在python中”**”是什么意思？

What does “**” mean in python?

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Possible Duplicate:
What does ** and * do for python parameters?
What does *args and **kwargs mean?

简单程序：

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storyFormat ="""
Once upon a time, deep in an ancient jungle,
there lived a {animal}. This {animal}
liked to eat {food}, but the jungle had
very little {food} to offer. One day, an
explorer found the {animal} and discovered
it liked {food}. The explorer took the
{animal} back to {city}, where it could
eat as much {food} as it wanted. However,
the {animal} became homesick, so the
explorer brought it back to the jungle,
leaving a large supply of {food}.

The End
"""

def tellStory():
userPicks = dict()
addPick('animal', userPicks)
addPick('food', userPicks)
addPick('city', userPicks)
story = storyFormat.format(**userPicks)
print(story)

def addPick(cue, dictionary):
'''Prompt for a user response using the cue string,
and place the cue-response pair in the dictionary.
'''
prompt = 'Enter an example for ' + cue + ': '
response = input(prompt).strip() # 3.2 Windows bug fix
dictionary[cue] = response

tellStory()
input("Press Enter to end the program.")

关注这一行：

1	story = storyFormat.format(**userPicks)

**是什么意思？为什么不直接通过一个普通的userPicks？

"**"接受dict并提取其内容，并将其作为参数传递给函数。以这个函数为例：

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def func(a=1, b=2, c=3):
print a
print b
print b

现在通常可以这样调用这个函数：

1	func(1, 2, 3)

但您也可以使用存储的参数来填充字典，如下所示：

1	params = {'a': 2, 'b': 3, 'c': 4}

现在可以将此传递给函数：

1	func(**params)

有时您会在函数定义中看到这种格式：

1 2	def func(args, *kwargs): ...

*args提取位置参数，**kwargs提取关键字参数。