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)
1
2
3
4
Notice how all the parameters i passed to foo are collected in the list
args.
**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="[EMAIL PROTECTED]")
{'name': 'Jordan', 'email': '[EMAIL PROTECTED]'}
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="[EMAIL PROTECTED]")
Positional arguments:
1 monkey 7.5
Non-positional argument list:
(10, 11, 12)
10
11
12
Keyword argument list:
{'name': 'jordan', 'email': '[EMAIL PROTECTED]'}
Key: name
Data: jordan
Key: email
Data: [EMAIL PROTECTED]
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
mine!
Anyway, I hope this helps!
-Jordan Greenberg
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