What does "**" mean in python? [duplicate]

Question

This question already has answers here:

What does ** (double star/asterisk) and * (star/asterisk) do for parameters? (28 answers)

Closed 3 years ago.

Simple program:

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.") 

Focus on this line:

 story = storyFormat.format(**userPicks)

What does the ** mean? Why not just pass a plain userPicks?

Answer 1

'**' takes a dict and extracts its contents and passes them as parameters to a function. Take this function for example:

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

Now normally you could call this function like this:

func(1, 2, 3)

But you can also populate a dictionary with those parameters stored like so:

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

Now you can pass this to the function:

func(**params)

Sometimes you'll see this format in function definitions:

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

*args extracts positional parameters and **kwargs extract keyword parameters.

• DNB5brims: So, it can take the dictionary key to map back the params, is that right? What is this function/feature called? I found it is very funny and powerful, is this unique for the python ONLY?
• johnsimer: @DNB5brims, I believe this is called "destructuring". It is finding its way into other languages such as Javascript (ECMAScript 2015).
• Minh Tran: Note that the key values of the params dict must match the names of the optional parameters listed in the definition of func, otherwise a TypeError will occur.

Answer 2

**means kwargs. here is a good article about it.
read this: http://www.saltycrane.com/blog/2008/01/how-to-use-args-and-kwargs-in-python/