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

Doc/library/datetime.rst

@@ -1076,13 +1076,12 @@ Other constructors, all class methods:
    ISO 8601 format, with the following exceptions:
 
    1. Time zone offsets may have fractional seconds.
-   2. The ``T`` separator may be replaced by any single unicode character.
-   3. Fractional hours and minutes are not supported.
-   4. Reduced precision dates are not currently supported (``YYYY-MM``,
+   2. Fractional hours and minutes are not supported.
+   3. Reduced precision dates are not currently supported (``YYYY-MM``,
       ``YYYY``).
-   5. Extended date representations are not currently supported
+   4. Extended date representations are not currently supported
       (``±YYYYYY-MM-DD``).
-   6. Ordinal dates are not currently supported (``YYYY-OOO``).
+   5. Ordinal dates are not currently supported (``YYYY-OOO``).
 
    Examples::
 
@@ -1111,6 +1110,8 @@ Other constructors, all class methods:
    .. versionchanged:: 3.11
       Previously, this method only supported formats that could be emitted by
       :meth:`date.isoformat` or :meth:`datetime.isoformat`.
+   .. versionchanged:: next
+      Separators other than ``T`` are deprecated.
 
 
 .. classmethod:: datetime.fromisocalendar(year, week, day)

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 +493,14 @@ 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
+            )
+
         tz_comps = _parse_hh_mm_ss_ff(tzstr)
 
         if all(x == 0 for x in tz_comps):
@@ -1933,6 +1941,11 @@ def fromisoformat(cls, date_string):
         # Split this at the separator
         try:
             separator_location = _find_isoformat_datetime_separator(date_string)
+            if separator_location != len(date_string) and date_string[separator_location] != 'T':
+                import warnings
+                warnings.warn("Support of date/time separators other than "
+                              "[\"T\"] is deprecated in accordance with ISO "
+                              "8601:2 and will be removed in 3.15", DeprecationWarning)
             dstr = date_string[0:separator_location]
             tstr = date_string[(separator_location+1):]
 
@@ -1943,7 +1956,10 @@ 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

@@ -1130,6 +1130,14 @@ 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",
+                     1);
+    }
+
     rv = parse_hh_mm_ss_ff(tzinfo_pos, p_end, &tzhour, &tzminute, &tzsecond,
                            tzmicrosecond);
 
@@ -5893,6 +5901,11 @@ datetime_fromisoformat(PyObject *cls, PyObject *dtstr)
     const Py_ssize_t separator_location = _find_isoformat_datetime_separator(
             dt_ptr, len);
 
+    if (separator_location != -1 && dt_ptr[separator_location] != 'T') {
+        PyErr_WarnEx(PyExc_DeprecationWarning,
+            "Support of date/time separators other than [\"T\"] is deprecated in "
+            "accordance with ISO 8601:2 and will be removed in 3.15", 1);
+    }
 
     const char *p = dt_ptr;