generator expressions: performance anomaly?

Nick Coghlan ncoghlan at
Sun Jan 16 06:20:28 EST 2005

John Machin wrote:
> Background: There was/is a very recent thread about ways of removing
> all instances of x from a list. /F proposed a list comprehension to
> build the result list. Given a requirement to mutate the original list,
> this necessitates the assignment to lst[:]. I tried a generator
> expression as well. However while the listcomp stayed competitive up to
> a million-element list, the genexp went into outer space, taking about
> 20 times as long. The above timeit runs show a simpler scenario where
> the genexp also seems to be going quadratic.
> Comments, clues, ... please.

Py> lc = [x for x in range(100)]
Py> len(lc)
Py> ge = (x for x in range(100))
Py> len(ge)
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
TypeError: len() of unsized object

It would be nice if unconditional ge's with known length inputs propagated 
__len__, but that is not currently the case. There's a similar performance 
glitch associated with constructing a tuple from a generator expression (with 
vanilla 2.4, detouring via list is actually faster)


Nick Coghlan   |   ncoghlan at   |   Brisbane, Australia

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