[Tutor] (*args, **kwargs)

Jordan Greenberg jordangreenberg at gmail.com
Fri Aug 4 18:38:54 CEST 2006

Matt Williams wrote:
> Dear All,
> I have learnt to do bits of python, but one of the things I cannot get 
> my head around is the *args, **kwargs syntax.
> I have tried reading stuff on the web, and I have a copy of the python 
> cookbook (which uses it as a recipe early on) but I still don't 
> understand it.
> Please could someone explain _very_ slowly?
> Apologies for the gross stupidity,
> Matt

Basically, *args and **kwargs allows you to collect arguments.
Normally, you'd do something like:
*args collects any arguments (other then the positional ones) into a
list, and **kwargs collects arguments into a dictionary.
In [6]: def foo(a, b, c):
   ...:         print a, b, c

In [7]: foo(1, 2, 3)
1 2 3

but, say you wanted to make it so it could accept any number of
arguments, you could use *args like so:
In [4]: def foo(*args):
   ...:         print args
   ...:         for each in args:
   ...:                 print each

In [5]: foo(1, 2, 3, 4)
(1, 2, 3, 4)

Notice how all the parameters i passed to foo are collected in the list

**kwargs collects arguments of the form key=value into a dictionary,
like so:

In [15]: def foo(**kwargs):
   ....:        print kwargs

In [16]: foo(name="Jordan", email="jordangreenberg at gmail.com")
{'name': 'Jordan', 'email': 'jordangreenberg at gmail.com'}

Your functions can then use the list/dictionary as normal.
You can also use these in conjunction with normal positional parameters:

In [27]: def foo(a, b, c, *args, **kwargs):
   ....:        print "Positional arguments:"
   ....:        print a, b, c
   ....:        print "Non-positional argument list:"
   ....:        print args
   ....:        for each in args:
   ....:                print each
   ....:        print "Keyword argument list:"
   ....:        print kwargs
   ....:        for key in kwargs.keys():
   ....:                print "Key: ", key
   ....:                print "Data: ", kwargs[key]

In [28]: foo(1, "monkey", 7.5, 10, 11, 12, name="jordan",
email="jordangreenberg at gmail.com")
Positional arguments:
1 monkey 7.5
Non-positional argument list:
(10, 11, 12)
Keyword argument list:
{'name': 'jordan', 'email': 'jordangreenberg at gmail.com'}
Key:  name
Data:  jordan
Key:  email
Data:  jordangreenberg at gmail.com

So, to summarize, *args and **kwargs are basically there so you can
build your functions so that they can accept a variable number of arguments.

We had a thread about this not terribly long ago too, and Kent Johnson
had a great explanation (as per usual) so I expect he'll be along
shortly with either a link to that thread or a better explanation then

Anyway, I hope this helps!
-Jordan Greenberg

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