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