diff options
| -rw-r--r-- | Doc/library/datetime.rst | 37 | ||||
| -rw-r--r-- | Lib/_strptime.py | 21 | ||||
| -rw-r--r-- | Lib/test/datetimetester.py | 13 | ||||
| -rw-r--r-- | Lib/test/test_time.py | 8 | ||||
| -rw-r--r-- | Lib/test/test_unittest/test_assertions.py | 10 | ||||
| -rw-r--r-- | Lib/unittest/case.py | 22 | ||||
| -rw-r--r-- | Misc/NEWS.d/next/Library/2024-03-20-16-10-29.gh-issue-70647.FpD6Ar.rst | 7 |
7 files changed, 117 insertions, 1 deletions
diff --git a/Doc/library/datetime.rst b/Doc/library/datetime.rst index 1905c9e..047427d 100644 --- a/Doc/library/datetime.rst +++ b/Doc/library/datetime.rst @@ -1079,6 +1079,24 @@ Other constructors, all class methods: time tuple. See also :ref:`strftime-strptime-behavior` and :meth:`datetime.fromisoformat`. + .. versionchanged:: 3.13 + + If *format* specifies a day of month without a year a + :exc:`DeprecationWarning` is now emitted. This is to avoid a quadrennial + leap year bug in code seeking to parse only a month and day as the + default year used in absence of one in the format is not a leap year. + Such *format* values may raise an error as of Python 3.15. The + workaround is to always include a year in your *format*. If parsing + *date_string* values that do not have a year, explicitly add a year that + is a leap year before parsing: + + .. doctest:: + + >>> from datetime import datetime + >>> date_string = "02/29" + >>> when = datetime.strptime(f"{date_string};1984", "%m/%d;%Y") # Avoids leap year bug. + >>> when.strftime("%B %d") # doctest: +SKIP + 'February 29' Class attributes: @@ -2657,6 +2675,25 @@ Notes: for formats ``%d``, ``%m``, ``%H``, ``%I``, ``%M``, ``%S``, ``%j``, ``%U``, ``%W``, and ``%V``. Format ``%y`` does require a leading zero. +(10) + When parsing a month and day using :meth:`~.datetime.strptime`, always + include a year in the format. If the value you need to parse lacks a year, + append an explicit dummy leap year. Otherwise your code will raise an + exception when it encounters leap day because the default year used by the + parser is not a leap year. Users run into this bug every four years... + + .. doctest:: + + >>> month_day = "02/29" + >>> datetime.strptime(f"{month_day};1984", "%m/%d;%Y") # No leap year bug. + datetime.datetime(1984, 2, 29, 0, 0) + + .. deprecated-removed:: 3.13 3.15 + :meth:`~.datetime.strptime` calls using a format string containing + a day of month without a year now emit a + :exc:`DeprecationWarning`. In 3.15 or later we may change this into + an error or change the default year to a leap year. See :gh:`70647`. + .. rubric:: Footnotes .. [#] If, that is, we ignore the effects of Relativity diff --git a/Lib/_strptime.py b/Lib/_strptime.py index 798cf9f..e42af75 100644 --- a/Lib/_strptime.py +++ b/Lib/_strptime.py @@ -10,6 +10,7 @@ FUNCTIONS: strptime -- Calculates the time struct represented by the passed-in string """ +import os import time import locale import calendar @@ -250,12 +251,30 @@ class TimeRE(dict): format = regex_chars.sub(r"\\\1", format) whitespace_replacement = re_compile(r'\s+') format = whitespace_replacement.sub(r'\\s+', format) + year_in_format = False + day_of_month_in_format = False while '%' in format: directive_index = format.index('%')+1 + format_char = format[directive_index] processed_format = "%s%s%s" % (processed_format, format[:directive_index-1], - self[format[directive_index]]) + self[format_char]) format = format[directive_index+1:] + match format_char: + case 'Y' | 'y' | 'G': + year_in_format = True + case 'd': + day_of_month_in_format = True + if day_of_month_in_format and not year_in_format: + import warnings + warnings.warn("""\ +Parsing dates involving a day of month without a year specified is ambiguious +and fails to parse leap day. The default behavior will change in Python 3.15 +to either always raise an exception or to use a different default year (TBD). +To avoid trouble, add a specific year to the input & format. +See https://github.com/python/cpython/issues/70647.""", + DeprecationWarning, + skip_file_prefixes=(os.path.dirname(__file__),)) return "%s%s" % (processed_format, format) def compile(self, format): diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py index 31fc383..c772639 100644 --- a/Lib/test/datetimetester.py +++ b/Lib/test/datetimetester.py @@ -2793,6 +2793,19 @@ class TestDateTime(TestDate): newdate = strptime(string, format) self.assertEqual(newdate, target, msg=reason) + def test_strptime_leap_year(self): + # GH-70647: warns if parsing a format with a day and no year. + with self.assertRaises(ValueError): + # The existing behavior that GH-70647 seeks to change. + self.theclass.strptime('02-29', '%m-%d') + with self.assertWarnsRegex(DeprecationWarning, + r'.*day of month without a year.*'): + self.theclass.strptime('03-14.159265', '%m-%d.%f') + with self._assertNotWarns(DeprecationWarning): + self.theclass.strptime('20-03-14.159265', '%y-%m-%d.%f') + with self._assertNotWarns(DeprecationWarning): + self.theclass.strptime('02-29,2024', '%m-%d,%Y') + def test_more_timetuple(self): # This tests fields beyond those tested by the TestDate.test_timetuple. t = self.theclass(2004, 12, 31, 6, 22, 33) diff --git a/Lib/test/test_time.py b/Lib/test/test_time.py index fb234b7..293799f 100644 --- a/Lib/test/test_time.py +++ b/Lib/test/test_time.py @@ -277,6 +277,8 @@ class TimeTestCase(unittest.TestCase): 'j', 'm', 'M', 'p', 'S', 'U', 'w', 'W', 'x', 'X', 'y', 'Y', 'Z', '%'): format = '%' + directive + if directive == 'd': + format += ',%Y' # Avoid GH-70647. strf_output = time.strftime(format, tt) try: time.strptime(strf_output, format) @@ -299,6 +301,12 @@ class TimeTestCase(unittest.TestCase): time.strptime('19', '%Y %') self.assertIs(e.exception.__suppress_context__, True) + def test_strptime_leap_year(self): + # GH-70647: warns if parsing a format with a day and no year. + with self.assertWarnsRegex(DeprecationWarning, + r'.*day of month without a year.*'): + time.strptime('02-07 18:28', '%m-%d %H:%M') + def test_asctime(self): time.asctime(time.gmtime(self.t)) diff --git a/Lib/test/test_unittest/test_assertions.py b/Lib/test/test_unittest/test_assertions.py index 5c1a28e..1dec947 100644 --- a/Lib/test/test_unittest/test_assertions.py +++ b/Lib/test/test_unittest/test_assertions.py @@ -386,6 +386,16 @@ class TestLongMessage(unittest.TestCase): '^UserWarning not triggered$', '^UserWarning not triggered : oops$']) + def test_assertNotWarns(self): + def warn_future(): + warnings.warn('xyz', FutureWarning, stacklevel=2) + self.assertMessagesCM('_assertNotWarns', (FutureWarning,), + warn_future, + ['^FutureWarning triggered$', + '^oops$', + '^FutureWarning triggered$', + '^FutureWarning triggered : oops$']) + def testAssertWarnsRegex(self): # test error not raised self.assertMessagesCM('assertWarnsRegex', (UserWarning, 'unused regex'), diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py index 001b640..36daa61 100644 --- a/Lib/unittest/case.py +++ b/Lib/unittest/case.py @@ -332,6 +332,23 @@ class _AssertWarnsContext(_AssertRaisesBaseContext): self._raiseFailure("{} not triggered".format(exc_name)) +class _AssertNotWarnsContext(_AssertWarnsContext): + + def __exit__(self, exc_type, exc_value, tb): + self.warnings_manager.__exit__(exc_type, exc_value, tb) + if exc_type is not None: + # let unexpected exceptions pass through + return + try: + exc_name = self.expected.__name__ + except AttributeError: + exc_name = str(self.expected) + for m in self.warnings: + w = m.message + if isinstance(w, self.expected): + self._raiseFailure(f"{exc_name} triggered") + + class _OrderedChainMap(collections.ChainMap): def __iter__(self): seen = set() @@ -811,6 +828,11 @@ class TestCase(object): context = _AssertWarnsContext(expected_warning, self) return context.handle('assertWarns', args, kwargs) + def _assertNotWarns(self, expected_warning, *args, **kwargs): + """The opposite of assertWarns. Private due to low demand.""" + context = _AssertNotWarnsContext(expected_warning, self) + return context.handle('_assertNotWarns', args, kwargs) + def assertLogs(self, logger=None, level=None): """Fail unless a log message of level *level* or higher is emitted on *logger_name* or its children. If omitted, *level* defaults to diff --git a/Misc/NEWS.d/next/Library/2024-03-20-16-10-29.gh-issue-70647.FpD6Ar.rst b/Misc/NEWS.d/next/Library/2024-03-20-16-10-29.gh-issue-70647.FpD6Ar.rst new file mode 100644 index 0000000..a9094df --- /dev/null +++ b/Misc/NEWS.d/next/Library/2024-03-20-16-10-29.gh-issue-70647.FpD6Ar.rst @@ -0,0 +1,7 @@ +Start the deprecation period for the current behavior of +:func:`datetime.datetime.strptime` and :func:`time.strptime` which always +fails to parse a date string with a :exc:`ValueError` involving a day of +month such as ``strptime("02-29", "%m-%d")`` when a year is **not** +specified and the date happen to be February 29th. This should help avoid +users finding new bugs every four years due to a natural mistaken assumption +about the API when parsing partial date values. |