map - lambda - problem

Alex Martelli aleaxit at
Sun Mar 11 14:44:48 CET 2001

"Gregor Lingl" <aon.912502367 at> wrote in message
news:3AAB6078.EE50AE8D at
> In direct mode I got:
> >>> r = []
> >>> for i in range(len(n[0])-1,-1,-1):
>         r.append(map(lambda x: x[i],n))

Here, i is in the global namespace, and therefore available to
functions (including lambdas).

> If i put this into a function:
> def rotate(piece):
>     r = []
>     for i in range(len(piece[0])-1,-1,-1):
>         r.append(map(lambda x: x[i], piece))

Here, i is a local variable and thus (in Python 2.0) not visible
in nested scopes (including lambdas).

> Why, exactly, this behaviour. Is i now taken from some other namespace?

Local scopes don't nest (in Python 2.0).  i must come from the
global namespace.

> If so, which one and why is it initialized with 0?
> How can I work around this?

Use lambda x, i=i: x[i] to inject i into the lambda's local namespace
(or, switch to Python 2.1, which does let local scopes nest).


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