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python——内置类型——函数——参数

2017-04-14 21:52 671 查看

必选参数

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

enroll('Bob', 'M')
enroll('Lily', 'F')

output:
name: Bob gender: M
name: Lily gender: F

默认参数

def enroll(name, gender, age = 8, city = 'Hangzhou'):
print 'name:', name, 'gender:', gender, 'age:', age, 'city:', city

enroll('Bob', 'M')
enroll('Lily', 'F', 9)
enroll('Jack', 'M', city = 'Shanghai')

output:
name: Bob gender: M age: 8 city: Hangzhou
name: Lily gender: F age: 9 city: Hangzhou
name: Jack gender: M age: 8 city: Shanghai

总结:
必选参数在前,默认参数在后
函数有多个参数时,变化大参数放前面,变化小参数放后面,变化小参数可设为默认参数
有多个默认参数时,既可按顺序提供默认参数,也可不按顺序提供部分默认参数,当不按顺序提供部分默认参数时,需说明参数名
def add_end(list=[]):
list.append('End')
return list

print add_end()
print add_end()
print add_end()

output:
['End']
['End', 'End']
['End', 'End', 'End']

解释:
函数是instance,函数定义时,默认参数指向对象就确定了,调用函数时,如果改变了默认参数指向对象值,默认参数就不再是函数定义时初值,因此默认参数必须指向不可变对象,否则运行会有逻辑错误

可变参数

def calSum(*args):
sum = 0
for num in args:
sum = sum + num
return sum

print calSum(), calSum(1), calSum(1, 9), calSum(1, 9, 19)
odd_list = [1, 3, 5, 7, 9]
even_tuple = (2, 4, 6, 8 ,10)
color_dict = {1:'red', 2:'green', 3:'blue'}
prime_set = ([2, 3, 5, 7])
print calSum(*odd_list), calSum(*even_tuple), calSum(*color_dict), calSum(*prime_set)

output:
0 1 10 29
25 30 6 17

总结:
定义可变形参用*变量名
可变实参本质上组装成tuple传给函数内部的可变形参
list,tuple,dict,set等容器类型,作为实参传给可变形参时,前面加*即可,便转换为tuple传给可变形参,dict和set是取key组装成tuple

关键字参数

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

enroll('Bob', 10)
enroll('Jack', 20, city = 'Hangzhou')
enroll('Martin', 30, gender = 'M', job = 'engineer')
dict = {'city': 'Shanghai', 'job': 'doctor'}
enroll('Tom', 40, **dict)

output:
name: Bob age: 10 other: {}
name: Jack age: 20 other: {'city': 'Hangzhou'}
name: Martin age: 30 other: {'gender': 'M', 'job': 'engineer'}
name: Tom age: 40 other: {'city': 'Shanghai', 'job': 'doctor'}

总结:
定义关键字参数用**变量名
关键字实参本质上组装成dict传给函数内部的关键字形参
dict作为实参传给关键字形参时,前面加**即可

参数应用

def mixArg(a, b, c = 0, *args, **kw):
print 'a =', a, 'b =', b, 'c =', c, 'args =', args, 'kw =', kw
mixArg(1, 2)
mixArg(1, 2, c = 3)
mixArg(1, 2, 3, 'a', 'b')
mixArg(1, 2, 3, 'a', 'b', x = 58)
args = (1, 2, 3, 4)
kw = {'x': 98}
mixArg(*args, **kw)

output:
a = 1 b = 2 c = 0 args = () kw = {}
a = 1 b = 2 c = 3 args = () kw = {}
a = 1 b = 2 c = 3 args = ('a', 'b') kw = {}
a = 1 b = 2 c = 3 args = ('a', 'b') kw = {'x': 58}
a = 1 b = 2 c = 3 args = (4,) kw = {'x': 98}

总结:
必选参数、默认参数、可变参数、关键字参数可任意组合使用,但参数定义顺序必须是:必选参数、默认参数、可变参数、关键字参数
函数调用时,解释器自动按照参数位置和参数名把对应参数传进去
任意函数,都可通过类似mixArg(*args, **kw)形式调用,无论参数是如何定义的
默认参数一定要用不可变对象,如果是可变对象,运行会有逻辑错误
*args是可变参数,args接收的是一个tuple,可变参数既可直接传入,也可先组装list,tuple,dict,set等容器类型,再通过*args传入
**kw是关键字参数,kw接收的是一个dict,关键字参数既可直接传入,也可先组装dict,再通过**kw传入
*args和**kw是python习惯写法,当然也可用其他参数名,但最好使用习惯写法
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