Make default __format__ be equivalent to __str__

@rhettinger

BPO	28974
Nosy	@rhettinger, @ericvsmith, @serhiy-storchaka
PRs	bpo-28974: `object.__format__(x, '')` is now equivalent to `str(x)` #506
Files	simpler-object-format.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 = None
closed_at = <Date 2017-05-13.09:41:23.437>
created_at = <Date 2016-12-14.21:52:10.509>
labels = ['interpreter-core', 'type-feature', '3.7']
title = 'Make default __format__ be equivalent to __str__'
updated_at = <Date 2017-05-13.09:41:23.436>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2017-05-13.09:41:23.436>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2017-05-13.09:41:23.437>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2016-12-14.21:52:10.509>
creator = 'serhiy.storchaka'
dependencies = []
files = ['45902']
hgrepos = []
issue_num = 28974
keywords = ['patch']
message_count = 4.0
messages = ['283220', '293605', '293607', '293609']
nosy_count = 3.0
nosy_names = ['rhettinger', 'eric.smith', 'serhiy.storchaka']
pr_nums = ['506']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue28974'
versions = ['Python 3.7']

Originally PEP-3101 defined the default __format__ implementation, object.__format__ as

     def __format__(self, format_spec):
         return format(str(self), format_spec)

After few changes (bpo-7994, bpo-28385) it now looks as

     def __format__(self, format_spec):
         assert format_spec == ''
         return format(str(self), '')

Proposed patch makes it yet simpler:

     def __format__(self, format_spec):
         assert format_spec == ''
         return str(self)

This is equivalent to the previous form except obscure case when str() returns not exact str, but strict subclass with overridden __format__.

The benefit of this change is simpler semantical model of the default implementation.

See the start of the discussion in bpo-28385 and the discussion on Python-Dev: https://mail.python.org/pipermail/python-dev/2016-October/146765.html.

What are your thoughts about this Eric?

I'm okay with this change.

New changeset 7e19dbc by Serhiy Storchaka in branch 'master':
bpo-28974: object.__format__(x, '') is now equivalent to str(x) (#506)
7e19dbc

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Make default format be equivalent to str #73160

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Make default __format__ be equivalent to __str__ #73160

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serhiy-storchaka commented on Dec 14, 2016

serhiy-storchaka commented on May 13, 2017

ericvsmith commented on May 13, 2017

serhiy-storchaka commented on May 13, 2017

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Make default format be equivalent to str #73160