实例讲解Python中函数的调用与定义

调用函数：

    #!/usr/bin/env python3 
    # -*- coding: utf-8 -*- 

    # 函数调用 
    >>> abs(100) 
    100 
    >>> abs(-110) 
    110 
    >>> abs(12.34) 
    12.34 
    >>> abs(1, 2) 
    Traceback (most recent call last): 
     File "<stdin>", line 1, in <module> 
    TypeError: abs() takes exactly one argument (2 given) 
    >>> abs('a') 
    Traceback (most recent call last): 
     File "<stdin>", line 1, in <module> 
    TypeError: bad operand type for abs(): 'str' 
    >>> max(1, 2) 
    2 
    >>> max(2, 3, 1, -5) 
    3 
    >>> int('123') 
    123 
    >>> int(12.34) 
    12 
    >>> str(1.23) 
    '1.23' 
    >>> str(100) 
    '100' 
    >>> bool(1) 
    True 
    >>> bool('') 
    False 
    >>> a = abs # 变量a指向abs函数，相当于引用 
    >>> a(-1) # 所以也可以通过a调用abs函数 
    1 

    >>> n1 = 255 
    >>> n2 = 1000 
    >>> print(hex(n1)) 
    0xff 
    >>> print(hex(n2)) 
    0x3e8 

定义函数：

    #!/usr/bin/env python3 
    # -*- coding: utf-8 -*- 

    #函数定义 
    def myAbs(x): 
     if x >= 0: 
      return x 
     else: 
      return -x 

    a = 10 
    myAbs(a) 

    def nop(): # 空函数 
     pass 

pass语句什么都不做 。
实际上pass可以用来作为占位符，比如现在还没想好怎么写函数代码，就可以先写一个pass，让代码运行起来。

    if age >= 18: 
     pass 
    #缺少了pass，代码就会有语法错误 
    >>> if age >= 18: 
    ... 
     File "<stdin>", line 2 

     ^ 
    IndentationError: expected an indented block 

    >>> myAbs(1, 2) 
    Traceback (most recent call last): 
     File "<stdin>", line 1, in <module> 
    TypeError: myAbs() takes 1 positional argument but 2 were given 
    >>> myAbs('A') 
    Traceback (most recent call last): 
     File "<stdin>", line 1, in <module> 
     File "<stdin>", line 2, in myAbs 
    TypeError: unorderable types: str() >= int() 
    >>> abs('A') 
    Traceback (most recent call last): 
     File "<stdin>", line 1, in <module> 
    TypeError: bad operand type for abs(): 'str' 

    def myAbs(x): 
     if not isinstance(x, (int, float)): 
      raise TypeError('bad operand type') 
     if x >= 0: 
      return x 
     else: 
      return -x 

    >>> myAbs('A') 
    Traceback (most recent call last): 
     File "<stdin>", line 1, in <module> 
     File "<stdin>", line 3, in myAbs 
    TypeError: bad operand type 

返回两个值？

    import math 
    def move(x, y, step, angle = 0): 
     nx = x + step * math.cos(angle) 
     ny = y - step * math.sin(angle) 
     return nx, ny 

    >>> x, y = move(100, 100, 60, math.pi / 6) 
    >>> print(x, y) 
    151.96152422706632 70.0 

其实上面只是一种假象，Python函数返回的仍然是单一值 。

    >>> r = move(100, 100, 60, math.pi / 6) 
    >>> print(r) 
    (151.96152422706632, 70.0) 

实际上返回的是一个tuple！
但是，语法上，返回一个tuple可以省略括号， 而多个变量可以同时接受一个tuple，按位置赋给对应的值。
所以，Python的函数返回多值实际就是返回一个tuple，但是写起来更方便。
函数执行完毕也没有return语句时，自动return None。

练习 ：

    import math 
    def quadratic(a, b, c): 
     x1 = (-b + math.sqrt(b * b - 4 * a * c)) / (2 * a) 
     x2 = (-b - math.sqrt(b * b - 4 * a * c)) / (2 * a) 
     return x1, x2 

    x1, x2 = quadratic(2, 5, 1) 
    print(x1, x2) 

    >>> import math 
    >>> def quadratic(a, b, c): 
    ...  x1 = (-b + math.sqrt(b * b - 4 * a * c)) / (2 * a) 
    ...  x2 = (-b - math.sqrt(b * b - 4 * a * c)) / (2 * a) 
    ...  return x1, x2 
    ... 
    >>> x1, x2 = quadratic(2, 5, 1) 
    >>> print(x1, x2) 
    -0.21922359359558485 -2.2807764064044154