[Python-ideas] Allow function __globals__ to be arbitrary mapping not just dict

[Yury Selivanov]

+1 on the idea.  Would be interesting to play with frozendict 
(if it's accepted someday)

On 2012-03-18, at 8:27 AM, Steven D'Aprano wrote:

> Currently, if you try to construct a function from parts, the mapping that becomes func.__globals__ must be an actual dict:
> 
> 
> py> class Mapping:
> ...     def __getitem__(self, key):
> ...             if key == 'y':
> ...                     return 42
> ...             raise KeyError(key)
> ...
> py> from types import FunctionType
> py> f = lambda x: x + y
> py> g = FunctionType(f.__code__, Mapping(), 'g')
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
> TypeError: function() argument 2 must be dict, not Mapping
> 
> 
> I propose to allow function.__globals__ to accept any mapping type.
> 
> That, plus the new collections.ChainMap class in Python 3.3, would allow some interesting experiments with namespaces and scoping rules.
> 
> E.g. if I want to write a function with a custom namespace, I have to do something like this:
> 
> 
> ns = ChainMap( ... )  # set up a namespace
> def func(a, ns=ns):
>    x = a + ns['b']
>    y = ns['some_func'](ns['c'])
>    z = ns['another_func'](x, y)
>    ns['d'] = (x, y, z)
>    return ns['one_last_thing'](d)
> 
> 
> which is not a very natural way of writing code. But if we could use non-dict mappings as __globals__, I could write that function like this:
> 
> 
> ns = ChainMap( ... )  # set up a namespace
> def func(a):
>    global d
>    x = a + b
>    y = some_func(c)
>    z = another_func(x, y)
>    d = (x, y, z)
>    return one_last_thing(d)
> 
> 
> # This could be a decorator.
> func = FunctionType(func.__code__, ns, func.__name__)
> 
> 
> 
> (By the way, ChainMap is only one possible example namespace.)
> 
> 
> 
> 
> -- 
> Steven
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