Function returns a function

[James Stroud]

Hello,

I want a "function factory" function in python. To do this I made a class like
such (this is for Tkinter, in case your wondering):

class FunctionMaker:
def __init__(self, avar, function, label)
self.var = avar
self.function = function
self.label = label
def __call__(self, e=None):
self.avar.set(s elf.label)
self.function()

It seems that it would make more sense to make this a function and add it to
the mixin I am writing. But I'm not quite sure how to do this without making
lambdas, which seem a little messy and also look to be on the brink of
deprecation (really not trying to start a lambda pro/con thread here).

Any ideas on how to do this with a regular function, or is the way I've done
it the pythonic choice?

James

--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/

[Paul Rubin]

Re: Function returns a function

James Stroud <jstroud@mbi.uc la.edu> writes:[color=blue]
> Any ideas on how to do this with a regular function, or is the way I've done
> it the pythonic choice?[/color]

I think you're trying to do something like this:

def FunctionMaker(a var, func, label):
def callback():
avar.set(label)
func()
return callback

[James Stroud]

Re: Function returns a function

Thank you Paul, this makes much more sense.

James

On Tuesday 06 September 2005 02:16 pm, Paul Rubin wrote:[color=blue]
>     def FunctionMaker(a var, func, label):
>        def callback():
>           avar.set(label)
>           func()
>        return callback[/color]

--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/

[Gregory Bond]

Re: Function returns a function

Paul Rubin wrote:
[color=blue]
>
> def FunctionMaker(a var, func, label):
> def callback():
> avar.set(label)
> func()
> return callback[/color]

I've seen this idiom a lot and I know _what_ it does..... but I can't
work _how_ it does it. How does the name binding work so that "avar" in
the returned function still references the object passed into the
(long-distant!) FunctionMaker() call?

Web pages / turotial ptrs etc gladly accepted.

[Aldo Cortesi]

Re: Function returns a function

Thus spake Gregory Bond (gnb@itga.com.a u):
[color=blue]
> Paul Rubin wrote:
>[color=green]
> >
> > def FunctionMaker(a var, func, label):
> > def callback():
> > avar.set(label)
> > func()
> > return callback[/color]
>
> I've seen this idiom a lot and I know _what_ it does.....
> but I can't
> work _how_ it does it. How does the name binding work so
> that "avar" in
> the returned function still references the object passed
> into the
> (long-distant!) FunctionMaker() call?
>
> Web pages / turotial ptrs etc gladly accepted.[/color]

The lexical scope within which a function is declared is
made available to the function when it is run. This is done
by storing the values of free variables within the declared
function in the func_closure attribute of the created
function object.

If you want to read more, the keyword you're looking for is
"closure" - a quick search on Google for "python closure"
should satisfy your curiosity handily. There is also a neat
recipe for inspecting the values kept in the func_closure
attribute of function objects directly:

http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/439096

Cheers,


Aldo



--
Aldo Cortesi
aldo@nullcube.c om

http://www.nullcube.com

Off: (02) 9283 1131
Mob: 0419 492 863

[Paul Rubin]

Re: Function returns a function

Aldo Cortesi <aldo@nullcube. com> writes:[color=blue]
> The lexical scope within which a function is declared is
> made available to the function when it is run. This is done
> by storing the values of free variables within the declared
> function in the func_closure attribute of the created
> function object.[/color]

Actually not quite right:

a = []
for i in range(5):
a.append(lambda : i) # i is free in the lambda
print [f() for f in a]

prints [4,4,4,4,4] instead of [0,1,2,3,4]. The value of i has not
been copied into the closure. Rather, the closure has a cell bound to
the same place that i is bound. So:

for i in range(5):
def f():
j = i # bind a new variable "j" and assign the value from i
return lambda: j # j is free and its value doesn't change
a.append(f())
print [f() for f in a]

prints [0,1,2,3,4]. There's a Pythonic idiom using default args:

a = []
for i in range(5):
a.append(lambda i=i: i) # the "inner" i is free in the lambda
print [f() for f in a]

Closures like may look a little surprising in Python but they're a
standard technique in Scheme. The classic textbook "Structure and
Interpretation of Computer Programming" explains it all. Full text is
online at:

http://mitpress.mit.edu/sicp/

[Gregory Bond]

Re: Function returns a function

Paul Rubin wrote:[color=blue]
> Aldo Cortesi <aldo@nullcube. com> writes:[/color]


Thanks to Paul and Aldo... one more question on the implementation.

Why is the func_closure a tuple of Cells and not just a tuple of
objects? Why the extra level of indirection?