Re: lambda closure question

[Kent Johnson]

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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