Issue using datetime with format()

[JordanS]

BPO	7789
Nosy	@birkenfeld, @abalkin, @ericvsmith, @bitdancer
Superseder	bpo-8913: Document that datetime.format is datetime.strftime

^{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/birkenfeld'
closed_at = <Date 2010-10-04.14:15:09.680>
created_at = <Date 2010-01-26.19:21:56.128>
labels = ['type-bug', 'docs']
title = 'Issue using datetime with format()'
updated_at = <Date 2010-10-04.14:15:09.679>
user = 'https://bugs.python.org/JordanS'

bugs.python.org fields:

activity = <Date 2010-10-04.14:15:09.679>
actor = 'belopolsky'
assignee = 'georg.brandl'
closed = True
closed_date = <Date 2010-10-04.14:15:09.680>
closer = 'belopolsky'
components = ['Documentation']
creation = <Date 2010-01-26.19:21:56.128>
creator = 'JordanS'
dependencies = []
files = []
hgrepos = []
issue_num = 7789
keywords = []
message_count = 8.0
messages = ['98358', '98362', '98363', '98365', '98367', '98369', '98370', '117949']
nosy_count = 5.0
nosy_names = ['georg.brandl', 'belopolsky', 'eric.smith', 'r.david.murray', 'JordanS']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = None
status = 'closed'
superseder = '8913'
type = 'behavior'
url = 'https://bugs.python.org/issue7789'
versions = ['Python 2.6', 'Python 3.1', 'Python 2.7', 'Python 3.2']

[JordanS]

format() cannot handle datetime.DateTime objects, returns the format_spec instead, without applying formatting to it, perhaps default behaviour in case of unknown type. Different modifications, ie: using str.format() syntax produce same behaviour.

Sample code:

import datetime

#data
row = {0: 1, 'BeamId': 218, 2: 0.0, 3: 0.0, 4: datetime.datetime(2006, 7, 18, 0, 12), 5: datetime.datetime(2007, 2, 23, 18, 26, 55, 450000), 6: 32637.774406455803, 1: 218, 'DateAndTime': datetime.datetime(2007, 2, 23, 18, 26, 55, 450000), 'AvgFlexuralStrengthDown': 32637.774406455803, 'WarmUpTime': datetime.datetime(2006, 7, 18, 0, 12), 'AvgFlexuralStrengthUp': 15916.5463146028, 'IceSheetId': 1, 'Y': 0.0, 'X': 0.0, 7: 15916.5463146028}

titles = ['BeamId', 'DateAndTime', 'AvgFlexuralStrengthDown', 'WarmUpTime', 'AvgFlexuralStrengthUp', 'IceSheetId', 'Y', 'X']

#attempt to print datetime: ignores formatting, doesn't print datetime value, prints format_spec instead
for key in titles:
    asLine = "{0:*<30}".format(row[key])
    print(asLine),
print '\n'

#prints a repr of datetime
for key in titles:
    asLine = "{0!r:*<30}".format(row[key])
    print(asLine),
print '\n'

#prints datetime as string
for key in titles:
    asLine = "{0!s:*<30}".format(row[key])
    print(asLine),
print '\n'

[JordanS]

sorry, first time posting anything like this:
versions:

Python 2.6.4 (r264:75708, Oct 26 2009, 08:23:19) [MSC v.1500 32 bit (Intel)] on win32

[ericvsmith]

datetime.datetime passes its format string to strftime:

>>> import datetime
>>> x = datetime.datetime(2001, 1, 2, 3, 4)
>>> x.strftime('%Y-%m-%d')
'2001-01-02'
>>> '{0:%Y-%m-%d}'.format(x)
'2001-01-02'

I'll check to make sure this is documented.

[bitdancer]

If it is, it isn't any place obvious. I thought I remembered something about using strftime strings in format, but when I looked in the docs for datetime and the section on the format mini language I couldn't find it, so I ended up doing '{} ...'.format(x.strftime("...") in my code...

[ericvsmith]

I don't think this is documented (that I can find, at least), so I'll assign it to Georg.

I think the correct thing to do is something like this, in the datetime, date, and time object descriptions:

date.__format__(fmt)
    For a date d, format(d, fmt) is equivalent to d.strftime(fmt).

Ditto for date.__format__. But maybe there's a better, more obvious place to document this.

[ericvsmith]

The documentation for this belongs in the mini-language specification, since that just address built-in types. Each type can define its own format specification language.

So I think adding documentation of __format__ to each non-builtin type that implements __format__ is probably the best way to go.

[ericvsmith]

Eric Smith wrote:

The documentation for this belongs in the mini-language specification, ...

Oops. "does NOT belong in the mini-language specification".

[abalkin]

The original bug report is invalid and the documentation issue is a duplicate of bpo-8913.