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,Python **(双星/星号)和*(星/星号)对参数有什么作用?,python,syntax,parameter-passing,variadic-functions,argument-unpacking,Python,Syntax,Parameter Passing,Variadic Functions,Argument Unpacking,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
*
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
,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dict
- “现在,
中元素的顺序与关键字参数传递给函数的顺序相对应。”-**kwargs
- 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
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")