Python **（双星/星号）和*（星/星号）对参数有什么作用？

在以下方法定义中，

*

和

**

对

param2

有什么作用

def foo(param1, *param2):
def bar(param1, **param2):

---

从Python文档中：

如果位置参数多于形式参数槽，则会引发TypeError异常，除非存在使用语法“*标识符”的形式参数；在本例中，该形式参数接收一个包含多余位置参数的元组（如果没有多余位置参数，则接收一个空元组）

如果任何关键字参数与形式参数名称不对应，则会引发TypeError异常，除非存在使用语法“**标识符”的形式参数；在本例中，该形式参数接收一个包含多余关键字参数的字典（使用关键字作为键，参数值作为对应值），或者如果没有多余关键字参数，则接收一个（新的）空字典

---

*args

和

**kwargs

是一种常见的习惯用法，允许对函数使用任意数量的参数，如Python文档中的一节所述

*args

将为您提供所有功能参数：

**kwargs

将为您提供所有 关键字参数与形式参数（如字典）相对应的参数除外

def bar(**kwargs):
    for a in kwargs:
        print(a, kwargs[a])  

bar(name='one', age=27)
# name one
# age 27

这两种习惯用法都可以与普通参数混合使用，以允许使用一组固定参数和一些可变参数：

def foo(kind, *args, **kwargs):
   pass

>>> x = xrange(3) # create our *args - an iterable of 3 integers
>>> xrange(*x)    # expand here
xrange(0, 2, 2)

也可以反过来使用：

def foo(a, b, c):
    print(a, b, c)

obj = {'b':10, 'c':'lee'}

foo(100,**obj)
# 100 10 lee

*l

习惯用法的另一个用法是在调用函数时解压缩参数列表

def foo(bar, lee):
    print(bar, lee)

l = [1,2]

foo(*l)
# 1 2

在Python3中，可以在赋值（）的左侧使用

*l

，尽管在这种上下文中它给出的是一个列表而不是元组：

first, *rest = [1,2,3,4]
first, *l, last = [1,2,3,4]

Python 3还添加了新的语义（请参阅）：

这样的函数只接受3个位置参数，

*

之后的所有内容只能作为关键字参数传递

def foo(arg, kwarg=None, *, kwarg2=None, **kwargs): 
    return arg, kwarg, kwarg2, kwargs

注:

• 语义上用于传递关键字参数的Python

  dict

  ，可以任意排序。然而，在Python3.6中，关键字参数保证记住插入顺序
• “现在，

  **kwargs

  中元素的顺序与关键字参数传递给函数的顺序相对应。”-
• 事实上，CPython 3.6中的所有DICT都会将插入顺序作为实现细节记住，这在Python 3.7中成为标准

---

单*表示可以有任意数量的额外位置参数

foo（）

可以像

foo（1,2,3,4,5）

那样调用。在foo（）的主体中，param2是一个包含2-5的序列

双**表示可以有任意数量的额外命名参数

bar（）

可以像调用

bar（1，a=2，b=3）

一样调用。在bar（）param2的主体中有一个包含{'a'：2，'b'：3}的字典

使用以下代码：

def foo(param1, *param2):
    print(param1)
    print(param2)

def bar(param1, **param2):
    print(param1)
    print(param2)

foo(1,2,3,4,5)
bar(1,a=2,b=3)

输出是

1
(2, 3, 4, 5)
1
{'a': 2, 'b': 3}

---

还值得注意的是，在调用函数时也可以使用

*

和

**

。这是一种快捷方式，允许您直接使用列表/元组或字典将多个参数传递给函数。例如，如果您具有以下功能：

def foo(x,y,z):
    print("x=" + str(x))
    print("y=" + str(y))
    print("z=" + str(z))

def args(normal_arg, *argv):
    print("normal argument:", normal_arg)

    for arg in argv:
        print("Argument in list of arguments from *argv:", arg)

args('animals', 'fish', 'duck', 'bird')

你可以这样做：

>>> mylist = [1,2,3]
>>> foo(*mylist)
x=1
y=2
z=3

>>> mydict = {'x':1,'y':2,'z':3}
>>> foo(**mydict)
x=1
y=2
z=3

>>> mytuple = (1, 2, 3)
>>> foo(*mytuple)
x=1
y=2
z=3

注意：

mydict

中的键的命名必须与函数

foo

的参数完全相同。否则，它将抛出一个

TypeError

：

>>> mydict = {'x':1,'y':2,'z':3,'badnews':9}
>>> foo(**mydict)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: foo() got an unexpected keyword argument 'badnews'

mydict={'x'：1，'y'：2，'z'：3，'badnews'：9} >>>foo（**mydict） 回溯（最近一次呼叫最后一次）： 文件“”，第1行，在 TypeError:foo（）获得意外的关键字参数“badnews”

---

*

和

***

在函数参数列表中有特殊用法<代码>* 表示参数是一个列表，

**

表示参数 这是一本字典。这允许函数执行任意数量的 论据

**

（双星）和

*

（星）对参数有什么作用？ 它们允许定义函数以接受，并允许用户传递任意数量的参数、位置（

*

）和关键字（

**

）

定义功能

*args

允许任意数量的可选位置参数（参数），这些参数将分配给名为

args

的元组

**kwargs

允许任意数量的可选关键字参数（参数），这些参数将位于名为

kwargs

的dict中

您可以（也应该）选择任何合适的名称，但如果希望参数具有非特定语义，

args

和

kwargs

是标准名称

展开，传递任意数量的参数 您还可以使用

*args

和

**kwargs

分别从列表（或任何iterable）和dicts（或任何映射）传入参数

接收参数的函数不必知道它们正在被扩展

例如，Python2的xrange没有显式地期望

*args

，但因为它接受3个整数作为参数：

def foo(kind, *args, **kwargs):
   pass

>>> x = xrange(3) # create our *args - an iterable of 3 integers
>>> xrange(*x)    # expand here
xrange(0, 2, 2)

另一个例子是，我们可以在

str.format

中使用dict扩展：

>>> foo = 'FOO'
>>> bar = 'BAR'
>>> 'this is foo, {foo} and bar, {bar}'.format(**locals())
'this is foo, FOO and bar, BAR'

Python3中的新功能：使用仅关键字参数定义函数 您可以在

*args

之后使用，例如，此处，

kwarg2

必须作为关键字参数提供，而不是按位置提供：

def foo(arg, kwarg=None, *args, kwarg2=None, **kwargs): 
    return arg, kwarg, args, kwarg2, kwargs

def bar(*, kwarg=None): 
    return kwarg

用法：

>>> foo(1,2,3,4,5,kwarg2='kwarg2', bar='bar', baz='baz')
(1, 2, (3, 4, 5), 'kwarg2', {'bar': 'bar', 'baz': 'baz'})

另外，

*

本身也可以用来表示后面跟着的参数只有关键字，而不允许有无限的位置参数

def foo(arg, kwarg=None, *, kwarg2=None, **kwargs): 
    return arg, kwarg, kwarg2, kwargs

这里，

kwarg2

必须是一个显式命名的关键字参数：

>>> foo(1,2,kwarg2='kwarg2', foo='foo', bar='bar')
(1, 2, 'kwarg2', {'foo': 'foo', 'bar': 'bar'})

我们不能再接受无限的位置参数，因为我们没有

*args*

：

>>> foo(1,2,3,4,5, kwarg2='kwarg2', foo='foo', bar='bar')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: foo() takes from 1 to 2 positional arguments 
    but 5 positional arguments (and 1 keyword-only argument) were given

在本例中，我们看到，如果尝试按位置传递

kwarg

，我们会得到一个错误：

>>> bar('kwarg')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: bar() takes 0 positional arguments but 1 was given

Python 2兼容演示

foo(a, b=10, *args, **kwargs)

a is a required argument, and its value is 1
b not required, its default value is 10, actual value: 2
args is of type <type 'tuple'> and length 2
unknown arg: 3
unknown arg: 4
kwargs is of type <type 'dict'> and length 3
unknown kwarg - kw: e, arg: 5
unknown kwarg - kw: g, arg: 7
unknown kwarg - kw: f, arg: 6
Args or kwargs can be passed without knowing what they are.
e.g. max(a, b, *args) 
4
e.g. dict(**kwargs) (same as dict(e=5, f=6, g=7)) returns: 
{'e': 5, 'g': 7, 'f': 6}

def bar(a):
    b, c, d, e, f = 2, 3, 4, 5, 6
    # dumping every local variable into foo as a keyword argument 
    # by expanding the locals dict:
    foo(**locals()) 

a is a required argument, and its value is 100
b not required, its default value is 10, actual value: 2
args is of type <type 'tuple'> and length 0
kwargs is of type <type 'dict'> and length 4
unknown kwarg - kw: c, arg: 3
unknown kwarg - kw: e, arg: 5
unknown kwarg - kw: d, arg: 4
unknown kwarg - kw: f, arg: 6
Args or kwargs can be passed without knowing what they are.
e.g. max(a, b, *args) 
100
e.g. dict(**kwargs) (same as dict(c=3, d=4, e=5, f=6)) returns: 
{'c': 3, 'e': 5, 'd': 4, 'f': 6}

def foo(a, b, c, d=0, e=100):
    # imagine this is much more code than a simple function call
    preprocess() 
    differentiating_process_foo(a,b,c,d,e)
    # imagine this is much more code than a simple function call
    postprocess()

def bar(a, b, c=None, d=0, e=100, f=None):
    preprocess()
    differentiating_process_bar(a,b,c,d,e,f)
    postprocess()

def baz(a, b, c, d, e, f):
    ... and so on

def decorator(function):
    '''function to wrap other functions with a pre- and postprocess'''
    @functools.wraps(function) # applies module, name, and docstring to wrapper
    def wrapper(*args, **kwargs):
        # again, imagine this is complicated, but we only write it once!
        preprocess()
        function(*args, **kwargs)
        postprocess()
    return wrapper

@decorator
def foo(a, b, c, d=0, e=100):
    differentiating_process_foo(a,b,c,d,e)

@decorator
def bar(a, b, c=None, d=0, e=100, f=None):
    differentiating_process_bar(a,b,c,d,e,f)

@decorator
def baz(a, b, c=None, d=0, e=100, f=None, g=None):
    differentiating_process_baz(a,b,c,d,e,f, g)

@decorator
def quux(a, b, c=None, d=0, e=100, f=None, g=None, h=None):
    differentiating_process_quux(a,b,c,d,e,f,g,h)

def __init__(self, *args, **kwargs):
    for attribute_name, value in zip(self._expected_attributes, args):
        setattr(self, attribute_name, value)
        if kwargs.has_key(attribute_name):
            kwargs.pop(attribute_name)

    for attribute_name in kwargs.viewkeys():
        setattr(self, attribute_name, kwargs[attribute_name])

class RetailItem(Item):
    _expected_attributes = Item._expected_attributes + ['name', 'price', 'category', 'country_of_origin']

class FoodItem(RetailItem):
    _expected_attributes = RetailItem._expected_attributes +  ['expiry_date']

food_item = FoodItem(name = 'Jam', 
                     price = 12.0, 
                     category = 'Foods', 
                     country_of_origin = 'US', 
                     expiry_date = datetime.datetime.now())

class ElectronicAccessories(RetailItem):
    _expected_attributes = RetailItem._expected_attributes +  ['specifications']
    # Depend on args and kwargs to populate the data as needed.
    def __init__(self, specifications = None, *args, **kwargs):
        self.specifications = specifications  # Rest of attributes will make sense to parent class.
        super(ElectronicAccessories, self).__init__(*args, **kwargs)

usb_key = ElectronicAccessories(name = 'Sandisk', 
                                price = '$6.00', 
                                category = 'Electronics',
                                country_of_origin = 'CN',
                                specifications = '4GB USB 2.0/USB 3.0')

>>> (0, *range(1, 4), 5, *range(6, 8))
(0, 1, 2, 3, 5, 6, 7)
>>> [0, *range(1, 4), 5, *range(6, 8)]
[0, 1, 2, 3, 5, 6, 7]
>>> {0, *range(1, 4), 5, *range(6, 8)}
{0, 1, 2, 3, 5, 6, 7}
>>> d = {'one': 1, 'two': 2, 'three': 3}
>>> e = {'six': 6, 'seven': 7}
>>> {'zero': 0, **d, 'five': 5, **e}
{'five': 5, 'seven': 7, 'two': 2, 'one': 1, 'three': 3, 'six': 6, 'zero': 0}

>>> range(*[1, 10], *[2])
range(1, 10, 2)

def test(a,b,c):
     print(a)
     print(b)
     print(c)

test(1,2,3)
#output:
1
2
3

def test(a,b,c):
     print(a)
     print(b)
     print(c)

test(a=1,b=2,c=3)
#output:
1
2
3

def test(a=0,b=0,c=0):
     print(a)
     print(b)
     print(c)
     print('-------------------------')

test(a=1,b=2,c=3)
#output :
1
2
3
-------------------------

def test(a=0,b=0,c=0):
    print(a)
    print(b)
    print(c)
    print('-------------------------')

test(1,2,3)
# output :
1
2
3
---------------------------------

def sum(a,b):  #receive args from function calls as sum(1,2) or sum(a=1,b=2)
    print(a+b)

my_tuple = (1,2)
my_list = [1,2]
my_dict = {'a':1,'b':2}

# Let us unpack data structure of list or tuple or dict into arguments with help of '*' operator
sum(*my_tuple)   # becomes same as sum(1,2) after unpacking my_tuple with '*'
sum(*my_list)    # becomes same as sum(1,2) after unpacking my_list with  '*'
sum(**my_dict)   # becomes same as sum(a=1,b=2) after unpacking by '**' 

# output is 3 in all three calls to sum function.

def sum(*args): #pack the received positional args into data structure of tuple. after applying '*' - def sum((1,2,3,4))
    sum = 0
    for a in args:
        sum+=a
    print(sum)

sum(1,2,3,4)  #positional args sent to function sum
#output:
10

def sum(**args): #pack keyword args into datastructure of dict after applying '**' - def sum({a:1,b:2,c:3,d:4})
    sum=0
    for k,v in args.items():
        sum+=v
    print(sum)

sum(a=1,b=2,c=3,d=4) #positional args sent to function sum

>>> def foo(*arg,**kwargs):
...     print arg
...     print kwargs
>>>
>>> a = (1, 2, 3)
>>> b = {'aa': 11, 'bb': 22}
>>>
>>>
>>> foo(*a,**b)
(1, 2, 3)
{'aa': 11, 'bb': 22}
>>>
>>>
>>> foo(a,**b) 
((1, 2, 3),)
{'aa': 11, 'bb': 22}
>>>
>>>
>>> foo(a,b) 
((1, 2, 3), {'aa': 11, 'bb': 22})
{}
>>>
>>>
>>> foo(a,*b)
((1, 2, 3), 'aa', 'bb')
{}

x = [1, 2, 3]
y = [4, 5, 6]

unzip_x, unzip_y = zip(*zip(x, y))

zip(*zip(x,y)) -> zip((1, 4), (2, 5), (3, 6))

class base(object):
    def __init__(self, base_param):
        self.base_param = base_param


class child1(base): # inherited from base class
    def __init__(self, child_param, *args) # *args for non-keyword args
        self.child_param = child_param
        super(child1, self).__init__(*args) # call __init__ of the base class and initialize it with a NON-KEYWORD arg

class child2(base):
    def __init__(self, child_param, **kwargs):
        self.child_param = child_param
        super(child2, self).__init__(**kwargs) # call __init__ of the base class and initialize it with a KEYWORD arg

c1 = child1(1,0)
c2 = child2(1,base_param=0)
print c1.base_param # 0
print c1.child_param # 1
print c2.base_param # 0
print c2.child_param # 1

            In function construction         In function call
=======================================================================
          |  def f(*args):                 |  def f(a, b):
*args     |      for arg in args:          |      return a + b
          |          print(arg)            |  args = (1, 2)
          |  f(1, 2)                       |  f(*args)
----------|--------------------------------|---------------------------
          |  def f(a, b):                  |  def f(a, b):
**kwargs  |      return a + b              |      return a + b
          |  def g(**kwargs):              |  kwargs = dict(a=1, b=2)
          |      return f(**kwargs)        |  f(**kwargs)
          |  g(a=1, b=2)                   |
-----------------------------------------------------------------------

def args(normal_arg, *argv):
    print("normal argument:", normal_arg)

    for arg in argv:
        print("Argument in list of arguments from *argv:", arg)

args('animals', 'fish', 'duck', 'bird')

def who(**kwargs):
    if kwargs is not None:
        for key, value in kwargs.items():
            print("Your %s is %s." % (key, value))

who(name="Nikola", last_name="Tesla", birthday="7.10.1856", birthplace="Croatia")  

def f(x, y, *myArgs, **myKW):
    print("# x      = {}".format(x))
    print("# y      = {}".format(y))
    print("# myArgs = {}".format(myArgs))
    print("# myKW   = {}".format(myKW))
    print("# ----------------------------------------------------------------------")

# Define a list for demonstration purposes
myList    = ["Left", "Right", "Up", "Down"]
# Define a dictionary for demonstration purposes
myDict    = {"Wubba": "lubba", "Dub": "dub"}
# Define a dictionary to feed y
myArgDict = {'y': "Why?", 'y0': "Why not?", "q": "Here is a cue!"}

# The 1st elem of myList feeds y
f("myEx", *myList, **myDict)
# x      = myEx
# y      = Left
# myArgs = ('Right', 'Up', 'Down')
# myKW   = {'Wubba': 'lubba', 'Dub': 'dub'}
# ----------------------------------------------------------------------

# y is matched and fed first
# The rest of myArgDict becomes additional arguments feeding myKW
f("myEx", **myArgDict)
# x      = myEx
# y      = Why?
# myArgs = ()
# myKW   = {'y0': 'Why not?', 'q': 'Here is a cue!'}
# ----------------------------------------------------------------------

# The rest of myArgDict becomes additional arguments feeding myArgs
f("myEx", *myArgDict)
# x      = myEx
# y      = y
# myArgs = ('y0', 'q')
# myKW   = {}
# ----------------------------------------------------------------------

# Feed extra arguments manually and append even more from my list
f("myEx", 4, 42, 420, *myList, *myDict, **myDict)
# x      = myEx
# y      = 4
# myArgs = (42, 420, 'Left', 'Right', 'Up', 'Down', 'Wubba', 'Dub')
# myKW   = {'Wubba': 'lubba', 'Dub': 'dub'}
# ----------------------------------------------------------------------

# Without the stars, the entire provided list and dict become x, and y:
f(myList, myDict)
# x      = ['Left', 'Right', 'Up', 'Down']
# y      = {'Wubba': 'lubba', 'Dub': 'dub'}
# myArgs = ()
# myKW   = {}
# ----------------------------------------------------------------------

def foo(*args):
    for arg in args:
        print(arg)

foo("two", 3)

two
3

def bar(**kwargs):
    for key in kwargs:
        print(key, kwargs[key])

bar(dic1="two", dic2=3)

dic1 two
dic2 3

accessModifier methodName(datatype… arg) {
    // method body
}

def foo(param1, *param2):
    print(param1)
    print(param2)


def bar(param1, **param2):
    print(param1)
    print(param2)


def three_params(param1, *param2, **param3):
    print(param1)
    print(param2)
    print(param3)


foo(1, 2, 3, 4, 5)
print("\n")
bar(1, a=2, b=3)
print("\n")
three_params(1, 2, 3, 4, s=5)

1
(2, 3, 4, 5)

1
{'a': 2, 'b': 3}

1
(2, 3, 4)
{'s': 5}

def f(a, b, /, c, d, *, e, f):
    pass

sum = lambda x, y, z: x + y + z
sum(1,2,3) # sum 3 items

sum([1,2,3]) # error, needs 3 items, not 1 list

x = [1,2,3][0]
y = [1,2,3][1]
z = [1,2,3][2]
sum(x,y,z) # ok

sum(*[1,2,3]) # ok, 1 list becomes 3 items

def any_param(*param):
    return type(param)

any_param(1) #tuple
any_param() # tuple

def any(*param):
    param[0] # correct

def any(*param):
    *param[0] # incorrect

def func(**any):
    return type(any) # dict

def func(**any):
    return any

func(width="10",height="20") # {width="10",height="20")