try again!'):    while True:        ok = input(prompt)        if ok in ('y', 'ye', 'yes'):            return True        if ok in ('n', 'no', 'nop', 'nope'):            return False        retries = retries - 1        if retries < 0:            raise ValueError('invalid user response')        print(reminder)

示例中有三种调用方式：

• 必须给一个参数：ask_ok('Do you really want to quit?')
• 给两个参数： ask_ok('OK to overwrite the file?', 2)
• 给三个参数： ask_ok('OK to overwrite the file?', 2, 'Come on, only yes or no!')
  示例中还展示了in关键字，该关键字用来测试某个元素是否在该序列中。

i = 5def f(arg=i):    print(arg)i = 6f()

参数的默认值只会计算一次，所以上示例中会打印5；但如果默认值是可变的，情况就有可能不同。如下示例

def f(a, L=[]):    L.append(a)    return Lprint(f(1))print(f(2))print(f(3))# 输出，因为L是指向[]的位置，而[]中的值是可变的[1][1, 2][1, 2, 3]# 下面方式可以避免def f(a, L=None):    if L is None:        L = []    L.append(a)    return L

关键字参数

函数可通过kwarg=value传值。

def parrot(voltage, state='a stiff', action='voom', type='Norwegian Blue'):    print("-- This parrot wouldn't", action, end=' ')    print("if you put", voltage, "volts through it.")    print("-- Lovely plumage, the", type)    print("-- It's", state, "!")

调用上面函数，必须的参数有一个，可选的有两个。
可通过一下方式调用：

parrot(1000)                                          # 1 positional argumentparrot(voltage=1000)                                  # 1 keyword argumentparrot(voltage=1000000, action='VOOOOOM')             # 2 keyword argumentsparrot(action='VOOOOOM', voltage=1000000)             # 2 keyword argumentsparrot('a million', 'bereft of life', 'jump')         # 3 positional argumentsparrot('a thousand', state='pushing up the daisies')  # 1 positional, 1 keyword

下面是错误的调用

parrot()                     # required argument missingparrot(voltage=5.0, 'dead')  # 位置参数要放前面，关键字参数必须放位置参数后面parrot(110, voltage=220)     # duplicate value for the same argumentparrot(actor='John Cleese')  # unknown keyword argument

*name可以接受一个tuple以及将多余的参数收进去；**name可以接受一个字典；前者必须在后者前面

def cheeseshop(kind, *arguments, **keywords):    print("-- Do you have any", kind, "?")    print("-- I'm sorry, we're all out of", kind)    for arg in arguments:        print(arg)    print("-" * 40)    keys = sorted(keywords.keys())    for kw in keys:        print(kw, ":", keywords[kw])

调用如下

cheeseshop("Limburger", "It's very runny, sir.",           "It's really very, VERY runny, sir.",           shopkeeper="Michael Palin",           client="John Cleese",           sketch="Cheese Shop Sketch")

打印如下

-- Do you have any Limburger ?-- I'm sorry, we're all out of LimburgerIt's very runny, sir.It's really very, VERY runny, sir.----------------------------------------client : John Cleeseshopkeeper : Michael Palinsketch : Cheese Shop Sketch

任意参数的列表

>>> def concat(*args, sep="/"):...     return sep.join(args)...>>> concat("earth", "mars", "venus")'earth/mars/venus'>>> concat("earth", "mars", "venus", sep=".")'earth.mars.venus'

参数列表拆包

是用或者*分别对list与dict进行拆包传入

>>> list(range(3, 6))            # normal call with separate arguments[3, 4, 5]>>> args = [3, 6]>>> list(range(*args))            # call with arguments unpacked from a list[3, 4, 5]

>>> def parrot(voltage, state='a stiff', action='voom'):...     print("-- This parrot wouldn't", action, end=' ')...     print("if you put", voltage, "volts through it.", end=' ')...     print("E's", state, "!")...>>> d = {"voltage": "four million", "state": "bleedin' demised", "action": "VOOM"}>>> parrot(**d)-- This parrot wouldn't VOOM if you put four million volts through it. E's bleedin' demised !