Python中的**kwargs有什么用途?

我知道你可以在表上执行objects.filter并传递一个**kwargs参数。 

我也可以这样做来指定时间增量,即时间增量(小时=时间1)吗?

它到底是如何工作的?是否归类为“开箱”?比如a,b=1,2?


当前回答

下面是一个我希望有帮助的例子:

#! /usr/bin/env python
#
def g( **kwargs) :
  print ( "In g ready to print kwargs" )
  print kwargs
  print ( "in g, calling f")
  f ( **kwargs )
  print ( "In g, after returning from f")

def f( **kwargs ) :
  print ( "in f, printing kwargs")
  print ( kwargs )
  print ( "In f, after printing kwargs")


g( a="red", b=5, c="Nassau")

g( q="purple", w="W", c="Charlie", d=[4, 3, 6] )

运行程序时,您将获得:

$ python kwargs_demo.py 
In g ready to print kwargs
{'a': 'red', 'c': 'Nassau', 'b': 5}
in g, calling f
in f, printing kwargs
{'a': 'red', 'c': 'Nassau', 'b': 5}
In f, after printing kwargs
In g, after returning from f
In g ready to print kwargs
{'q': 'purple', 'c': 'Charlie', 'd': [4, 3, 6], 'w': 'W'}
in g, calling f
in f, printing kwargs
{'q': 'purple', 'c': 'Charlie', 'd': [4, 3, 6], 'w': 'W'}
In f, after printing kwargs
In g, after returning from f

这里的关键是调用中命名参数的可变数量转换为函数中的字典。

其他回答

这是了解python解包的简单示例,

>>> def f(*args, **kwargs):
...    print 'args', args, 'kwargs', kwargs

eg1:

>>>f(1, 2)
>>> args (1,2) kwargs {} #args return parameter without reference as a tuple
>>>f(a = 1, b = 2)
>>> args () kwargs {'a': 1, 'b': 2} #args is empty tuple and kwargs return parameter with reference as a dictionary

基于一个好的示例有时比一个长的论述更好,我将使用所有python变量参数传递工具(位置和命名参数)编写两个函数。你应该能够很容易地自己看到它的作用:

def f(a = 0, *args, **kwargs):
    print("Received by f(a, *args, **kwargs)")
    print("=> f(a=%s, args=%s, kwargs=%s" % (a, args, kwargs))
    print("Calling g(10, 11, 12, *args, d = 13, e = 14, **kwargs)")
    g(10, 11, 12, *args, d = 13, e = 14, **kwargs)

def g(f, g = 0, *args, **kwargs):
    print("Received by g(f, g = 0, *args, **kwargs)")
    print("=> g(f=%s, g=%s, args=%s, kwargs=%s)" % (f, g, args, kwargs))

print("Calling f(1, 2, 3, 4, b = 5, c = 6)")
f(1, 2, 3, 4, b = 5, c = 6)

下面是输出:

Calling f(1, 2, 3, 4, b = 5, c = 6)
Received by f(a, *args, **kwargs) 
=> f(a=1, args=(2, 3, 4), kwargs={'c': 6, 'b': 5}
Calling g(10, 11, 12, *args, d = 13, e = 14, **kwargs)
Received by g(f, g = 0, *args, **kwargs)
=> g(f=10, g=11, args=(12, 2, 3, 4), kwargs={'c': 6, 'b': 5, 'e': 14, 'd': 13})

此外,您还可以在调用kwargs函数时混合使用不同的用法:

def test(**kwargs):
    print kwargs['a']
    print kwargs['b']
    print kwargs['c']


args = { 'b': 2, 'c': 3}

test( a=1, **args )

给出以下输出:

1
2
3

注意,**kwargs必须是最后一个参数

kwargs是一种语法糖,可以将名称参数作为字典传递(对于func),或将字典作为命名参数传递(对于func)

下面是一个我希望有帮助的例子:

#! /usr/bin/env python
#
def g( **kwargs) :
  print ( "In g ready to print kwargs" )
  print kwargs
  print ( "in g, calling f")
  f ( **kwargs )
  print ( "In g, after returning from f")

def f( **kwargs ) :
  print ( "in f, printing kwargs")
  print ( kwargs )
  print ( "In f, after printing kwargs")


g( a="red", b=5, c="Nassau")

g( q="purple", w="W", c="Charlie", d=[4, 3, 6] )

运行程序时,您将获得:

$ python kwargs_demo.py 
In g ready to print kwargs
{'a': 'red', 'c': 'Nassau', 'b': 5}
in g, calling f
in f, printing kwargs
{'a': 'red', 'c': 'Nassau', 'b': 5}
In f, after printing kwargs
In g, after returning from f
In g ready to print kwargs
{'q': 'purple', 'c': 'Charlie', 'd': [4, 3, 6], 'w': 'W'}
in g, calling f
in f, printing kwargs
{'q': 'purple', 'c': 'Charlie', 'd': [4, 3, 6], 'w': 'W'}
In f, after printing kwargs
In g, after returning from f

这里的关键是调用中命名参数的可变数量转换为函数中的字典。