Aral Balkan

I can't believe I've been using Python for several months now without really understanding the extended call syntax.

You know how in ActionScript you can do functionRef.apply(thisObj, argumentsArray) if you need to call a function with a dynamic list of arguments? I was looking for a way to do this in Python and googled for "apply". Lo and behold, I found that it was deprecated.

Instead of using a separate function, you can simply pass in your arguments (and keyword arguments) to the function itself.

e.g., to call function add_numbers(first_number=a,second_number=b) with the list special_arguments=[2,2], you'd write:

add_numbers(*special_arguments)

And, for keyword arguments, where my_awesome_keyword_arguments = {'first_number':2, 'second_number':2}:

add_numbers(**my_awesome_keyword_arguments)

Finally, you can mix both positional and keyword arguments (say positional=[2] and keyword={'second_number':2}):

add_numbers(*positional, **keyword)

The crazy thing is that I've been using methods that use extended syntax for months now and yet I didn't actually grok exactly what was going on until today. Ah, I love it when something clicks. (And did I mention that the more I use Python, the more I love it?) :)

Comments

• widged: In python can also do that by: def func(*args): # now `args` is a tuple print length(args) print args

  by bsdemon on 2008-08-07 11:32:03

• @widged: Yep, but it's not the same thing. That's the way I'd been using extended call syntax in Python not understanding that you can also _call_ it via an argument object. For it to be equivalent, we should be able to do the following in AS3 (based on your example): args = [1,2,3] myFunc = function(x,y,z){ trace(x + ", " + y + ", " + z) } myFunc(...args) or, borrowing Python syntax: myFunc(*args) We can't do that yet in AS3. That said, Python and AS3 are very similar in flavor (both dynamic languages) and we do have apply which does the same thing. I just find Python's syntax more concise in this case and quite elegant.

  by Aral on 2008-08-07 17:33:31

• Hi Aral, Note that in AS3 you can now pass a variable number of arguments to the function itself: function passAnything(...statements):void { trace(statements.length +": "+ statements); } [Example borrowed from this AS3 tutorial by senocular]

  by widged on 2008-08-07 11:11:20

• The second part of the awesomeness is that if you want a function bound to a particular scope (thisObj) you only need to call thisObj.function(*args, **kwargs). thisObj.function is always bound to the scope thisObj. You can also use an unbound method and bind it later by passing in an additional first argument for self, e.g.: class Foo(object): bar = "" def __init__(self, bar): self.bar = bar def foo(self): return self.bar f = Foo("bar") print "bound method Foo('bar').foo() %s" % f.foo() print "unbound method Foo.foo(f) %s" % Foo.foo(f)

  by Jesse on 2010-06-20 04:48:56

• Thanks! Been really useful. I did not understand what you were saying at start. Here's a little recap: func(*[a, b]) func(a, b) func(**{key: value}) func(key=value)

  by vjeux on 2010-09-22 08:46:13