python的函数参数

#x和n都是必选参数
def new_power(x,n):
    s=1
    while n>0:
        n=n-1
        s=s*x
    return(s)

#n=2就是默认参数
def new_power(x,n=2):
    s=1
    while n>0:
        n=n-1
        s=s*x
    return(s)

#调用的时候可以不写n=2
new_power(5)  #25

#当于默认参数不符合时，需要提供默认参数的值
def enroll(name, gender, age=6, city=‘Beijing‘):
    print(‘name:‘, name)
    print(‘gender:‘, gender)
    print(‘age:‘, age)
    print(‘city:‘, city)

#必须传入默认参数的新值
enroll(‘Tom‘,‘M‘,city=‘Tianjing‘)

enroll(‘Tom‘,‘M‘,7)

#要注意这种情况,定义默认参数要牢记一点：默认参数必须指向不变对象！
def add_end(L=[]):
    L.append(‘END‘)
    return L

#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
add_end()  #[‘END‘]
add_end()  #[‘END‘, ‘END‘]
add_end()  #[‘END‘, ‘END‘, ‘END‘]

#当变量是可变的，如变量是一个list或者tuple
def calc(numbers):
    sum = 0
    for n in numbers:
        sum = sum + n * n
    return sum

#调用时，需要先组装一个一个list或tuple
calc([1,2,3])
calc((1,2,3,4))

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


#调用
calc(1,2,3,4)

#方法一
num=[1,2,3]
calc(num[0],num[1],num[2])



#方法二
num=[1,2,3]
calc(*num)

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


#调用，只传入必选参数
person(‘Michael‘, 30)  #name: Michael age: 30 other: {}

#传入任意个关键字参数
person(‘Bob‘, 35, city=‘Beijing‘)  #name: Bob age: 35 other: {‘city‘: ‘Beijing‘}
person(‘Adam‘, 45, gender=‘M‘, job=‘Engineer‘)  #name: Adam age: 45 other: {‘gender‘: ‘M‘, ‘job‘: ‘Engineer‘}

#和可变参数类似，也可以先组装出一个dict，然后，把该dict转换为关键字参数传进去
extra = {‘city‘: ‘Beijing‘, ‘job‘: ‘Engineer‘}
 person(‘Jack‘, 24, city=extra[‘city‘], job=extra[‘job‘])  #name: Jack age: 24 other: {‘city‘: ‘Beijing‘, ‘job‘: ‘Engineer‘}

#上面的也可以这样调用
person(‘Jack‘, 24, **extra)

#仍以person()函数为例，我们希望检查是否有city和job参数：
def person(name, age, **kw):
    if ‘city‘ in kw:
        # 有city参数
        pass
    if ‘job‘ in kw:
        # 有job参数
        pass
    print(‘name:‘, name, ‘age:‘, age, ‘other:‘, kw)

#但是调用者仍可以传入不受限制的关键字参数
person(‘Jack‘, 24, city=‘Beijing‘, addr=‘Chaoyang‘, zipcode=123456)  #name: Jack age: 24 other: {‘city‘: ‘Beijing‘, ‘addr‘: ‘Chaoyang‘, ‘zipcode‘: 123456}

#这种方式定义的函数如下：
def person(name, age, *, city, job):
    print(name, age, city, job)

#调用方式：
person(‘Jack‘, 24, city=‘Beijing‘, job=‘Engineer‘)

def person(name, age, *args, city, job):
    print(name, age, args, city, job)

#1.命名关键字参数必须传入参数名，如果没有传入参数名，调用将报错：
 person(‘Jack‘, 24, ‘Beijing‘, ‘Engineer‘)
输出：Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: person() takes 2 positional arguments but 4 were given


#2.命名关键字参数可以有缺省值，从而简化调用：
def person(name, age, *, city=‘Beijing‘, job):
    print(name, age, city, job)

#由于命名关键字参数city具有默认值，调用时，可不传入city参数：
person(‘Jack‘, 24, job=‘Engineer‘)

#3.使用命名关键字参数时，要特别注意，如果没有可变参数，就必须加一个*作为特殊分隔符。如果缺少*，Python解释器将无法识别位置参数和命名关键字参数：
def person(name, age, city, job):
    # 缺少 *，city和job被视为位置参数
    pass

#比如定义一个函数，包含上述若干种参数：
def f1(a, b, c=0, *args, **kw):
    print(‘a =‘, a, ‘b =‘, b, ‘c =‘, c, ‘args =‘, args, ‘kw =‘, kw)

def f2(a, b, c=0, *, d, **kw):
    print(‘a =‘, a, ‘b =‘, b, ‘c =‘, c, ‘d =‘, d, ‘kw =‘, kw)


f1(1, 2)  #a = 1 b = 2 c = 0 args = () kw = {}
f1(1, 2, c=3)  #a = 1 b = 2 c = 3 args = () kw = {}
f1(1, 2, 3, ‘a‘, ‘b‘)  #a = 1 b = 2 c = 3 args = (‘a‘, ‘b‘) kw = {}
f1(1, 2, 3, ‘a‘, ‘b‘, x=99)  #a = 1 b = 2 c = 3 args = (‘a‘, ‘b‘) kw = {‘x‘: 99}
f2(1, 2, d=99, ext=None)  #a = 1 b = 2 c = 0 d = 99 kw = {‘ext‘: None}

#例子
args = (1, 2, 3, 4)
kw = {‘d‘: 99, ‘x‘: ‘#‘}
f1(*args, **kw)  #a = 1 b = 2 c = 3 args = (4,) kw = {‘d‘: 99, ‘x‘: ‘#‘}


args = (1, 2, 3)
kw = {‘d‘: 88, ‘x‘: ‘#‘}
f2(*args, **kw)  #a = 1 b = 2 c = 3 d = 88 kw = {‘x‘: ‘#‘}