Python3基础之函数用法
一般来说，函数（function）是组织好的、可重复使用的、具有一定功能的代码段。函数能提高应用的模块性和代码的重复利用率，在Python中已经提供了很多的内建函数，比如print()，同时Python还允许用户自定义函数。
本文就来实例总结一下Python3的函数用法，具体内容如下：
一、定义
定义函数使用关键字def，后接函数名和放在圆括号( )中的可选参数列表，函数内容以冒号起始并且缩进。一般格式如下：
def 函数名(参数列表): """文档字符串""" 函数体 return [expression]
注意：参数列表可选，文档字符串可选，return语句可选。
示例：
def fib(n):
"""Print a Fibonacci series"""
a, b = 0, 1
while b < n:
print(b, end=' ')
a, b = b, a+b
print()
fib(2000) # call
f = fib # assignment
f(2000)
函数名的值是一种用户自定义的函数类型。函数名的值可以被赋予另一个名字，使其也能作为函数使用。
二、函数变量作用域
在函数内部定义的变量拥有一个局部作用域，在函数外定义的拥有全局作用域。注意：在函数内部可以引用全局变量，但无法对其赋值（除非用global进行声明）。
a = 5 # 全局变量a
def func1():
print('func1() print a =', a)
def func2():
a = 21 # 局部变量a
print('func2() print a =', a)
def func3():
global a
a = 10 # 修改全局变量a
print('func3() print a =', a)
func1()
func2()
func3()
print('the global a =', a)
三、函数调用
1、普通调用
与其他语言中函数调用一样，Python中在调用函数时，需要给定和形参相同个数的实参并按顺序一一对应。
def fun(name, age, gender):
print('Name:',name,'Age:',age,'Gender:',gender,end=' ')
print()
fun('Jack', 20, 'man') # call
2、使用关键字参数调用函数
函数也可以通过keyword=value 形式的关键字参数来调用，因为我们明确指出了对应关系，所以参数的顺序也就无关紧要了。
def fun(name, age, gender):
print('Name:',name,'Age:',age,'Gender:',gender,end=' ')
print()
fun(gender='man', name='Jack', age=20) # using keyword arguments
3、调用具有默认实参的函数
Python中的函数也可以给一个或多个参数指定默认值，这样在调用时可以选择性地省略该参数：
def fun(a, b, c=5): print(a+b+c) fun(1,2) fun(1,2,3)
注意：通常情况下默认值只被计算一次，但如果默认值是一个可变对象时会有所不同, 如列表, 字典, 或大多类的对象时。例如，下面的函数在随后的调用中会累积参数值：
def fun(a, L=[]): L.append(a) print(L) fun(1) # 输出[1] fun(2) # 输出[1, 2] fun(3) # 输出[1, 2, 3]
4、调用可变参数函数
通过在形参前加一个星号（*）或两个星号（**）来指定函数可以接收任意数量的实参。
def fun(*args):
print(type(args))
print(args)
fun(1,2,3,4,5,6)
# 输出：
# <class 'tuple'>
# (1, 2, 3, 4, 5, 6)
def fun(**args):
print(type(args))
print(args)
fun(a=1,b=2,c=3,d=4,e=5)
# 输出：
# <class 'dict'>
# {'d': 4, 'e': 5, 'b': 2, 'c': 3, 'a': 1}
从两个示例的输出可以看出：当参数形如*args时，传递给函数的任意个实参会按位置被包装进一个元组（tuple）；当参数形如**args时，传递给函数的任意个key=value实参会被包装进一个字典（dict）。
5、通过解包参数调用函数
上一点说到传递任意数量的实参时会将它们打包进一个元组或字典，当然有打包也就有解包（unpacking）。通过 单星号和双星号对List、Tuple和Dictionary进行解包：
def fun(a=1, b=2, c=3):
print(a+b+c)
fun() # 正常调用
list1 = [11, 22, 33]
dict1 = {'a':40, 'b':50, 'c':60}
fun(*list1) # 解包列表
fun(**dict1) # 解包字典
# 输出：
# 6
# 66
# 150
注：*用于解包Sequence，**用于解包字典。解包字典会得到一系列的key=value，故本质上就是使用关键字参数调用函数。
四、lambda表达式
lambda关键词能创建小型匿名函数。lambda函数能接收任何数量的参数但只能返回一个表达式的值，它的一般形式如下：
lambda [arg1 [,arg2,.....argn]] : expression
lambda表达式可以在任何需要函数对象的地方使用，它们在语法上被限制为单一的表达式：
f = lambda x, y: x+y print(f(10, 20)) def make_fun(n): return lambda x: x+n f = make_fun(15) print(f(5))
五、文档字符串
函式体的第一个语句可以是三引号括起来的字符串， 这个字符串就是函数的文档字符串，或称为docstring 。我们可以使用print(function.__doc__)输出文档：
def fun(): """Some information of this function. This is documentation string.""" return print(fun.__doc__)
文档字符串主要用于描述一些关于函数的信息，让用户交互地浏览和输出。建议养成在代码中添加文档字符串的好习惯。
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