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

Guido van Rossum guido at python.org
Fri Apr 30 08:16:32 EDT 2004

> 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:
> * doesn't use the result immediately
> * contains free variables
> This is less that 10% of the cases.  However, for each of them, I
> had to check if the free variables had a chance of being modified
> after the genexpr.  This is pretty rare, admittedly, but there is an
> example in test/test_random.py (test_avg_std).  There are a number
> of other examples that just don't happen to modify the free
> variables, by chance; e.g. in optparse.py:
>     metavar = option.metavar or option.dest.upper()
>     short_opts = [sopt + metavar for sopt in option._short_opts]
>     long_opts = [lopt + "=" + metavar for lopt in option._long_opts]
> If we find out later that long_opts actually needs a slightly
> different value for metavar, it would be tempting to do:
>     metavar = option.metavar or option.dest.upper()
>     short_opts = [sopt + metavar for sopt in option._short_opts]
>     metavar = option.metavar or option.dest.capitalize()
>     long_opts = [lopt + "=" + metavar for lopt in option._long_opts]
> Replace these with genexprs and it doesn't work any more: the 2nd
> metavar is unexpectedly used in the first genexpr as well.  In
> general I find it strange to have to look if a given variable could
> be modified much later in the same function to be sure of what a
> genexpr really means.  Early binding is closer to the idea that
> turning a listcomp into a genexprs should just work if you only
> iterate once on the result.

Thanks for the analysis.  My comment on this: why on earth would you
want to replace the perfectly sensible list comprehension with a
generator comprehension in this particular case?  (Note that the
remainder of that function proceeds to do list concatenation of the
two lists.)

A more general question would be: have you found any examples of list
comprehensions that weren't immediately used where there would be a
compelling reason to use a generator expression instead?  In this
example, I can't see any of the touted advantages -- the set of
command line options will never be long enough to have to worry about
the memory consumption.  How about the other examples you found?

--Guido van Rossum (home page: http://www.python.org/~guido/)

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