Bugs item #1517509, was opened at 2006-07-05 08:09
Message generated for change (Comment added) made by rhettinger
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Category: Python Interpreter Core
Group: Python 2.5
>Status: Closed
>Resolution: Wont Fix
Priority: 5
Submitted By: Collin Winter (collinwinter)
Assigned to: Nobody/Anonymous (nobody)
Summary: filter() implementation does not match docs
Initial Comment:
The docs for the built-in function filter() claim that
"filter(function, list) is equivalent to [item for item
in list if function(item)] if function is not None and
[item for item in list if item] if function is None".
>>> class infinite_str(str):
... def __getitem__(self, index):
... return "a"
...
>>> filter(None, infinite_str("1234"))
'aaaa'
Now, if we translate this to a listcomp according to
the docs:
>>> [x for x in infinite_str("1234") if x]
The listcomp version proceeds to chew up memory until
it exhausts the system resources or is killed by the user.
If the docs are to be believed, the filter() version
should do the same thing.
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>Comment By: Raymond Hettinger (rhettinger)
Date: 2006-07-05 09:58
Message:
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user_id=80475
Please take a larger view when reviewing the docs.
The listcomp analogy is very helpful in explaining what
filter() does and readers would not benefit by its removal.
Throughtout the docs, the phrase "is equivalent to" does
not mean "is identical to" or "exactly the same as". In
this case, you have isolated a non-guaranteed
implementation detail that is almost always irrelevant.
When an object such as infinite_str lies about its length,
the consequent behavior is undefined. It is not hard to
produce weird results when objects violate basic
invariants such as len(istr)!=len(list(istr)) or the
expected relation between __eq__ and __hash__.
----------------------------------------------------------------------
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