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__length_hint__ isn't a hint for list() #73126

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ronaldoussoren opened this issue Dec 11, 2016 · 1 comment
Open

__length_hint__ isn't a hint for list() #73126

ronaldoussoren opened this issue Dec 11, 2016 · 1 comment
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interpreter-core (Objects, Python, Grammar, and Parser dirs)

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@ronaldoussoren
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BPO 28940
Nosy @ronaldoussoren
PRs
  • bpo-28940: Don't fail early when __length_hint__() is too big for preallocation. #8635
    		•

    Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = None
closed_at = None
created_at = <Date 2016-12-11.20:53:16.843>
labels = ['interpreter-core']
title = "__length_hint__ isn't a hint for list()"
updated_at = <Date 2018-08-02.19:00:20.255>
user = 'https://github.com/ronaldoussoren'

    bugs.python.org fields:

    activity = <Date 2018-08-02.19:00:20.255>
actor = 'sir-sigurd'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2016-12-11.20:53:16.843>
creator = 'ronaldoussoren'
dependencies = []
files = []
hgrepos = []
issue_num = 28940
keywords = ['patch']
message_count = 1.0
messages = ['282946']
nosy_count = 1.0
nosy_names = ['ronaldoussoren']
pr_nums = ['8635']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue28940'
versions = ['Python 3.5', 'Python 3.6']
    @ronaldoussoren
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    The following code raises MemoryError instead of creating an empty list:

    # START
import sys

class CustomIter:
    def __iter__(self):
        return self
    def __next__(self):
        raise StopIteration
    def __length_hint__(self):
        return sys.maxsize

l = list(CustomIter())
#END

    That's because this empty iterator has a __length_hint__ that claims it returns a very large number of methods.

    The function listextend in Objects/listobject.c already ignores __length_hint__() when using it would overflow the size of the list, it would IMHO also be better to ignore the length hint when the attempt to resize fails as __length_hint__() is documented as a hint that may be incorrect.

    @ronaldoussoren ronaldoussoren added the interpreter-core (Objects, Python, Grammar, and Parser dirs) label Dec 11, 2016
    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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