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gh-155175: Reject fractional seconds without a decimal mark in C fromisoformat #155177

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gh-155175: Reject fractional seconds without a decimal mark in C fromisoformat #155177

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10 changes: 10 additions & 0 deletions Lib/test/datetimetester.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3762,6 +3762,11 @@ def test_fromisoformat_fails_datetime(self):
'2009-04-19T12:30:45.-05:00', # Empty fraction before offset
'2009-04-19T12:30:45.Z', # Empty fraction before Z
'2009-04-19T12:30:45,+05:00', # Empty fraction (comma) before offset
'2009-04-19T12304578', # Fraction without a decimal mark
'2009-04-19T123045789', # Fraction without a decimal mark
'2009-04-19T123045123456789', # Fraction without a decimal mark
'2009-04-19T123045+00000000', # Offset fraction without decimal mark
'2009-04-19T12:30:45+00000000', # Offset fraction without decimal mark
]

for bad_str in bad_strs:
Expand Down Expand Up @@ -5042,6 +5047,11 @@ def test_fromisoformat_fails(self):
'12:30:45.-05:00', # Empty fraction before offset
'12:30:45.Z', # Empty fraction before Z
'12:30:45,+05:00', # Empty fraction (comma) before offset
'12304578', # Fraction without a decimal mark
'123045789', # Fraction without a decimal mark
'123045123456789', # Fraction without a decimal mark
'123045+00000000', # Offset fraction without decimal mark
'12:30:45+00000000', # Offset fraction without decimal mark
]

for bad_str in bad_strs:
Expand Down
5 changes: 5 additions & 0 deletions Misc/NEWS.d/next/Library/2026-08-04-19-40-00.gh-issue-155175.Kf3xQ1.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
Fix :meth:`datetime.time.fromisoformat` and
:meth:`datetime.datetime.fromisoformat` in the C implementation accepting a
fractional seconds component that is not introduced by a decimal mark, such
as ``'12345678'`` (previously parsed as ``12:34:56.780000``). Such strings
are now rejected, matching the pure-Python implementation.
10 changes: 10 additions & 0 deletions Modules/_datetimemodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1021,6 +1021,7 @@ parse_hh_mm_ss_ff(const char *tstr, const char *tstr_end, int *hour,
int *vals[3] = {hour, minute, second};
// This is initialized to satisfy an erroneous compiler warning.
unsigned char has_separator = 1;
unsigned char has_fraction = 0;

// Parse [HH[:?MM[:?SS]]]
for (size_t i = 0; i < 3; ++i) {
Expand All @@ -1041,6 +1042,7 @@ parse_hh_mm_ss_ff(const char *tstr, const char *tstr_end, int *hour,
if (p >= p_end) {
return -3; // Decimal mark not followed by any digit
}
has_fraction = 1;
break;
}
else if (p >= p_end) {
Expand All @@ -1060,6 +1062,14 @@ parse_hh_mm_ss_ff(const char *tstr, const char *tstr_end, int *hour,
}
}

// A fractional component must be introduced by a decimal mark. Falling
// out of the loop above without having seen one means the basic format
// left unconsumed characters after SS (e.g. "12345678"), which would
// otherwise be silently parsed as a fraction.
if (!has_fraction) {
return -4; // Malformed microsecond separator
}

// Parse fractional components
size_t len_remains = p_end - p;
size_t to_parse = len_remains;
Expand Down
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