[Python-Dev] An issue recently brought up in patch #872326(generator expression) (original) (raw)

Tim Peters tim.one at comcast.net
Mon Mar 22 22:54:14 EST 2004

[Greg Ewing]

... I'm disturbed by the number of special rules we seem to be needing to make up in the name of getting generator expressions to DWIM.

Yup.

First we have free variables getting captured, which is unprecedented anywhere else;

Not sure that's it. In some sense it's also arbitrary that Python decides in

def f(x=DEFAULT_X_VALUE):
    ...

to capture the binding of DEFAULT_X_VALUE (which is a free vrbl so far as the defn of f is concerned) at the time f is defined and reuse it on each call; it would have been possible to define the language to use whatever binding is current each time f is called.

Outside of explicitly burying something inside a lambda body, there's no precendent in Python for "delaying" evaluation of anything in a Python expression either. So generator expressions aren't like any non-lambda expressions in Python either: the time at which their guts get evaluated can be arbitrarily far removed from the time the statement holding the guts gets executed.

now we have some iterators being treated more equally than others.

I'm getting an "architecture smell" here. Something is wrong somewhere, and I don't think we're tinkering in the right place to fix it properly.

When scopes and lifetimes get intricate, it's easier to say what you mean in Scheme (which is a good argument for not letting scopes and lifetimes get intricate ).

[Guido]

I'm not disagreeing -- I was originally vehemently against the idea of capturing free variables, but Tim gave an overwhelming argument that whenever it made a difference that was the desired semantics.

Na, and that's a big part of the problem we're having here: I didn't make an argument, I just trotted out non-insane examples. They all turned out to suck under "delay evaluation" semantics, to such an extent that wholly delayed evaluation appeared to be a downright foolish choice. But decisions driven purely by use cases can be mysterious. I was hoping to see many more examples, on the chance that a clarifying principle would reveal itself.

But I always assumed that the toplevel iterable would be different.

Jiwon's example certainly suggests that it must be. But why <0.7 wink>?

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