Re: lambda closure question

From: Kent Johnson (kent37_at_tds.net)
Date: 02/19/05 

Date: Sat, 19 Feb 2005 08:54:38 -0500

Ted Lilley wrote:
> What I want to do is pre-load functions with arguments by iterating
> through a list like so:
>
>
>>>>class myclass:
>
> ... pass
>
>>>>def func(self, arg):
>
> ... print arg
>
>>>>mylist = ["my", "sample", "list"]
>>>>for item in mylist:
>
> ... setattr(myclass, item, lamdba self: func(self, item))
>
> This attaches a list of functions to the class, making them bound
> methods when called from a class instance.
>
>
>>>>obj = myclass()
>>>>obj.list()
>
> list
> Unfortunately, it doesn't work. It seems the closure keeps track of
> the variable fed to it dynamically - if the variable changes after the
> lambda is created, the lambda still references the _variable_ not the
> original _value_ and so gets the new value like so:
>
>
>>>>obj.sample()
>
> list
>
>>>>obj.my()
>
> list

The closure isn't bound until the scope containing it is exited. A simple workaround is to bind item
as a default argument to the lambda:
for item in mylist:
     setattr(myclass, item, lambda self, item=item: func(self, item))

You can also make a helper function that returns the closure, the closure will be bound each time
the helper returns:

def make_f(item):
     def f(self): func(self, item)
     return f

mylist = ["my", "sample", "list"]
for item in mylist:
     setattr(myclass, item, make_f(item))

Kent

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