[Python-Dev] Visibility scope for "for/while/if" statements

[Nick Coghlan]

Andrew Koenig wrote:
>>Interestingly enough, not all C++ compilers (Microsoft) hid variables
>>created in for loops
>>(http://www.devx.com/cplus/10MinuteSolution/28908/0/page/2).
> 
> 
> That's because the C++ spec changed during standardization, when the
> standards committee realized the original idea was a mistake.
> 
> One of the convincing cases:
> 
> 	if (x)
> 		for (int i = 0; i != 10; ++i) { }
> 
> Is I in scope after the if statement?  If so, what value does it have if x
> is false?  If not, then apparently the subject of an "if" statement is a
> scope...so why can't I write this?
> 
> 	if (x)
> 		int i = 42;
> 
> and have i go out of scope?

The difference is that C++ uses {} to delineate a new scope, whereas Python 
uses only def statements.

The behaviour in C++ and C99 can indeed be quite handy, and aligns perfectly 
with the static type system of those languages. Importantly, creating a new 
scope is cheap - it's just a matter of moving the stack pointer around a bit 
(and maybe invoking some destructors that would have been invoked eventually 
anyway).

Python, however, uses a dynamic name binding system and scopes are expensive 
because they require setting up all of the machinery to support nested 
visibility. That is, additional inline scoping would add significant *runtime* 
overhead. So a different design choice was made in Python, but that choice is 
still internally consistent.

Using the above example:

     if x:
         for i in range(10): pass
     print i

     if x:
         i = 42
     print i

What happens on the 'print i' line in Python if 'x' is false? Easy: attempting 
to access 'i' will result in "UnboundLocalError" being raised at runtime, just 
as it would if the 'print i' line was *before* the if statement. That's 
because the name 'i' *is* in scope - it just hasn't been bound to anything yet.

As Guido has said, this is really a discussion for c.l.p. Even before taking 
it there, however, I suggest reviewing the c.l.p discussion from January 
regarding the idea of 'statement local namespaces' [1] (and also checking the 
archives to see if the discussion has moved on since then, as I haven't been 
following c.l.p. for a good long while).

Regards,
Nick.

[1] http://mail.python.org/pipermail/python-list/2005-January/259556.html

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
---------------------------------------------------------------
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