gh-124529: Fix _strptime to make %c/%x accept a year with fewer digits

Doc/library/datetime.rst

@@ -2648,6 +2648,17 @@ Notes:
  example, "month/day/year" versus "day/month/year"), and the output may
  contain non-ASCII characters.
 
+ .. versionchanged:: 3.14
+ The :meth:`~.datetime.strptime` method, if used with the ``%c``
+ or ``%x`` format code, no longer requires the *year* part of the
+ input to be zero-padded to the usual width (which is either 4 or
+ 2 digits, depending on the format code and current locale). In
+ previous versions, a :exc:`ValueError` was raised if a narrower
+ (i.e., not zero-padded) year representation was part of the input
+ (and it is worth noting that, depending on the platform/locale,
+ such inputs may be produced by using :meth:`~.datetime.strftime`
+ with ``%c`` or ``%x``).
+
 (2)
  The :meth:`~.datetime.strptime` method can parse years in the full [1, 9999] range, but
  years < 1000 must be zero-filled to 4-digit width.

Lib/_strptime.py

@@ -199,8 +199,6 @@ def __init__(self, locale_time=None):
  'V': r"(?P<V>5[0-3]|0[1-9]|[1-4]\d|\d)",
  # W is set below by using 'U'
  'y': r"(?P<y>\d\d)",
- #XXX: Does 'Y' need to worry about having less or more than
- # 4 digits?
  'Y': r"(?P<Y>\d\d\d\d)",
  'z': r"(?P<z>[+-]\d\d:?[0-5]\d(:?[0-5]\d(\.\d{1,6})?)?|(?-i:Z))",
  'A': self.__seqToRE(self.locale_time.f_weekday, 'A'),
@@ -213,8 +211,10 @@ def __init__(self, locale_time=None):
  'Z'),
  '%': '%'})
  base.__setitem__('W', base.__getitem__('U').replace('U', 'W'))
- base.__setitem__('c', self.pattern(self.locale_time.LC_date_time))
- base.__setitem__('x', self.pattern(self.locale_time.LC_date))
+ base.__setitem__(
+ 'c', self.__pattern_with_lax_year(self.locale_time.LC_date_time))
+ base.__setitem__(
+ 'x', self.__pattern_with_lax_year(self.locale_time.LC_date))
  base.__setitem__('X', self.pattern(self.locale_time.LC_time))
 
  def __seqToRE(self, to_convert, directive):
@@ -236,6 +236,27 @@ def __seqToRE(self, to_convert, directive):
  regex = '(?P<%s>%s' % (directive, regex)
  return '%s)' % regex
 
+ def __pattern_with_lax_year(self, format):
+ """Like pattern(), but making %Y and %y accept also fewer digits.
+
+ Necessary to ensure that strptime() is able to parse strftime()'s
+ output when %c or %x is used -- considering that for some locales
+ and platforms (e.g., 'C.UTF-8' on Linux) formatting with %c or %x
+ may produce a year number representation which is not zero-padded
+ and, consequently (if the number is small enough), shorter than
+ the usual four or two digits (e.g., '999' rather than `0999', or
+ '9' rather than of '09').
+
+ Note that this helper is intended to be used to prepare regex
+ patterns *only* for the %c and %x format codes (and *not* for
+ %y, %Y or %G).
+
+ """
+ pattern = self.pattern(format)
+ pattern = pattern.replace(self['Y'], r"(?P<Y>\d{1,4})")
+ pattern = pattern.replace(self['y'], r"(?P<y>\d{1,2})")
+ return pattern
+
  def pattern(self, format):
  """Return regex pattern for the format string.
 
@@ -374,6 +395,7 @@ def _strptime(data_string, format="%a %b %d %H:%M:%S %Y"):
  # U, W
  # worthless without day of the week
  if group_key == 'y':
+ # 1 or 2 digits (1 only for directive c or x; see TimeRE.__init__)
  year = int(found_dict['y'])
  # Open Group specification for strptime() states that a %y
  #value in the range of [00, 68] is in the century 2000, while
@@ -383,6 +405,7 @@ def _strptime(data_string, format="%a %b %d %H:%M:%S %Y"):
  else:
  year += 1900
  elif group_key == 'Y':
+ # 1-4 digits (1-3 only for directive c or x; see TimeRE.__init__)
  year = int(found_dict['Y'])
  elif group_key == 'G':
  iso_year = int(found_dict['G'])

Lib/test/datetimetester.py

@@ -2124,6 +2124,84 @@ def test_fromisocalendar_type_errors(self):
  with self.assertRaises(TypeError):
  self.theclass.fromisocalendar(*isocal)
 
+ def test_strftime_strptime_roundtrip_concerning_locale_specific_year(self):
+ # gh-124529
+ concerned_formats = '%c', '%x'
+
+ def run_subtest():
+ input_obj = sample.replace(year=year)
+ reason = (f"test strftime/strptime roundtrip concerning "
+ f"locale-specific year representation "
+ f"- for{fmt=} and{input_obj=}")
+ fail_msg = f"{reason} - failed"
+ with self.subTest(reason=reason):
+ formatted = input_obj.strftime(fmt)
+ try:
+ parsed = self.theclass.strptime(formatted, fmt)
+ except ValueError as exc:
+ self.fail(f"{fail_msg}; parsing error:{exc!r}")
+ self.assertEqual(parsed, input_obj, fail_msg)
+
+ sample = self.theclass.strptime('1999-03-17', '%Y-%m-%d')
+ for fmt in concerned_formats:
+ with self.subTest(fmt=fmt):
+ sample_str = sample.strftime(fmt)
+ if '1999' in sample_str:
+ for year in [
+ 1000, 1410, 1989, 2024, 2095, 9999,
+ # gh-124529:
+ 1, 9, 10, 99, 100, 999,
+ ]:
+ run_subtest()
+ elif '99' in sample_str:
+ for year in [
+ 1969, 1999, 2068,
+ # gh-124529:
+ 2000, 2001, 2009,
+ ]:
+ run_subtest()
+ else:
+ self.fail(f"{sample!r}.strftime({fmt!r})={sample_str!r} "
+ f"does not include year={sample.year!r} in "
+ f"any expected format (is there something "
+ f"severely wrong with the current locale?)")
+
+ def test_strptime_accepting_locale_specific_year_with_fewer_digits(self):
+ # gh-124529
+ concerned_formats = '%c', '%x'
+
+ def run_subtest():
+ input_str = sample_str.replace(sample_year_digits, year_digits)
+ reason = (f"test strptime accepting locale-specific "
+ f"year representation with fewer digits "
+ f"- for{fmt=} and{input_str=} ({year=})")
+ fail_msg = f"{reason} - failed"
+ expected = sample.replace(year=year)
+ with self.subTest(reason=reason):
+ try:
+ parsed = self.theclass.strptime(input_str, fmt)
+ except ValueError as exc:
+ self.fail(f"{fail_msg}; parsing error:{exc!r}")
+ self.assertEqual(parsed, expected, fail_msg)
+
+ sample = self.theclass.strptime('1999-03-17', '%Y-%m-%d')
+ for fmt in concerned_formats:
+ with self.subTest(fmt=fmt):
+ sample_str = sample.strftime(fmt)
+ if (sample_year_digits := '1999') in sample_str:
+ for year in [1, 9, 10, 99, 100, 999]:
+ year_digits = str(year)
+ run_subtest()
+ elif (sample_year_digits := '99') in sample_str:
+ for year in [2000, 2001, 2009]:
+ year_digits = str(year - 2000)
+ run_subtest()
+ else:
+ self.fail(f"{sample!r}.strftime({fmt!r})={sample_str!r} "
+ f"does not include year={sample.year!r} in "
+ f"any expected format (is there something "
+ f"severely wrong with the current locale?)")
+
 
 #############################################################################
 # datetime tests

Lib/test/test_strptime.py

@@ -161,11 +161,42 @@ def test_compile(self):
  for directive in ('a','A','b','B','c','d','G','H','I','j','m','M','p',
  'S','u','U','V','w','W','x','X','y','Y','Z','%'):
  fmt = "%d %Y" if directive == 'd' else "%" + directive
+ input_string = time.strftime(fmt)
  compiled = self.time_re.compile(fmt)
- found = compiled.match(time.strftime(fmt))
- self.assertTrue(found, "Matching failed on '%s' using '%s' regex" %
- (time.strftime(fmt),
- compiled.pattern))
+ found = compiled.match(input_string)
+ self.assertTrue(found,
+ (f"Matching failed on '{input_string}' "
+ f"using '{compiled.pattern}' regex"))
+ for directive in ('c', 'x'):
+ # gh-124529
+ fmt = "%" + directive
+ with self.subTest(f"{fmt!r} should match input containing "
+ f"year with fewer digits than usual"):
+ try:
+ (input_string,
+ _) = _get_data_to_test_strptime_with_few_digits_year(fmt)
+ except AssertionError as exc:
+ self.fail(str(exc))
+ compiled = self.time_re.compile(fmt)
+ found = compiled.match(input_string)
+ self.assertTrue(found,
+ (f"Matching failed on '{input_string}' "
+ f"using '{compiled.pattern}' regex"))
+ for directive in ('y', 'Y', 'G'):
+ fmt = "%" + directive
+ with self.subTest(f"{fmt!r} should not match input containing "
+ f"year with fewer digits than usual"):
+ try:
+ (input_string,
+ _) = _get_data_to_test_strptime_with_few_digits_year(fmt)
+ except AssertionError as exc:
+ self.fail(str(exc))
+ compiled = self.time_re.compile(fmt)
+ found = compiled.match(input_string)
+ self.assertFalse(found,
+ (f"Matching unexpectedly succeeded "
+ f"on '{input_string}' using "
+ f"'{compiled.pattern}' regex"))
 
  def test_blankpattern(self):
  # Make sure when tuple or something has no values no regex is generated.
@@ -299,6 +330,28 @@ def helper(self, directive, position):
  (directive, strf_output, strp_output[position],
  self.time_tuple[position]))
 
+ def helper_for_directives_accepting_few_digits_year(self, directive):
+ fmt = "%" + directive
+
+ try:
+ (input_string,
+ expected_year,
+ ) = _get_data_to_test_strptime_with_few_digits_year(fmt)
+ except AssertionError as exc:
+ self.fail(str(exc))
+
+ try:
+ output_year = _strptime._strptime(input_string, fmt)[0][0]
+ except ValueError as exc:
+ # See: gh-124529
+ self.fail(f"testing of{directive!r} directive failed; "
+ f"{input_string!r} -> exception:{exc!r}")
+ else:
+ self.assertEqual(output_year, expected_year,
+ (f"testing of{directive!r} directive failed; "
+ f"{input_string!r} -> output including year "
+ f"{output_year!r} !={expected_year!r}"))
+
  def test_year(self):
  # Test that the year is handled properly
  for directive in ('y', 'Y'):
@@ -312,6 +365,17 @@ def test_year(self):
  "'y' test failed; passed in '%s' "
  "and returned '%s'" % (bound, strp_output[0]))
 
+ def test_bad_year(self):
+ for directive, bad_inputs in (
+ ('y', ('9', '100', 'ni')),
+ ('Y', ('7', '42', '999', '10000', 'SPAM')),
+ ):
+ fmt = "%" + directive
+ for input_val in bad_inputs:
+ with self.subTest(directive=directive, input_val=input_val):
+ with self.assertRaises(ValueError):
+ _strptime._strptime_time(input_val, fmt)
+
  def test_month(self):
  # Test for month directives
  for directive in ('B', 'b', 'm'):
@@ -454,11 +518,21 @@ def test_date_time(self):
  for position in range(6):
  self.helper('c', position)
 
+ def test_date_time_accepting_few_digits_year(self): # gh-124529
+ # Test %c directive with input containing year
+ # number consisting of fewer digits than usual
+ self.helper_for_directives_accepting_few_digits_year('c')
+
  def test_date(self):
  # Test %x directive
  for position in range(0,3):
  self.helper('x', position)
 
+ def test_date_accepting_few_digits_year(self): # gh-124529
+ # Test %x directive with input containing year
+ # number consisting of fewer digits than usual
+ self.helper_for_directives_accepting_few_digits_year('x')
+
  def test_time(self):
  # Test %X directive
  for position in range(3,6):
@@ -769,5 +843,59 @@ def test_TimeRE_recreation_timezone(self):
  _strptime._strptime_time(oldtzname[1], '%Z')
 
 
+def _get_data_to_test_strptime_with_few_digits_year(fmt):
+ # This helper, for the given format string (fmt), returns a 2-tuple:
+ # (<strptime input string>, <expected year>)
+ # where:
+ # * <strptime input string> -- is a `strftime(fmt)`-result-like str
+ # containing a year number which is *shorter* than the usual four
+ # or two digits, because the number is small and *not* 0-padded
+ # (namely: here the contained year number consist of one digit: 7
+ # -- that's an arbitrary choice);
+ # * <expected year> -- is an int representing the year number that
+ # is expected to be part of the result of a `strptime(<strptime
+ # input string>, fmt)` call (namely: either 7 or 2007, depending
+ # on the given format string and current locale...).
+ # Note: AssertionError (with an appropriate failure message) is
+ # raised if <strptime input string> does *not* contain the year
+ # part (for the given format string and current locale).
+
+ # 1. Prepare auxiliary sample time data (note that the magic values
+ # we use here are guaranteed to be compatible with `time.strftime()`,
+ # and are also intended to be well distinguishable within formatted
+ # strings, thanks to the fact that the amount of overloaded numbers
+ # is minimized, as in `_strptime.LocaleTime.__calc_date_time()`):
+ sample_year = 1999
+ sample_tt = (sample_year, 3, 17, 22, 44, 55, 2, 76, 0)
+ sample_string = time.strftime(fmt, sample_tt)
+ sample_year_4digits = str(sample_year)
+ sample_year_2digits = str(sample_year)[-2:]
+
+ # 2. Pick an arbitrary year number representation that
+ # is always *shorter* than the usual four or two digits:
+ year_1digit = '7'
+
+ # 3. Obtain the resultant 2-tuple:
+ if sample_year_4digits in sample_string:
+ input_string = sample_string.replace(sample_year_4digits, year_1digit)
+ if sample_year_2digits in input_string:
+ raise RuntimeError(f"the{fmt!r} format is not supported by "
+ f"this helper (a{fmt!r}-formatted string, "
+ f"{sample_string!r}, seems to include both a "
+ f"2-digit and 4-digit year number)")
+ expected_year = int(year_1digit)
+ elif sample_year_2digits in sample_string:
+ input_string = sample_string.replace(sample_year_2digits, year_1digit)
+ expected_year = 2000 + int(year_1digit)
+ else:
+ raise AssertionError(f"time.strftime({fmt!r}, ...)={sample_string!r} "
+ f"does not include the year{sample_year!r} in "
+ f"any expected format (are{fmt!r}-formatted "
+ f"strings supposed to include a year number? "
+ f"if they are, isn't there something severely "
+ f"wrong with the current locale?)")
+ return input_string, expected_year
+
+
 if __name__ == '__main__':
  unittest.main()

Lib/test/test_time.py

@@ -307,6 +307,42 @@ def test_strptime_leap_year(self):
  r'.*day of month without a year.*'):
  time.strptime('02-07 18:28', '%m-%d %H:%M')
 
+ def test_strptime_accepting_locale_specific_year_with_fewer_digits(self):
+ # GH-124529
+ concerned_formats = '%c', '%x'
+
+ def run_subtest():
+ input_str = sample_str.replace(sample_year_digits, year_digits)
+ reason = (f"test strptime accepting locale-specific "
+ f"year representation with fewer digits "
+ f"- for{fmt=} and{input_str=} ({year=})")
+ fail_msg = f"{reason} - failed"
+ expected = (year,) + sample_tt[1:6]
+ with self.subTest(reason=reason):
+ try:
+ parsed_tt = time.strptime(input_str, fmt)
+ except ValueError as exc:
+ self.fail(f"{fail_msg}; parsing error:{exc!r}")
+ self.assertEqual(parsed_tt[:6], expected, fail_msg)
+
+ sample_tt = (1999, 3, 17) + (0,) * 6
+ for fmt in concerned_formats:
+ with self.subTest(fmt=fmt):
+ sample_str = time.strftime(fmt, sample_tt)
+ if (sample_year_digits := '1999') in sample_str:
+ for year in [1, 9, 10, 99, 100, 999]:
+ year_digits = str(year)
+ run_subtest()
+ elif (sample_year_digits := '99') in sample_str:
+ for year in [2000, 2001, 2009]:
+ year_digits = str(year - 2000)
+ run_subtest()
+ else:
+ self.fail(f"time.strftime({fmt!r}, ...)={sample_str!r} "
+ f"does not include year={sample_tt[0]!r} in "
+ f"any expected format (is there something "
+ f"severely wrong with the current locale?)")
+
  def test_asctime(self):
  time.asctime(time.gmtime(self.t))
 

Misc/NEWS.d/next/Library/2024-09-30-04-49-22.gh-issue-124529.hDDlMH.rst

@@ -0,0 +1,5 @@
+Fix :meth:`datetime.datetime.strptime`, :meth:`datetime.date.strptime`
+and :func:`time.strptime` to make ``%c`` and ``%x`` accept year numbers
+with fewer digits than the usual 4 or 2 (not zero-padded). This prevents
+:exc:`ValueError` in certain cases of ``strftime/strptime`` round trips.
+Patch by Jan Kaliszewski.