Add Enum class that supports deprecated members (#88)

This commit introduces a new `frequenz.core.enum` module that provides a
drop-in replacement for the standard library's `enum.Enum` class, with
added support for deprecating enum members.

Author: llucax

.github/ISSUE_TEMPLATE/bug.yml

@@ -52,8 +52,9 @@ body:
         - Collections (part:collections)
         - Date and time utilities (part:datetime)
         - Documentation (part:docs)
+        - Enum utilities (part:enum)
         - IDs (part:id)
-        - Mathemathics utilities (part:math)
+        - Mathematics utilities (part:math)
         - Module utilities (part:module)
         - Logging utilities (part:logging)
         - Type hints and typing (part:typing)

.github/keylabeler.yml

@@ -16,6 +16,7 @@ labelMappings:
   "part:collections": "part:collections"
   "part:datetime": "part:datetime"
   "part:docs": "part:docs"
+  "part:enum": "part:enum"
   "part:id": "part:id"
   "part:logging": "part:logging"
   "part:math": "part:math"

.github/labeler.yml

@@ -63,6 +63,14 @@
       - "examples/**"
       - LICENSE
 
+"part:enum":
+  - changed-files:
+    - any-glob-to-any-file:
+      - "src/frequenz/core/enum.py"
+      - "src/frequenz/core/enum/**"
+      - "tests/test_enum.py"
+      - "tests/enum/**"
+
 "part:id":
   - changed-files:
     - any-glob-to-any-file:

README.md

@@ -92,6 +92,25 @@ positive = Interval(0, None)  # [0, ∞]
 assert 1000 in positive  # True
 ```
 
+### `Enum` with deprecated members
+
+Define enums with deprecated members that raise deprecation warnings when
+accessed:
+
+```python
+from frequenz.core.enum import Enum, DeprecatedMember
+
+class TaskStatus(Enum):
+   OPEN = 1
+   IN_PROGRESS = 2
+   PENDING = DeprecatedMember(1, "PENDING is deprecated, use OPEN instead")
+   DONE = DeprecatedMember(3, "DONE is deprecated, use FINISHED instead")
+   FINISHED = 4
+
+status1 = TaskStatus.PENDING  # Warns: "PENDING is deprecated, use OPEN instead"
+assert status1 is TaskStatus.OPEN
+```
+
 ### Typing Utilities
 
 Disable class constructors to enforce factory pattern usage:

RELEASE_NOTES.md

@@ -1,17 +1,21 @@
 # Frequenz Core Library Release Notes
 
-## Summary
-
-<!-- Here goes a general summary of what this release is about -->
-
-## Upgrading
+## New Features
 
-<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
+- A new `frequenz.core.enum` module was added, providing a drop-in replacement `Enum` that supports deprecating members.
 
-## New Features
+   Example:
 
-<!-- Here goes the main new features and examples or instructions on how to use them -->
+   ```python
+   from frequenz.core.enum import Enum, DeprecatedMember
 
-## Bug Fixes
+   class TaskStatus(Enum):
+       OPEN = 1
+       IN_PROGRESS = 2
+       PENDING = DeprecatedMember(1, "PENDING is deprecated, use OPEN instead")
+       DONE = DeprecatedMember(3, "DONE is deprecated, use FINISHED instead")
+       FINISHED = 4
 
-* `BaseId` will now log instead of raising a warning when a duplicate prefix is detected. This is to fix [a problem with code examples](https://github.com/frequenz-floss/frequenz-repo-config-python/issues/421) being tested using sybil and the class being imported multiple times, which caused the exception to be raised. We first tried to use `warn()` but that complicated the building process for all downstream projects, requiring them to add an exception for exactly this warning.
+   status1 = TaskStatus.PENDING  # Warns: "PENDING is deprecated, use OPEN instead"
+   assert status1 is TaskStatus.OPEN
+   ```

src/frequenz/core/datetime.py

@@ -1,7 +1,7 @@
 # License: MIT
 # Copyright © 2024 Frequenz Energy-as-a-Service GmbH
 
-"""Timeseries basic types."""
+"""Date and time utilities."""
 
 from datetime import datetime, timezone
 from typing import Final

src/frequenz/core/enum.py

@@ -0,0 +1,207 @@
+# License: MIT
+# Copyright © 2025 Frequenz Energy-as-a-Service GmbH
+
+"""Enum utilities with support for deprecated members.
+
+This module provides an [`Enum`][frequenz.core.enum.Enum] base class that extends the
+standard library's [`enum.Enum`][] to support marking certain members as deprecated.
+
+See the [class documentation][frequenz.core.enum.Enum] for details and examples.
+"""
+
+from __future__ import annotations
+
+import enum
+import warnings
+from collections.abc import Mapping
+from typing import TYPE_CHECKING, Any, ClassVar, Self, TypeVar, cast
+
+# Note: This module contains more casts and uses of Any than what's typically
+# ideal. This is because type hinting EnumType and Enum subclasses is quite
+# challenging, as there is a lot of special behavior in `mypy` for these classes.
+#
+# The resulting enum should be treated as a regular enum by mypy, so hopefully everthing
+# still works as expected.
+
+EnumT = TypeVar("EnumT", bound=enum.Enum)
+"""Type variable for enum types."""
+
+
+class DeprecatedMemberWarning(DeprecationWarning):
+    """Warning category for deprecated enum members."""
+
+
+class DeprecatedMember:
+    """Marker used in enum class bodies to declare deprecated members.
+
+    Please read the [`Enum`][frequenz.core.enum.Enum] documentation for details and
+    examples.
+    """
+
+    # Using slots is just an optimization to make the class more lightweight and avoid
+    # the creation of a `__dict__` for each instance and its corresponding lookup.
+    __slots__ = ("value", "message")
+
+    def __init__(self, value: Any, message: str) -> None:
+        """Initialize this instance."""
+        self.value = value
+        self.message = message
+
+
+class DeprecatingEnumType(enum.EnumType):
+    """Enum metaclass that supports `DeprecatedMember` wrappers.
+
+    Tip:
+        Normally it is not necessary to use this class directly, use
+        [`Enum`][frequenz.core.enum.Enum] instead.
+
+    Behavior:
+
+    - In the class body, members may be declared as `NAME = DeprecatedMember(value, msg)`.
+    - During class creation, these wrappers are replaced with `value` so that
+      a normal enum member or alias is created by [`EnumType`][enum.EnumType].
+    - The deprecated names are recorded so that:
+
+        * `MyEnum.NAME` warns (attribute access by name)
+        * `MyEnum["NAME"]` warns (lookup by name)
+        * `MyEnum(value)` warns **only if** the resolved member has **no**
+            non-deprecated aliases (all names for that member are deprecated).
+    """
+
+    def __new__(  # pylint: disable=too-many-locals
+        mcs,
+        name: str,
+        bases: tuple[type[EnumT], ...],
+        classdict: Mapping[str, Any],
+        **kw: Any,
+    ) -> type[EnumT]:
+        """Create the new enum class, rewriting `DeprecatedMember` instances."""
+        deprecated_names: dict[str, str] = {}
+        prepared = super().__prepare__(name, bases, **kw)
+
+        # Unwrap DeprecatedMembers and record them as deprecated
+        for key, value in classdict.items():
+            if isinstance(value, DeprecatedMember):
+                deprecated_names[key] = value.message
+                prepared[key] = value.value
+            else:
+                prepared[key] = value
+
+        cls = cast(type[EnumT], super().__new__(mcs, name, bases, prepared, **kw))
+
+        # Build alias groups: member -> list of names
+        member_to_names: dict[EnumT, list[str]] = {}
+        member: EnumT
+        for member_name, member in cls.__members__.items():
+            member_to_names.setdefault(member, []).append(member_name)
+
+        warned_by_member: dict[EnumT, str] = {}
+        for member, names in member_to_names.items():
+            # warn on value only if all alias names are deprecated
+            deprecated_aliases = [n for n in names if n in deprecated_names]
+            if deprecated_aliases and len(deprecated_aliases) == len(names):
+                warned_by_member[member] = deprecated_names[deprecated_aliases[0]]
+
+        # Inject maps quietly
+        type.__setattr__(cls, "__deprecated_names__", deprecated_names)
+        type.__setattr__(cls, "__deprecated_value_map__", warned_by_member)
+
+        return cls
+
+    @staticmethod
+    def _name_map(cls_: type[Any]) -> Mapping[str, str]:
+        """Map from member names to deprecation messages."""
+        return cast(
+            Mapping[str, str],
+            type.__getattribute__(cls_, "__dict__").get("__deprecated_names__", {}),
+        )
+
+    @staticmethod
+    def _value_map(cls_: type[Any]) -> Mapping[Any, str]:
+        """Map from enum members to deprecation messages."""
+        return cast(
+            Mapping[Any, str],
+            type.__getattribute__(cls_, "__dict__").get("__deprecated_value_map__", {}),
+        )
+
+    def __getattribute__(cls, name: str) -> Any:
+        """Resolve `name` to a member, warning if the member is deprecated."""
+        if name in ("__deprecated_names__", "__deprecated_value_map__"):
+            return type.__getattribute__(cls, name)
+        deprecated = DeprecatingEnumType._name_map(cls)
+        if name in deprecated:
+            warnings.warn(deprecated[name], DeprecatedMemberWarning, stacklevel=2)
+        return super().__getattribute__(name)
+
+    def __getitem__(cls, name: str) -> Any:
+        """Resolve `name` to a member, warning if the member is deprecated."""
+        deprecated = DeprecatingEnumType._name_map(cls)
+        if name in deprecated:
+            warnings.warn(deprecated[name], DeprecatedMemberWarning, stacklevel=2)
+        return super().__getitem__(name)
+
+    def __call__(cls, value: Any, *args: Any, **kwargs: Any) -> Any:
+        """Resolve `value` to a member, warning if the member is purely deprecated."""
+        member = super().__call__(value, *args, **kwargs)
+        value_map: Mapping[Any, str] = DeprecatingEnumType._value_map(cls)
+        msg = value_map.get(member)
+        if msg is not None:
+            warnings.warn(msg, DeprecatedMemberWarning, stacklevel=2)
+        return member
+
+
+if TYPE_CHECKING:
+    # Make type checkers treat it as a plain Enum (so member checks work), if we don't
+    # do this, mypy will consider the resulting enum completely dynamic and never
+    # complain if an unexisting member is accessed.
+
+    # pylint: disable-next=missing-class-docstring
+    class Enum(enum.Enum):  # noqa
+        __deprecated_names__: ClassVar[Mapping[str, str]]
+        __deprecated_value_map__: ClassVar[Mapping[Enum, str]]
+
+else:
+
+    class Enum(enum.Enum, metaclass=DeprecatingEnumType):
+        """Base class for enums that support DeprecatedMember.
+
+        This class extends the standard library's [`enum.Enum`][] to support marking
+        certain members as deprecated. Deprecated members can be accessed, but doing so
+        will emit a [`DeprecationWarning`][], specifically
+        a [`DeprecatedMemberWarning`][frequenz.core.enum.DeprecatedMemberWarning].
+
+        To declare a deprecated member, use the
+        [`DeprecatedMember`][frequenz.core.enum.DeprecatedMember] wrapper in the class body.
+
+        When using the enum constructor (i.e. `MyEnum(value)`), a warning is only emitted if
+        the resolved member has no non-deprecated aliases. If there is at least one
+        non-deprecated alias for the member, no warning is emitted.
+
+        Example:
+            ```python
+            from frequenz.core.enum import Enum, DeprecatedMember
+
+            class TaskStatus(Enum):
+                OPEN = 1
+                IN_PROGRESS = 2
+                PENDING = DeprecatedMember(1, "PENDING is deprecated, use OPEN instead")
+                DONE = DeprecatedMember(3, "DONE is deprecated, use FINISHED instead")
+                FINISHED = 4
+
+            # Accessing deprecated members:
+            status1 = TaskStatus.PENDING  # Warns: "PENDING is deprecated, use OPEN instead"
+            assert status1 is TaskStatus.OPEN
+
+            status2 = TaskStatus["DONE"]  # Warns: "DONE is deprecated, use FINISHED instead"
+            assert status2 is TaskStatus.FINISHED
+
+            status3 = TaskStatus(1)  # No warning, resolves to OPEN which has a non-deprecated alias
+            assert status3 is TaskStatus.OPEN
+
+            status4 = TaskStatus(3)  # Warns: "DONE is deprecated, use FINISHED instead"
+            assert status4 is TaskStatus.FINISHED
+            ```
+        """
+
+        __deprecated_names__: ClassVar[Mapping[str, str]]
+        __deprecated_value_map__: ClassVar[Mapping[Self, str]]

tests/test_enum.py

@@ -0,0 +1,84 @@
+# License: MIT
+# Copyright © 2025 Frequenz Energy-as-a-Service GmbH
+
+"""Tests for the `frequenz.core.enum` module."""
+
+import pytest
+
+from frequenz.core.enum import (
+    DeprecatedMember,
+    DeprecatedMemberWarning,
+    Enum,
+)
+
+
+class _TestEnum(Enum):
+    """A test enum with some deprecated members."""
+
+    OPEN = 1
+    IN_PROGRESS = 2
+    PENDING = DeprecatedMember(1, "Use OPEN instead")
+    DONE = DeprecatedMember(3, "Use FINISHED instead")
+    FINISHED = 4
+
+
+def _assert_deprecated_member(
+    recorder: pytest.WarningsRecorder, expected_msg: str
+) -> None:
+    """Assert that a single deprecation warning was recorded with the expected message."""
+    assert len(recorder.list) == 1
+    warning = recorder.pop().message
+    assert str(warning) == expected_msg
+    assert isinstance(warning, DeprecatedMemberWarning)
+
+
+def test_mypy_detects_deprecated_members() -> None:
+    """Test that mypy detects missing members as expected.
+
+    If mypy wouldn't detect this, it should complain about an unused type: ignore.
+    """
+    with pytest.raises(AttributeError):
+        _ = _TestEnum.I_DONT_EXIST  # type: ignore[attr-defined]
+
+
+def test_attribute_access_warns() -> None:
+    """Test accessing deprecated members as attributes triggers a deprecation warning."""
+    with pytest.deprecated_call() as recorder:
+        _ = _TestEnum.PENDING
+    _assert_deprecated_member(recorder, "Use OPEN instead")
+
+    with pytest.deprecated_call() as recorder:
+        _ = _TestEnum.DONE
+    _assert_deprecated_member(recorder, "Use FINISHED instead")
+
+
+def test_name_lookup_warns() -> None:
+    """Test accessing deprecated members by name triggers a deprecation warning."""
+    with pytest.deprecated_call() as recorder:
+        _ = _TestEnum["PENDING"]
+    _assert_deprecated_member(recorder, "Use OPEN instead")
+
+    with pytest.deprecated_call() as recorder:
+        _ = _TestEnum["DONE"]
+    _assert_deprecated_member(recorder, "Use FINISHED instead")
+
+
+def test_value_lookup_behavior_non_deprecated_alias() -> None:
+    """Test accessing members by value triggers no warnings when a non-deprecated alias exists."""
+    member = _TestEnum(1)
+    assert member is _TestEnum.OPEN
+
+
+def test_value_lookup_behavior_purely_deprecated() -> None:
+    """Test accessing members by value triggers warnings when there is no non-deprecated alias."""
+    with pytest.deprecated_call() as recorder:
+        member = _TestEnum(3)
+    _assert_deprecated_member(recorder, "Use FINISHED instead")
+    with pytest.deprecated_call():  # Avoid pytest showing the deprecation in the output
+        assert member is _TestEnum.DONE
+
+
+def test_members_integrity() -> None:
+    """Test that all enum members are present in __members__."""
+    names = list(_TestEnum.__members__.keys())
+    assert {"OPEN", "IN_PROGRESS", "PENDING", "DONE", "FINISHED"} <= set(names)