On Saturday, August 27, 2011 8:45:05 PM UTC-7, John O'Hagan wrote:
> Somewhat apropos of the recent "function principle" thread, I was recently
> surprised by this:
>
> funcs=[]
> for n in range(3):
> def f():
> return n
> funcs.append(f)
>
> [i() for i in funcs]
>
> The last expression, IMO surprisingly, is [2,2,2], not [0,1,2]. Google tells
> me I'm not the only one surprised, but explains that it's because "n" in the
> function "f" refers to whatever "n" is currently bound to, not what it was
> bound to at definition time (if I've got that right), and that there are at
> least two ways around it: ....
> My question is, is this an inescapable consequence of using closures, or is
> it by design, and if so, what are some examples of where this would be the
> preferred behaviour?
It is the preferred behavior for the following case.
def foo():
def printlocals():
print a,b,c,d
a = 1; b = 4; c = 5; d = 0.1
printlocals()
a = 2
printlocals()
When seeing a nested function, there are strong expectations by most people
that it will behave this way (not to mention it's a lot more useful). It's
only for the less common and much more advanced case of creating a closure in a
loop that the other behavior would be preferred.
Carl Banks
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