在前面我们已经见过一些Python内建函数，比如len和rang。现在我们来看看自定义函数，函数是通过def关键字来定义，后面跟函数名称和圆括号，括号内可以包含参数，该行以冒号结束，接下来是语句块，即函数体。
1. 简单的sayHello函数
def sayHello():
print("Hello world!")
#调用函数
sayHello()
2. 带形参函数
def printMax(a, b):
if a > b:
print(a, "is maximum.")
else:
print(b, "is maximum.")
printMax(3, 4)
3. 局部变量
def func(x):
print("x is ", x)
x = 2
print("Changed local x to ", x)
x = 50
func(x)
print("x is still ", x)
运行结果为：
x is 50
Changed local x to 2
x is still 50
4. 默认参数值
def say(message, times = 1):
print(message * times)
say("ha")
say("ha", 2)
运行结果为：
ha
haha
5. 关键参数
def func(a, b = 5, c = 10):
print("a is", a, "and b is", b, "and c is", c)
func(3, 7)
func(25, c = 24)
func(c = 50, a = 100)
运行结果为：
a is 3 and b is 7 and c is 10
a is 25 and b is 5 and c is 24
a is 100 and b is 5 and c is 50
6. return语句
def maximum(x, y):
if x > y:
return x
else:
return y
print(maximum(2, 3))
7. 文档字符串DocStrings
Python有一个奇妙的特性，就是文档字符串（DocStrings），它的主要作用就是帮助你的程序文档更加简单易懂。
def printMax(x, y):
'''Prints the maximum of two numbers.
The two values must be integers.'''
x = int(x)
y = int(y)
if x > y:
print(x, "is maximum.")
else:
print(y, "is maximum.")
printMax(3, 5)
print(printMax.__doc__)
help(printMax)
运行结果为：
5 is maximum.
Prints the maximum of two numbers.
The two values must be integers.
Help on function printMax in module __main__:
printMax(x, y)
Prints the maximum of two numbers.
The two values must be integers.