Idea: "Frozen" inner function

[Steve Horne]

On Sun, 26 Nov 2006 15:28:03 +0100, Michael Butscher
<mbutscher at gmx.de> wrote:

>Steve Horne wrote:

>> So for instance, in Python (which does use closures)...
>> 
>> a = 5
>> fn = lambda : a
>> a = 6
>> print fn()
>> 
>> The output should be 5, not 6 - the value when the lambda expression
>> was evaluated.
>
>Have you tried that? With Python 2.5 in an interactive session, I get:
>
>>>> def test():
>	a = 5
>	fn = lambda : a
>	a = 6
>	print fn()
>
>	
>>>> test()
>6

No, I didn't test, and now I'm a bit embarrassed, and confused.

I have tonnes of code that uses lambdas, and I'm sure some depends on
this behaviour. Code which is heavily used and which all seems to
work. Yet suddenly, I can't get a simple example to work right.

All I can say is that something's going on in Python that I don't know
about.


I've checked my favorite compiler-stuff text (Modern Compiler Design,
Dick Grune et al) and theres a section (6.3.5.5 Currying a routine)
that seems to agree with my closure definition more-or-less.

But this is about currying, not inner functions.

In the same book, section 6.3.5.3 is directly relevant (title
"Returning a nested routine as a value"). It describes the
using-dead-variables issue, but actually suggests keeping the local
variables for the outer function in a heap frame rather than on the
stack.

But...

1  This is the variables for the outer function, not the inner
   function. You put those variables in the stack frame whether
   the inner function object is actually created or not, and
   several inner function objects could be created that all refer
   to the same stack frame.

2  It doesn't use the term closure to describe this.

So basically, I thought I knew what I was talking about, but now I'm
not so sure.

Maybe Python is doing the (1) thing, but that leaves me wondering how
come my existing code is working.

Incidentally, Amazon seems to have the full text of the book if you
want to check exactly what it says, but stepping through the pages
one-by-one to get to the relevant section (around page 490-ish) is
painful.

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