What does the def-star-variable (or def-asterisk-parameter) syntax do in Python?
Inspired by Titus' blog post about Python's elusive at sign I thought I'd drop a little google egg for anyone who is learning Python and hasn't read the manual cover to cover yet (like I did ... yeah right).
If you see a def in python with asterisks (or stars), like so:
def foo(*args, **kw):
pass
...what does this syntax do? It is buried in the Python tutorial here, under More On Defining Functions > Arbitrary Argument Lists which would only appear in a google search if you already knew what the syntax does! The first form means any arbitrary number of arguments to the function foo will be provided to you in a tuple named args. And the second form means any arbitrary number of keyword arguments will be available in the dict kw. For example:
>>> def checkout(*items, **coupons):
... print items
... print coupons
...
>>> checkout('eggs', 'cheese', 'cereal',
... cereal='10% off', cheese='buy 1 get 1')
('eggs', 'cheese', 'cereal')
{'cheese': 'buy 1 get 1', 'cereal': '10% off'}
>>> The above section in the manual has the details on the order in which this is achieved.
DISCLAIMER: The author of this post does not condone lazily learning a programming language by trial and error, reading other peoples' code, or simply googling for an answer to something :)
Re: What does the def-star-variable (or def-asterisk-parameter) syntax do in Python?
posted by Tennessee Leeuwenburg on Wednesday Jun 20th, 2007 at 6:45p.m.
One thing which I've always been unsure of is what the * operator does in the following context
a = range(0,5)
b = range(6,10)
z = zip(a,b)
zip(*z)
This has always struck me as odd, because if I simply type
*z
in the interpreter, I get a SyntaxError.
So, what am I passing to zip?
Cheers,
-T
Re: What does the def-star-variable (or def-asterisk-parameter) syntax do in Python?
posted by Kumar McMillan on Wednesday Jun 20th, 2007 at 9:05p.m.
Yes, it is pretty strange that you can't type *z into the interpretter and see the result, I assume it is due to python internals and how it probably collides with star for multiplication.
All you are doing by calling zip(*z) is inverting the behavior of def zip(*z): [inside of function]. Behind the scenes, python is actually calling the zip() method like so:
zip((0, 6), (1, 7), (2, 8), (3, 9))
in other words, it takes the list or iterable, z, and expands each item to be a separate argument for the function call. I dug through some code and found this as a practical example:
clauses = [ Files.c.type.id==video_type.id ]
if time is not None:
clauses.append(Files.c.modified >= time)
files = Files.select(and_(*clauses))
Re: What does the def-star-variable (or def-asterisk-parameter) syntax do in Python?
posted by Tennessee Leeuwenburg on Wednesday Jun 20th, 2007 at 11:13p.m.
Thanks for that explanation, it makes sense. I always wondered.
-T
Re: What does the def-star-variable (or def-asterisk-parameter) syntax do in Python?
posted by Siew Kam Onn on Thursday Jun 21st, 2007 at 2:04a.m.
Succinct.
Re: What does the def-star-variable (or def-asterisk-parameter) syntax do in Python?
posted by Esteban on Thursday Jun 21st, 2007 at 6:39a.m.
> Yes, it is pretty strange that you can't type *z into the interpretter and see the result
And what would that result be? ;)
Re: What does the def-star-variable (or def-asterisk-parameter) syntax do in Python?
posted by Fredrik on Thursday Jun 21st, 2007 at 7:33a.m.
> Yes, it is pretty strange that you can't type *z into the interpretter and see the result
*z is function calling syntax, not expression syntax, so you need to use it in an calling context. to see it in action, enter
def f(*args, **kw): print args, kw
and then try calling the f() function in different ways (e.g. f(*z) etc).
Re: What does the def-star-variable (or def-asterisk-parameter) syntax do in Python?
posted by Kumar McMillan on Thursday Jun 21st, 2007 at 10:38a.m.
@Esteban :
>> Yes, it is pretty strange that you can't type *z into the interpretter and see the result
>
> And what would that result be? ;)
actually ... yeah thinking about it again it would make no sense! if z is a list then in an interactive terminal *z would be, errm, a list.
My knee-jerk response was really to the fact that to find out what zip(*z) is doing, you'd have to write your own zip method to simulate the scenario, which is slightly unintuitive to someone learning python through the terminal.
Tutorial
posted by Albert on Tuesday Jun 26th, 2007 at 10:58a.m.
I *never* used Python, I don't have *any* book about it, but I still know what this syntax means, simply because I read the entire tutorial :p. Well, OK, the tutorial is very long, and is referred to as the manual on this page, but still.
Re: What does the def-star-variable (or def-asterisk-parameter) syntax do in Python?
posted by Kevin on Monday Jul 9th, 2007 at 11:15a.m.
This was very helpful and I found it on google as you intended :) Just saying thanks.