datetime.strptime without a year fails on Feb 29

[SriramRajagopalan]

BPO	26460
Nosy	@gpshead, @abalkin, @pganssle, @tirkarthi, @nickzoic

^{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 = None
created_at = <Date 2016-02-29.18:02:35.000>
labels = ['3.7', '3.8', 'type-feature', 'library']
title = 'datetime.strptime without a year fails on Feb 29'
updated_at = <Date 2020-03-03.17:16:20.440>
user = 'https://bugs.python.org/SriramRajagopalan'

bugs.python.org fields:

activity = <Date 2020-03-03.17:16:20.440>
actor = 'nickzoic'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-02-29.18:02:35.000>
creator = 'Sriram Rajagopalan'
dependencies = []
files = []
hgrepos = []
issue_num = 26460
keywords = []
message_count = 13.0
messages = ['261014', '261024', '261027', '261028', '261033', '343085', '363123', '363202', '363215', '363217', '363223', '363257', '363280']
nosy_count = 8.0
nosy_names = ['gregory.p.smith', 'belopolsky', 'polymorphm', 'Sriram Rajagopalan', 'p-ganssle', 'xtreak', 'gerardw@alum.mit.edu', 'nickzoic']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue26460'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

Linked PRs

• gh-70647: Better promote how to safely parse yearless dates in datetime. #116179
• gh-70647: Add detail to error message when parsing a bad day of month. #116184
• GH-70647: Deprecate strptime day of month parsing without a year present to avoid leap-year bugs #117107
• gh-70647: Raise a more informative error for when date is out of range #131335
• [3.12] gh-70647: update docs to mention the datetime 1900 year default 2/29 issue #131534

[SriramRajagopalan]

$ python
    Python 3.5.1 (default, Dec  7 2015, 12:58:09) 
    [GCC 5.2.0] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>> 
    >>> 
    >>> 
    >>> import time
    >>> 
    >>> time.strptime("Feb 29", "%b %d")
    time.struct_time(tm_year=1900, tm_mon=2, tm_mday=29, tm_hour=0, tm_min=0, tm_sec=0, tm_wday=0, tm_yday=60, tm_isdst=-1)
    >>> 
    >>> 
    >>> import datetime
    >>> 
    >>> datetime.datetime.strptime("Feb 29", "%b %d")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/usr/lib/python3.5/_strptime.py", line 511, in _strptime_datetime
        return cls(*args)
    ValueError: day is out of range for month

The same issue is seen in all versions of Python

[gpshead]

Python's time.strptime() behavior is consistent with that of glibc 2.19:

======= strptime_c.c =======

#define _XOPEN_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

int
main(void)
{
  struct tm tm;
  char buf[255];
  memset(&tm, 0, sizeof(struct tm));
  strptime("Feb 29", "%b %d", &tm);
  strftime(buf, sizeof(buf), "%d %b %Y %H:%M", &tm);
  puts(buf);
  exit(EXIT_SUCCESS);
}

=======

$ gcc strptime_c.c 
$ ./a.out
29 Feb 1900 00:00

I'm not saying that the behavior is a good API, but given the unfortunate API at hand, parsing a date without specifying what year it is using strptime is a bad idea.

[abalkin]

This is not no more bug than

>>> from datetime import *
>>> datetime.strptime('0228', '%m%d')
datetime.datetime(1900, 2, 28, 0, 0)

Naturally, as long as datetime.strptime('0228', '%m%d') is the same as datetime.strptime('19000228', '%Y%m%d'), datetime.strptime('0229', '%m%d') should raise a ValueError as long as datetime.strptime('19000229', '%Y%m%d') does.

The only improvement, I can think of in this situation is to point the user to time.strptime() in the error message. The time.strptime method works just fine in the recent versions (see bpo-14157.)

>>> time.strptime('0229', '%m%d')[1:3]
(2, 29)

[abalkin]

Python's time.strptime() behavior is consistent with that of glibc 2.19

Gregory,

I believe OP is complaining about the way datetime.datetime.strptime() behaves, not time.strptime() which is mentioned as (preferred?) alternative.

See msg261015 in bpo-14157 for context.

[gpshead]

time.strptime() is "working" (not raising an exception) as it appears not to validate the day of the month when a year is not specified, yet the return value from either of these APIs is a date which has no concept of an ambiguous year.

## Via the admittedly old Python 2.7.6 from Ubuntu 14.04: ##
# 1900 was not a leap year as it is not divisible by 400.
>>> time.strptime("1900 Feb 29", "%Y %b %d")
ValueError: day is out of range for month
>>> time.strptime("Feb 29", "%b %d")
time.struct_time(tm_year=1900, tm_mon=2, tm_mday=29, tm_hour=0, tm_min=0, tm_sec=0, tm_wday=0, tm_yday=60, tm_isdst=-1)

So what should the validation behavior be?

>>> datetime.datetime.strptime("Feb 29", "%b %d")
ValueError: day is out of range for month
>>> datetime.datetime.strptime("2016 Feb 29", "%Y %b %d")
datetime.datetime(2016, 2, 29, 0, 0)
>>> datetime.datetime.strptime("1900 Feb 29", "%Y %b %d")
ValueError: day is out of range for month
>>> datetime.datetime(year=1900, month=2, day=29)
ValueError: day is out of range for month

datetime objects cannot be constructed with the invalid date (as the time.strptime return value allows).

Changing the API to assume the current year or a +/- 6 months from "now" when no year is parsed is likely to break existing code.

[tirkarthi]

See also bpo-19376. This behavior is now documented with 56027cc

[nickzoic]

I suspect this is going to come up about this time of every leap year :-/

The workaround is prepending "%Y " to the pattern and eg: "2020 " to the date string, but that's not very nice.

Would adding a kwarg "default_year" be an acceptable solution?
I can't think of any other situation other than leap years when this is going to come up. If both "default_year" and "%Y" are present throw an exception (maybe therefore just call the kwarg "year")

In the weird case where you want to do date maths involving the month as well, you can always use a safe choice like "default_year=2020" and then fix the year up afterwards:

    dt = datetime.strptime(date_str, "%b %d", default_year=2020)
    dt = dt.replace(year=2021 if dt.month > 6 else 2022)

[pganssle]

I don't think adding a default_year parameter is the right solution here.

The actual problem is that time.strptime, and by extension datetime.strptime has a strange and confusing interface. What should happen is either that year is set to None or some other marker of a missing value or datetime.strptime should raise an exception when it's being asked to construct something that does not contain a year.

Since there is no concept of a partial datetime, I think our best option would be to throw an exception, except that this has been baked into the library for ages and would start to throw exceptions even when the person has correctly handled the Feb 29th case.

I think one possible "solution" to this would be to raise a warning any time someone tries to use datetime.strptime without requesting a year to warn them that the thing they're doing only exists for backwards compatibility reasons. We could possibly eventually make that an exception, but I'm not sure it's totally worth a break in backwards compatibility when a warning should put people on notice.

[nickzoic]

Not disagreeing with you that "%b %d" timestamps with no "%Y" are excerable, but they're fairly common in the *nix world unfortunately.

People need to parse them, and the simple and obvious way to do this breaks every four years.

I like the idea of having a warning for not including %Y *and* not setting a default_year kwarg though.