Refactor to focus on datetime/date only with safe-datetime error code

Co-authored-by: m-aciek <9288014+m-aciek@users.noreply.github.com>

Author: Copilot

docs/source/error_code_list2.rst

@@ -700,26 +700,22 @@ Example:
         def add_forty_two(value: int) -> int:
             return value + 42
 
-.. _code-unsafe-subtype:
+.. _code-safe-datetime:
 
-Check for unsafe subtype relationships [unsafe-subtype]
---------------------------------------------------------
+Disallow datetime where date is expected [safe-datetime]
+---------------------------------------------------------
 
-If enabled with :option:`--enable-error-code unsafe-subtype <mypy --enable-error-code>`,
-mypy will block certain subtype relationships that are unsafe at runtime despite
-being valid in Python's type system.
-
-The primary use case is blocking the ``datetime.datetime`` to ``datetime.date``
-inheritance relationship. While ``datetime`` is a subclass of ``date`` at runtime,
-comparing a ``datetime`` with a ``date`` raises a ``TypeError``. When this error
-code is enabled, mypy will prevent ``datetime`` objects from being used where
-``date`` is expected, catching these errors at type-check time.
+If enabled with :option:`--enable-error-code safe-datetime <mypy --enable-error-code>`,
+mypy will prevent ``datetime.datetime`` objects from being used where ``datetime.date``
+is expected. While ``datetime`` is a subclass of ``date`` at runtime, comparing a
+``datetime`` with a ``date`` raises a ``TypeError``. This error code catches these
+errors at type-check time.
 
 Example:
 
 .. code-block:: python
 
-    # mypy: enable-error-code="unsafe-subtype"
+    # mypy: enable-error-code="safe-datetime"
     from datetime import date, datetime
 
     # Error: Incompatible types in assignment (expression has type "datetime", variable has type "date")
@@ -746,7 +742,7 @@ runtime:
     if dt < d:
         print("never reached")
 
-When ``unsafe-subtype`` is enabled, assignment and parameter passing are blocked,
+When ``safe-datetime`` is enabled, assignment and parameter passing are blocked,
 preventing the runtime error.
 
 **Note:** Equality comparisons (``==`` and ``!=``) still work between these types,

mypy/errorcodes.py

@@ -172,9 +172,9 @@ def __hash__(self) -> int:
 COMPARISON_OVERLAP: Final = ErrorCode(
     "comparison-overlap", "Check that types in comparisons and 'in' expressions overlap", "General"
 )
-UNSAFE_SUBTYPE: Final = ErrorCode(
-    "unsafe-subtype",
-    "Warn about unsafe subtyping relationships that may cause runtime errors",
+SAFE_DATETIME: Final = ErrorCode(
+    "safe-datetime",
+    "Disallow datetime where date is expected",
     "General",
     default_enabled=False,
 )

mypy/subtypes.py

@@ -85,11 +85,10 @@
 IS_VAR: Final = 4
 IS_EXPLICIT_SETTER: Final = 5
 
-# Known unsafe subtyping relationships that should trigger warnings.
-# Each tuple is (subclass_fullname, superclass_fullname).
-# These are cases where a class is a subclass at runtime but treating it
-# as a subtype can cause runtime errors.
-UNSAFE_SUBTYPING_PAIRS: Final = [("datetime.datetime", "datetime.date")]
+# Disallow datetime.datetime where datetime.date is expected when safe-datetime is enabled.
+# While datetime is a subclass of date at runtime, comparing them raises TypeError,
+# making this inheritance relationship problematic in practice.
+DATETIME_DATE_FULLNAMES: Final = ("datetime.datetime", "datetime.date")
 
 TypeParameterChecker: _TypeAlias = Callable[[Type, Type, int, bool, "SubtypeContext"], bool]
 
@@ -537,12 +536,12 @@ def visit_instance(self, left: Instance) -> bool:
             rname = right.type.fullname
             lname = left.type.fullname
 
-            # Check if this is an unsafe subtype relationship that should be blocked
-            if (
-                self.options
-                and codes.UNSAFE_SUBTYPE in self.options.enabled_error_codes
-                and (lname, rname) in UNSAFE_SUBTYPING_PAIRS
-            ):
+            # Check if this is the datetime/date relationship that should be blocked
+            # when safe-datetime is enabled
+            if (self.options
+                    and codes.SAFE_DATETIME in self.options.enabled_error_codes
+                    and lname == DATETIME_DATE_FULLNAMES[0]
+                    and rname == DATETIME_DATE_FULLNAMES[1]):
                 return False
 
             # Always try a nominal check if possible,

test-data/unit/check-unsafe-subtype.test test-data/unit/check-safe-datetime.testtest-data/unit/check-unsafe-subtype.test renamed to test-data/unit/check-safe-datetime.test

@@ -1,5 +1,5 @@
 [case testDatetimeVsDateComparison]
-# flags: --enable-error-code unsafe-subtype
+# flags: --enable-error-code safe-datetime
 from datetime import date, datetime
 
 dt: datetime
@@ -79,7 +79,7 @@ class datetime(date):
     def __ge__(self, other: datetime) -> bool: ...  # type: ignore[override]
 
 [case testDatetimeVsDateComparisonExplicitTypes]
-# flags: --enable-error-code unsafe-subtype
+# flags: --enable-error-code safe-datetime
 from datetime import date, datetime
 
 def compare_datetime_date(dt: datetime, d: date) -> bool:
@@ -107,7 +107,7 @@ class datetime(date):
     def __ge__(self, other: datetime) -> bool: ...  # type: ignore[override]
 
 [case testDatetimeComparisonOK]
-# flags: --enable-error-code unsafe-subtype
+# flags: --enable-error-code safe-datetime
 from datetime import date, datetime
 
 dt1: datetime
@@ -145,7 +145,7 @@ class datetime(date):
     def __ge__(self, other: datetime) -> bool: ...  # type: ignore[override]
 
 [case testDatetimeVsDateComparisonWithNow]
-# flags: --enable-error-code unsafe-subtype
+# flags: --enable-error-code safe-datetime
 from datetime import date, datetime
 
 if datetime.now() < date.today():  # E: Unsupported operand types for < ("datetime" and "date")
@@ -173,7 +173,7 @@ class datetime(date):
     def __ge__(self, other: datetime) -> bool: ...  # type: ignore[override]
 
 [case testDatetimeVsDateComparisonInExpression]
-# flags: --enable-error-code unsafe-subtype
+# flags: --enable-error-code safe-datetime
 from datetime import date, datetime
 
 dt: datetime
@@ -203,7 +203,7 @@ class datetime(date):
     def __ge__(self, other: datetime) -> bool: ...  # type: ignore[override]
 
 [case testDateVsDateMixedWithDatetime]
-# flags: --enable-error-code unsafe-subtype
+# flags: --enable-error-code safe-datetime
 from datetime import date, datetime
 
 def compare_dates(d1: date, d2: date) -> bool:
@@ -238,7 +238,7 @@ class datetime(date):
     def __ge__(self, other: datetime) -> bool: ...  # type: ignore[override]
 
 [case testInheritanceBlocking]
-# flags: --enable-error-code unsafe-subtype
+# flags: --enable-error-code safe-datetime
 from datetime import date, datetime
 
 # Assignment should be blocked