函数的参数

Python的函数定义非常简单，但灵活度却非常大。除了正常定义的必选参数外，还可以使用默认参数、可变参数和关键字参数，使得函数定义出来的接口，不但能处理复杂的参数，还可以简化调用者的代码。

默认参数

``````def power(x):
return x * x
``````

``````>>> power(5)
25
>>> power(15)
225
``````

``````def power(x, n):
s = 1
while n > 0:
n = n - 1
s = s * x
return s
``````

``````>>> power(5, 2)
25
>>> power(5, 3)
125
``````

``````>>> power(5)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: power() takes exactly 2 arguments (1 given)
``````

``````def power(x, n=2):
s = 1
while n > 0:
n = n - 1
s = s * x
return s
``````

``````>>> power(5)
25
>>> power(5, 2)
25
``````

``````def enroll(name, gender):
print 'name:', name
print 'gender:', gender
``````

``````>>> enroll('Sarah', 'F')
name: Sarah
gender: F
``````

``````def enroll(name, gender, age=6, city='Beijing'):
print 'name:', name
print 'gender:', gender
print 'age:', age
print 'city:', city
``````

``````>>> enroll('Sarah', 'F')
Student:
name: Sarah
gender: F
age: 6
city: Beijing
``````

``````enroll('Bob', 'M', 7)
``````

``````def add_end(L=[]):
L.append('END')
return L
``````

``````>>> add_end([1, 2, 3])
[1, 2, 3, 'END']
['x', 'y', 'z', 'END']
``````

``````>>> add_end()
['END']
``````

``````>>> add_end()
['END', 'END']
['END', 'END', 'END']
``````

Python函数在定义的时候，默认参数`L`的值就被计算出来了，即`[]`，因为默认参数`L`也是一个变量，它指向对象`[]`，每次调用该函数，如果改变了`L`的内容，则下次调用时，默认参数的内容就变了，不再是函数定义时的`[]`了。

``````def add_end(L=None):
if L is None:
L = []
L.append('END')
return L
``````

``````>>> add_end()
['END']
['END']
``````

可变参数

``````def calc(numbers):
sum = 0
for n in numbers:
sum = sum + n * n
return sum
``````

``````>>> calc([1, 2, 3])
14
>>> calc((1, 3, 5, 7))
84
``````

``````>>> calc(1, 2, 3)
14
>>> calc(1, 3, 5, 7)
84
``````

``````def calc(*numbers):
sum = 0
for n in numbers:
sum = sum + n * n
return sum
``````

``````>>> calc(1, 2)
5
>>> calc()
0
``````

``````>>> nums = [1, 2, 3]
>>> calc(nums[0], nums[1], nums[2])
14
``````

``````>>> nums = [1, 2, 3]
>>> calc(*nums)
14
``````

关键字参数

``````def person(name, age, **kw):
print 'name:', name, 'age:', age, 'other:', kw
``````

``````>>> person('Michael', 30)
name: Michael age: 30 other: {}
``````

``````>>> person('Bob', 35, city='Beijing')
name: Bob age: 35 other: {'city': 'Beijing'}
name: Adam age: 45 other: {'gender': 'M', 'job': 'Engineer'}
``````

``````>>> kw = {'city': 'Beijing', 'job': 'Engineer'}
>>> person('Jack', 24, city=kw['city'], job=kw['job'])
name: Jack age: 24 other: {'city': 'Beijing', 'job': 'Engineer'}
``````

``````>>> kw = {'city': 'Beijing', 'job': 'Engineer'}
>>> person('Jack', 24, **kw)
name: Jack age: 24 other: {'city': 'Beijing', 'job': 'Engineer'}
``````

参数组合

``````def func(a, b, c=0, *args, **kw):
print 'a =', a, 'b =', b, 'c =', c, 'args =', args, 'kw =', kw
``````

``````>>> func(1, 2)
a = 1 b = 2 c = 0 args = () kw = {}
>>> func(1, 2, c=3)
a = 1 b = 2 c = 3 args = () kw = {}
>>> func(1, 2, 3, 'a', 'b')
a = 1 b = 2 c = 3 args = ('a', 'b') kw = {}
>>> func(1, 2, 3, 'a', 'b', x=99)
a = 1 b = 2 c = 3 args = ('a', 'b') kw = {'x': 99}
``````

``````>>> args = (1, 2, 3, 4)
>>> kw = {'x': 99}
>>> func(*args, **kw)
a = 1 b = 2 c = 3 args = (4,) kw = {'x': 99}
``````

小结

Python的函数具有非常灵活的参数形态，既可以实现简单的调用，又可以传入非常复杂的参数。

`*args`是可变参数，args接收的是一个tuple；

`**kw`是关键字参数，kw接收的是一个dict。