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statistics.geometric_mean gives incorrect results for mixed int/float inputs #72514

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mdickinson opened this issue Oct 1, 2016 · 6 comments
Closed

statistics.geometric_mean gives incorrect results for mixed int/float inputs #72514

mdickinson opened this issue Oct 1, 2016 · 6 comments
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@stevendaprano
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3.7 (EOL) end of life type-bug An unexpected behavior, bug, or error

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@mdickinson
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BPO 28327
Nosy @mdickinson, @stevendaprano, @iritkatriel
Files
  • geometric_mean_int_float.patch
  • geometric_mean_int_float_v2.patch
    		•

    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 = 'https://github.com/stevendaprano'
closed_at = <Date 2021-08-20.10:02:48.549>
created_at = <Date 2016-10-01.12:18:35.892>
labels = ['type-bug', '3.7']
title = 'statistics.geometric_mean gives incorrect results for mixed int/float inputs'
updated_at = <Date 2021-08-20.10:02:48.548>
user = 'https://github.com/mdickinson'

    bugs.python.org fields:

    activity = <Date 2021-08-20.10:02:48.548>
actor = 'mark.dickinson'
assignee = 'steven.daprano'
closed = True
closed_date = <Date 2021-08-20.10:02:48.549>
closer = 'mark.dickinson'
components = []
creation = <Date 2016-10-01.12:18:35.892>
creator = 'mark.dickinson'
dependencies = []
files = ['44906', '44908']
hgrepos = []
issue_num = 28327
keywords = ['patch']
message_count = 6.0
messages = ['277808', '277809', '277814', '277815', '399948', '399951']
nosy_count = 3.0
nosy_names = ['mark.dickinson', 'steven.daprano', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28327'
versions = ['Python 3.7']
    @mdickinson
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    The following calculations should all be giving the same result:

    >>> import statistics
>>> statistics.geometric_mean([2, 3, 5, 7])
3.80675409583932
>>> statistics.geometric_mean([2, 3, 5, 7.0])
1.6265765616977859
>>> statistics.geometric_mean([2, 3, 5.0, 7.0])
2.4322992790977875
>>> statistics.geometric_mean([2, 3.0, 5.0, 7.0])
3.201085872943679
>>> statistics.geometric_mean([2.0, 3.0, 5.0, 7.0])
3.80675409583932

    (Correct result is 3.80675409583932.)

    The culprit is this line in statistics._product:

    mant, scale = 1, 0  #math.frexp(prod)  # FIXME

    ... and indeed, we should be starting from prod rather than 1 here. But simply using math.frexp has potential for failure if the accumulated integer product overflows a float.

    @mdickinson mdickinson added the 3.7 (EOL) end of life label Oct 1, 2016
    @mdickinson mdickinson added the type-bug An unexpected behavior, bug, or error label Oct 1, 2016
    @mdickinson
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    Here's a fix. I was planning to add tests, but as far as I can tell geometric_mean currently has no tests at all. Steve, is that correct? That seems like something that should be fixed before the 3.6 release.

    @stevendaprano
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    Looks good for me.

    Thanks for catching this: I knew it was a bug, but then I ran into the issue that I could no longer build 3.6 before I could fix it, and between that and various issues in the real world I never got back to this.

    @mdickinson
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    New patch, with a (very slightly) cleaner implementation of _frexp_gen.

    @iritkatriel
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    I can't reproduce this now:

    >>> statistics.geometric_mean([2, 3, 5, 7])
3.80675409583932
>>> statistics.geometric_mean([2, 3, 5, 7.0])
3.80675409583932
>>> statistics.geometric_mean([2, 3, 5.0, 7.0])
3.80675409583932
>>> statistics.geometric_mean([2, 3.0, 5.0, 7.0])
3.80675409583932
>>> statistics.geometric_mean([2.0, 3.0, 5.0, 7.0])
3.80675409583932
>>>

    The current geometric_mean was added in PR12638. Is this issue about a previous version?

    @mdickinson
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    Is this issue about a previous version?

    Yep. Sorry for failing to close this earlier.

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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    @mdickinson @iritkatriel @stevendaprano