[Python-Dev] PEP 289 - Generator Expressions - Let's Move Forward

[Samuele Pedroni]

At 12:29 30.04.2004 -0400, Edward Loper wrote:
>>Hello Guido,
>>I did a quick review of the stdlib, including the tests, to see which list
>>comprehensions could be replaced with generator expressions.  Thought I admit
>>I am biased towards early binding, I ended up looking for cases with the
>>following properties:  [...]
>
>Even if we switched to early binding, won't these issues still bite you 
>with mutable values?  E.g.:
>
>     some_dict = {'x': 1, 'y': 2}
>     iter1 = (some_dict.get(v, 3) for v in input1)
>     some_dict['z'] = 5
>     iter2 = (some_dict.get(v, 7) for v in input2)
>
>It seems like it would be surprising (to me, anyway) if this gave a 
>different result than:
>
>     some_dict = {'x': 1, 'y': 2}
>     iter1 = (some_dict.get(v, 3) for v in input1)
>     some_dict = {'x': 1, 'y': 2, 'z': 5}
>     iter2 = (some_dict.get(v, 7) for v in input2)

well, the deep problem here is that lazinesss and side-effects (or 
mutability, if one wants, inclusive of bindings) do not mix well. There is 
so much that we can do about that.

It's really a matter of, depending on our choices, how often the casual 
user will
be bitten by this and what kind of questions we want to get.

Otherwise we would have to give up about the whole idea.