Re: Problem with Lexical Scope

jslowery@xxxxxxxxx wrote: 
I am not completely knowledgable about the status of lexical scoping in
Python, but it was my understanding that this was added in a long time
ago around python2.1-python2.2

I am using python2.4 and the following code throws a "status variable"
not found in the inner-most function, even when I try to "global" it.

def collect(fields, reducer):
    def rule(record):
        status = True
        def _(x, y):
            cstat = reducer(x, y)
            if status and not cstat:
                status = False
            return y
        return reduce(_, [record[field] for field in fields])
    return rule

What gives?

What's happening is that the interpreter, when it compiles the inner function _(), sees an assignment to status and so assumes it is local to the _() function. Consequently, since you reference it inside _() before assignment you get (I presume) an exception reporting an unbound local variable.

The scoping rules do work when you obey them:

 >>> def f1(a, b):
 ...   s = a+b
 ...   def _(x):
 ...     return s+x
 ...   return _
 ...
 >>> func = f1(12, 13)
 >>> func(10)
35
 >>>

Here the nested lexical scopes rule isn't that helpful given the overriding nature of assignment within an inner scope. Using global will simply put the status variable at *module* scope, which also isn't what you want.

regards
 Steve
--
Steve Holden       +44 150 684 7255  +1 800 494 3119
Holden Web LLC                     www.holdenweb.com
PyCon TX 2006                  www.python.org/pycon/

.

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