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From: Peter Otten <__peter__@web.de>
Subject: Re: Using non-dict namespaces in functions
Date: Sun, 18 Mar 2012 08:50:46 +0100
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Xref: csiph.com comp.lang.python:21851

Steven D'Aprano wrote:

> On Sat, 17 Mar 2012 11:42:49 -0700, Eric Snow wrote:
> 
>> On Sat, Mar 17, 2012 at 4:18 AM, Steven D'Aprano
>> <steve+comp.lang.python@pearwood.info> wrote:
>>> Note that it is important for my purposes that MockChainMap does not
>>> inherit from dict.
>> 
>> Care to elaborate?
> 
> I want to use collections.ChainMap, or something very like it, and I
> don't want to be forced into an unnatural is-a relationship with dict if
> I don't have to.
> 
> 
> [...]
>> Regardless, you could also implement __call__() on a function look-alike
>> class to get what you're after.  It may not be as performant though.
> 
> I don't think that can work, because __call__ itself is a function, and I
> would need to change *its* globals. Which brings me back exactly where I
> started, trying to change globals in a function to a non-dict.

The key lookup code in ceval.c is inlined, so even subclassing dict and 
overriding __getitem__() won't help. Instead of 

def f(a):
  return a + b # b taken from some custom namespace

you have to resort to the conventional approach

def f(a, ns=magic()):
    return a + ns["b"]

or

def f(self, a):
    return a + self.b