Author furiel
Recipients furiel
Date 2019-07-24.18:18:16
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Message-id <1563992297.11.0.886326187816.issue37671@roundup.psfhosted.org>
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Reproducible with current master (3.9, 151b91dfd21a100ecb1eba9e293c0a8695bf3bf5)

I would expect itertools.combinations to be lazy in the sense that it should not exhaust the input iterator in constructor time.

import itertools;
itertools.combinations(itertools.count(),2)

should return instantly. Instead it "hangs" until the process is killed.

Similarly, one can reproduce with the following simple crafted generator:

Python 3.9.0a0 (heads/master-dirty:151b91d, Jul 24 2019, 19:51:53) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> def mygenerator_with_exception():
    yield 2
    yield 2
    yield 3
    raise Exception("Should not be raised")
... ... ... ... ... 
>>> g = mygenerator_with_exception()
>>> import itertools             
>>> itertools.combinations(g,2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in mygenerator_with_exception
Exception: Should not be raised

Could you please consider making itertools.combinations truely lazy?
History
Date User Action Args
2019-07-24 18:18:17furielsetrecipients: + furiel
2019-07-24 18:18:17furielsetmessageid: <1563992297.11.0.886326187816.issue37671@roundup.psfhosted.org>
2019-07-24 18:18:17furiellinkissue37671 messages
2019-07-24 18:18:16furielcreate