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Python

Posted by Jainesh Patel
Jainesh Patel
11,100 Points

*args and **kwargs

Can someone please explain to me what these two do. I am super confused

2 Answers

Darryl Mah
Darryl Mah
5,492 Points

Think of them as special variables that python created.

The *args allows your function to have the flexibility to take as many parameters your function might need. For example, if you have a class called “add” you want to take as many arguments the user provides to calculate a sum. The problem is you may not know how many arguments the user provided, so you can’t hard code the arguments into the function.

class Add:
    def __init__ (x, y):
        print x + y

This class could only take two arguments

class AddMultiple:
    def __init__ (*args):
        for x in args:
            total += x
        print total

This class could take as many arguments the user passes into AddMultiple and add them together.

The **kwargs does the same thing but puts those arguments into the for of key value pairs. So python is looking for an input by the user of “key=value” for example “name = John”

Hope this clears things up! Always good to ask questions to understand the fundamentals instead of just copying code :)

[MOD: added ```python formating -cf]

Chris Freeman
Chris Freeman
Treehouse Moderator 68,468 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,468 Points

Good answer. I would add that it is the single asterisk * and the double asterisk ** that have the meaning and the "args" and "kwargs" are only a convention in the same way "self" and "cls" are used for method parameters.

Alternatively, think of * as "tuple packing" and ** as "dictionary packing". Any positional arguments not put into existing parameters will be packed in the tuple "args" (or whatever name gets used). Any keyword/value pairs not put into existing keyword arguments will be packed in to the dictionary "kwargs" (or whatever name is used).

>>> def packing(pos1, pos2, kw1=None, kw2=None, *args, **kwargs):
...     print(f"pos1:{pos1}, pos2:{pos2}, kw1:{kw1}, kw2:{kw2}")
...     print(f"type args:{type(args)}, args:{args}")
...     print(f"type kwargs:{type(kwargs)}, kwargs:{kwargs}")
... 
>>> packing("p1", "p2", 101, 102, "a", "b", kw3=203, kw4=204)
pos1:p1, pos2:p2, kw1:101, kw2:102
type args:<class 'tuple'>, args:('a', 'b')
type kwargs:<class 'dict'>, kwargs:{'kw3': 203, 'kw4': 204}

>>> def packing(pos1, pos2, kw1=None, kw2=None, *foo, **kwbar):
...     print(f"pos1:{pos1}, pos2:{pos2}, kw1:{kw1}, kw2:{kw2}")
...     print(f"type foo:{type(foo)}, foo:{foo}")
...     print(f"type kwbar:{type(kwbar)}, kwbar:{kwbar}")
... 
>>> packing("p1", "p2", 101, 102, "a", "b", kw3=203, kw4=204)
pos1:p1, pos2:p2, kw1:101, kw2:102
type foo:<class 'tuple'>, foo:('a', 'b')
type kwbar:<class 'dict'>, kwbar:{'kw3': 203, 'kw4': 204}
Jainesh Patel
Jainesh Patel
11,100 Points
Jainesh Patel
Jainesh Patel
11,100 Points

Thank you this helped clear my misunderstanding