gh-115783: Deprecate support for invalid ISO formats in datetime.fromisoformat

Lib/_pydatetime.py

@@ -452,7 +452,7 @@ def _parse_hh_mm_ss_ff(tstr):
 
     return time_comps
 
-def _parse_isoformat_time(tstr):
+def _parse_isoformat_time(tstr, is_expanded=False):
     # Format supported is HH[:MM[:SS[.fff[fff]]]][+HH:MM[:SS[.ffffff]]]
     len_str = len(tstr)
     if len_str < 2:
@@ -493,6 +493,15 @@ def _parse_isoformat_time(tstr):
         if len(tzstr) in (0, 1, 3) or tstr[tz_pos-1] == 'Z':
             raise ValueError("Malformed time zone string")
 
+        if is_expanded and ":" not in tzstr and len(tzstr) > 2:
+            import warnings
+            warnings.warn(
+                "Support for partially expanded formats are deprecated in "
+                "accordance with ISO 8601:2 and will be removed in 3.15",
+                DeprecationWarning,
+                stacklevel=2,
+            )
+
         tz_comps = _parse_hh_mm_ss_ff(tzstr)
 
         if all(x == 0 for x in tz_comps):
@@ -1943,7 +1952,10 @@ def fromisoformat(cls, date_string):
 
         if tstr:
             try:
-                time_components, became_next_day, error_from_components = _parse_isoformat_time(tstr)
+                is_expanded = date_string[4] == "-"
+                time_components, became_next_day, error_from_components = (
+                    _parse_isoformat_time(tstr, is_expanded)
+                )
             except ValueError:
                 raise ValueError(
                     f'Invalid isoformat string: {date_string!r}') from None

Misc/NEWS.d/next/Library/2025-03-20-18-00-00.gh-issue-115783.befw32.rst

@@ -0,0 +1 @@
+Deprecate support for invalid ISO formats in :func:`datetime.datetime.fromisoformat`

Modules/_datetimemodule.c

@@ -1131,6 +1131,13 @@ parse_isoformat_time(const char *dtstr, size_t dtlen, int *hour, int *minute,
     int tzsign = (*tzinfo_pos == '-') ? -1 : 1;
     tzinfo_pos++;
     int tzhour = 0, tzminute = 0, tzsecond = 0;
+
+    if (tzinfo_pos + 2 < p_end && tzinfo_pos[2] != ':' && strlen(tzinfo_pos) > 2) {
+        PyErr_WarnEx(PyExc_DeprecationWarning,
+                     "Support for partially expanded formats is deprecated in "
+                     "accordance with ISO 8601:2 and will be removed in 3.15", 2);
+    }
+
     rv = parse_hh_mm_ss_ff(tzinfo_pos, p_end, &tzhour, &tzminute, &tzsecond,
                            tzmicrosecond);
 
@@ -5894,7 +5901,6 @@ datetime_fromisoformat(PyObject *cls, PyObject *dtstr)
     const Py_ssize_t separator_location = _find_isoformat_datetime_separator(
             dt_ptr, len);
 
-
     const char *p = dt_ptr;
 
     int year = 0, month = 0, day = 0;