datetime.strptime parses input wrong

@abalkin

BPO	12886
Nosy	@abalkin, @vstinner, @ezio-melotti
Files	issue12886.patch issue12886.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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assignee = None
closed_at = None
created_at = <Date 2011-09-02.14:33:26.942>
labels = ['type-bug', '3.8', '3.9', '3.10', '3.7', 'library']
title = 'datetime.strptime parses input wrong'
updated_at = <Date 2020-05-31.12:37:23.388>
user = 'https://bugs.python.org/heidarrafn'

bugs.python.org fields:

activity = <Date 2020-05-31.12:37:23.388>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2011-09-02.14:33:26.942>
creator = 'heidar.rafn'
dependencies = []
files = ['27675', '27688']
hgrepos = []
issue_num = 12886
keywords = ['patch']
message_count = 7.0
messages = ['143400', '143401', '143402', '143515', '143517', '173600', '173644']
nosy_count = 5.0
nosy_names = ['belopolsky', 'vstinner', 'ezio.melotti', 'heidar.rafn', 'hieu.nguyen']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue12886'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

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When using datetime.strptime or time.strptime to parse string representing timestamp with the format string '%Y%m%dT%H%M%S' then a strange behavior happens when the input string does not contain the seconds: the minute part is split and first digit used as minutes and second digit as seconds !

According to documentation %M shall contain Minute as a decimal number [00,59] and %S shall contain Second as a decimal number [00,61]

Here are few examples to show this:
------------------

Python 2.6.6 (r266:84292, Sep 15 2010, 16:22:56) 
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime
>>> datetime.datetime.strptime('20110817T1234','%Y%m%dT%H%M%S')
datetime.datetime(2011, 8, 17, 12, 3, 4)
=>ERROR no seconds in input string: minute=3, second=4
=>I would expect exception ValueError or datetime.datetime(2011, 8, 17, 12, 34, 00)

>>> datetime.datetime.strptime('20110817T123456','%Y%m%dT%H%M%S')
datetime.datetime(2011, 8, 17, 12, 34, 56)
=>CORRECT

>>> datetime.datetime.strptime('20110817T123456','%Y%m%dT%H%M')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/_strptime.py", line 328, in _strptime
    data_string[found.end():])
ValueError: unconverted data remains: 56
=>CORRECT

>>> datetime.datetime.strptime('20110817T1234','%Y%m%dT%H%M')
datetime.datetime(2011, 8, 17, 12, 34)
=> CORRECT

I have tested this with python 2.6 and 2.7
This also happens on when playing with %H%M format string and omit minutes from the input.

ERROR no seconds in input string: minute=3, second=4

If you specificy "%S" in the format string, strptime() *requires* seconds. If seconds are optional in your format, you should call strptime() twice: first without '%S', then with '%S' if the first failed.

I don't consider your examples as bugs.

Example:
---

import datetime

def test(text):
    try:
        d = datetime.datetime.strptime(text, '%Y%m%dT%H%M')
    except ValueError:
        pass
    else:
        print("%s without seconds => %s" % (text, d))
        return
    d = datetime.datetime.strptime(text, '%Y%m%dT%H%M%S')
    print("%s with seconds => %s" % (text, d))

test('20110817T1234')
test('20110817T12345')
test('20110817T123456')

Output:

20110817T1234 without seconds => 2011-08-17 12:34:00
20110817T12345 with seconds => 2011-08-17 12:34:05
20110817T123456 with seconds => 2011-08-17 12:34:56

My understanding of the python documentation and the ISO 8601 standard is that the digits in a timestamp representing hours, minutes and seconds shall always be in pairs of 2 digits (hh, mm, ss), i.e. when a number is less than 10 it should be preceded by 0.
In the example I give, the minute figure is split between minutes and seconds by the python library function which I consider a bug:
datetime.datetime.strptime('20110817T1234','%Y%m%dT%H%M%S')
gives
datetime.datetime(2011, 8, 17, 12, 3, 4)

This issue is also discussed here: http://bugs.python.org/issue5979

I have attached a patch for this issue, so that if the format of the input argument doesn't match ISO 8601 format, it will return ValueError: time data xxx does not match format xxx.

Attached another patch for clearer test cases against this fix (as suggested from Ezio).

gh-57095: Add note about input splitting in `datetime.*.strptime` (GH…

pythongh-57095: Add note about input splitting in `datetime.*.strptim…

[3.14] gh-57095: Add note about input splitting in `datetime.*.strpti…

[3.13] gh-57095: Add note about input splitting in `datetime.*.strpti…

pythongh-57095: Add note about input splitting in `datetime.*.strptim…

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datetime.strptime parses input wrong #57095

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heidarrafn commented on Sep 2, 2011

vstinner commented on Sep 2, 2011

vstinner commented on Sep 2, 2011

heidarrafn commented on Sep 5, 2011

heidarrafn commented on Sep 5, 2011

hieunguyen commented on Oct 23, 2012

hieunguyen commented on Oct 23, 2012

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