Implementation-depended pickling floats with protocol 0

[serhiy-storchaka]

BPO	23096
Nosy	@pitrou, @avassalotti, @serhiy-storchaka
Files	pickle_float_repr.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/serhiy-storchaka'
closed_at = <Date 2015-02-15.12:35:43.683>
created_at = <Date 2014-12-21.08:10:33.232>
labels = ['type-bug', 'library']
title = 'Implementation-depended pickling floats with protocol 0'
updated_at = <Date 2015-02-15.12:35:43.683>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2015-02-15.12:35:43.683>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2015-02-15.12:35:43.683>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2014-12-21.08:10:33.232>
creator = 'serhiy.storchaka'
dependencies = []
files = ['38039']
hgrepos = []
issue_num = 23096
keywords = ['patch']
message_count = 6.0
messages = ['232992', '232995', '233024', '233027', '235550', '236034']
nosy_count = 4.0
nosy_names = ['pitrou', 'alexandre.vassalotti', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue23096'
versions = ['Python 3.5']

[serhiy-storchaka]

Python and C implementations of pickle produce different results when pickle floats with protocol 0.

>>> pickle.dumps(4.2, 0)
b'F4.2000000000000002\n.'
>>> pickle._dumps(4.2, 0)
b'F4.2\n.'

[serhiy-storchaka]

Python implementation uses repr(value) and C implementation uses '%.17g' % value.

[avassalotti]

The repr for floats was changed some time ago to use a shorter decimal strings when possible. Both representations should yield the same float value once decoded. If we want to make the C and Python implementations of pickle consistent, then we should pick on what the Python version does currently: i.e., call repr on the float.

[pitrou]

This looks less like a bug than an enhancement request.

[serhiy-storchaka]

Here is a patch.

[python-dev]

New changeset 8c9121993eb5 by Serhiy Storchaka in branch 'default':
Issue bpo-23096: Pickle representation of floats with protocol 0 now is the same
https://hg.python.org/cpython/rev/8c9121993eb5