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Block datetime from being used where date is expected with optional safe-datetime check #1

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5a9efed
Initial plan
Copilot Dec 19, 2025
126ffab
Implement unsafe subtype checking for datetime/date comparisons
Copilot Dec 19, 2025
7ff17b2
Fix docstring indentation and line continuation style
Copilot Dec 19, 2025
857f0fa
Flag date vs date comparisons as unsafe when datetime subclass exists
Copilot Dec 19, 2025
ffc3592
Add detailed documentation for supertype comparison checking
Copilot Dec 19, 2025
901cdec
Block datetime/date inheritance when unsafe-subtype is enabled
Copilot Dec 19, 2025
cb83a2a
Move errorcodes import to module level for performance
Copilot Dec 19, 2025
1b55f52
Remove date/date comparison warnings as inheritance blocking makes th…
Copilot Dec 20, 2025
2d231f5
Remove redundant comparison check code in visit_comparison_expr
Copilot Dec 20, 2025
4a0a75d
Add documentation for unsafe-subtype error code
Copilot Dec 20, 2025
2b3e1b3
Add str/Iterable[str] to unsafe-subtype checks
Copilot Dec 20, 2025
3aa9743
Add sub-codes for fine-grained control of unsafe-subtype checks
Copilot Dec 26, 2025
e16b5ab
Refactor to focus on datetime/date only with safe-datetime error code
Copilot Jan 21, 2026
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Add str/Iterable[str] to unsafe-subtype checks 
Co-authored-by: m-aciek <9288014+m-aciek@users.noreply.github.com>
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@m-aciek
Copilot and m-aciek committed Dec 20, 2025
commit 2b3e1b36cf522a860c0a55a641b9f3d631fd8513
29 changes: 29 additions & 0 deletions docs/source/error_code_list2.rst
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Expand Up @@ -709,6 +709,8 @@ If enabled with :option:`--enable-error-code unsafe-subtype <mypy --enable-error
mypy will block certain subtype relationships that are unsafe at runtime despite
being valid in Python's type system.

**datetime/date incompatibility**

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
Expand Down Expand Up @@ -751,3 +753,30 @@ preventing the runtime error.

**Note:** Equality comparisons (``==`` and ``!=``) still work between these types,
as ``__eq__`` accepts ``object`` as its parameter.

**str/Iterable[str] incompatibility**

Another common issue is using a ``str`` where ``Iterable[str]`` is expected. While
``str`` is iterable and yields strings (characters), this often leads to bugs because
developers expect iteration over a collection of strings, not individual characters.

Example:

.. code-block:: python

# mypy: enable-error-code="unsafe-subtype"
from typing import Iterable

def process_items(items: Iterable[str]) -> None:
for item in items:
print(f"Item: {item}")

# Error: Argument 1 to "process_items" has incompatible type "str"; expected "Iterable[str]"
process_items("hello") # Would iterate over: 'h', 'e', 'l', 'l', 'o'

# OK: Pass a list of strings instead
process_items(["hello"]) # Iterates over: "hello"

This prevents a common source of bugs where a single string is accidentally treated
as a collection of strings, with iteration yielding individual characters instead of
the full string.
2 changes: 2 additions & 0 deletions mypy/subtypes.py
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Expand Up @@ -91,6 +91,8 @@
# as a subtype can cause runtime errors.
UNSAFE_SUBTYPING_PAIRS: Final = [
("datetime.datetime", "datetime.date"),
("builtins.str", "typing.Iterable"),
("builtins.str", "collections.abc.Iterable"),
]

TypeParameterChecker: _TypeAlias = Callable[[Type, Type, int, bool, "SubtypeContext"], bool]
Expand Down
26 changes: 26 additions & 0 deletions test-data/unit/check-unsafe-subtype.test
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Expand Up @@ -274,3 +274,29 @@ class datetime(date):
def __le__(self, other: datetime) -> bool: ... # type: ignore[override]
def __gt__(self, other: datetime) -> bool: ... # type: ignore[override]
def __ge__(self, other: datetime) -> bool: ... # type: ignore[override]

[case testStrIterableBlocking]
# flags: --enable-error-code unsafe-subtype
from typing import Iterable
from collections.abc import Iterable as ABCIterable

def process_typing(items: Iterable[str]) -> None:
pass

def process_abc(items: ABCIterable[str]) -> None:
pass

# str to Iterable[str] - blocked
s: str = "hello"
process_typing(s) # E: Argument 1 to "process_typing" has incompatible type "str"; expected "Iterable[str]"

# str to collections.abc.Iterable[str] - blocked
process_abc(s) # E: Argument 1 to "process_abc" has incompatible type "str"; expected "Iterable[str]"

# Assignment - blocked
items: Iterable[str] = "hello" # E: Incompatible types in assignment (expression has type "str", variable has type "Iterable[str]")

# list[str] to Iterable[str] - OK
lst: list[str] = ["hello", "world"]
process_typing(lst) # OK
[builtins fixtures/list.pyi]