closures and dynamic binding

Marc 'BlackJack' Rintsch bj_666 at gmx.net
Sun Sep 28 08:14:01 CEST 2008


On Sat, 27 Sep 2008 21:43:15 -0700, Aaron \"Castironpi\" Brady wrote:

> To me, this is a somewhat unintuitive behavior.  I want to discuss the
> parts of it I don't understand.
> 
>>>> f= [ None ]* 10
>>>> for n in range( 10 ):
> ...     f[ n ]= lambda: n
> ...
>>>> f[0]()
> 9
>>>> f[1]()
> 9

`n` is looked up at the time ``f[0]`` is called.  At that time it is 
bound to 9.

>>>> f= [ None ]* 10
>>>> for n in range( 10 ):
> ...     f[ n ]= (lambda n: ( lambda: n ) )( n ) ...
>>>> f[0]()
> 0
>>>> f[1]()
> 1
> 
> Which is of course the desired effect.  Why doesn't the second one just
> look up what 'n' is when I call f[0], and return 9?

It *does* look up `n` at the time you call ``f[0]`` but this time it's 
the local `n` of the outer ``lambda`` function and that is bound to 
whatever the function was called with.  At the time it was called the 
global `n` was bound to 0.  Maybe it get's more clear if you don't name 
it `n`:

In [167]: f = [None] * 10

In [168]: for n in xrange(10):
   .....:     f[n] = (lambda x: lambda: x)(n)
   .....:

In [169]: f[0]()
Out[169]: 0

Ciao,
	Marc 'BlackJack' Rintsch



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