Add a new predicate to skip messages that are equal to the previous one

The new `ChangedOnly` predicate is is just a special case of
`OnlyIfPrevious` that skips messages that are equal to the previous one.

This is such a common use case that it deserves its own predicate.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

RELEASE_NOTES.md

@@ -13,6 +13,7 @@
 ### Experimental
 
 - A new predicate, `OnlyIfPrevious`, to `filter()` messages based on the previous message.
+- A new special case of `OnlyIfPrevious`, `ChangedOnly`, to skip messages if they are equal to the previous message.
 
 ## Bug Fixes
 

src/frequenz/channels/experimental/__init__.py

@@ -10,10 +10,11 @@
 """
 
 from ._pipe import Pipe
-from ._predicates import OnlyIfPrevious
+from ._predicates import ChangedOnly, OnlyIfPrevious
 from ._relay_sender import RelaySender
 
 __all__ = [
+    "ChangedOnly",
     "OnlyIfPrevious",
     "Pipe",
     "RelaySender",

src/frequenz/channels/experimental/_predicates.py

@@ -127,3 +127,58 @@ def __str__(self) -> str:
     def __repr__(self) -> str:
         """Return a string representation of this instance."""
         return f"<{type(self).__name__}: {self._predicate!r} first_is_true={self._first_is_true!r}>"
+
+
+class ChangedOnly(OnlyIfPrevious[object]):
+    """A predicate to check if a message is different from the previous one.
+
+    This predicate can be used to filter out messages that are the same as the previous
+    one. This can be useful in cases where you want to avoid processing duplicate
+    messages.
+
+    Warning:
+        This predicate uses the `!=` operator to compare messages, which includes all
+        the weirdnesses of Python's equality comparison (e.g., `1 == 1.0`, `True == 1`,
+        `True == 1.0`, `False == 0` are all `True`).
+
+        If you need to use a different comparison, you can create a custom predicate
+        using [`OnlyIfPrevious`][frequenz.channels.experimental.OnlyIfPrevious].
+
+    Example:
+        ```python
+        from frequenz.channels import Broadcast
+        from frequenz.channels.experimental import ChangedOnly
+
+        channel = Broadcast[int](name="skip_duplicates_test")
+        receiver = channel.new_receiver().filter(ChangedOnly())
+        sender = channel.new_sender()
+
+        # This message will be received as it is the first message.
+        await sender.send(1)
+        assert await receiver.receive() == 1
+
+        # This message will be skipped as it is the same as the previous one.
+        await sender.send(1)
+
+        # This message will be received as it is different from the previous one.
+        await sender.send(2)
+        assert await receiver.receive() == 2
+        ```
+    """
+
+    def __init__(self, *, first_is_true: bool = True) -> None:
+        """Initialize this instance.
+
+        Args:
+            first_is_true: Whether the first message should be considered as different
+                from the previous one. Defaults to `True`.
+        """
+        super().__init__(lambda old, new: old != new, first_is_true=first_is_true)
+
+    def __str__(self) -> str:
+        """Return a string representation of this instance."""
+        return f"{type(self).__name__}"
+
+    def __repr__(self) -> str:
+        """Return a string representation of this instance."""
+        return f"{type(self).__name__}(first_is_true={self._first_is_true!r})"

tests/experimental/test_predicates_changed_only.py

@@ -0,0 +1,121 @@
+# License: MIT
+# Copyright © 2024 Frequenz Energy-as-a-Service GmbH
+
+"""Tests for the ChangedOnly implementation.
+
+Most testing is done in the OnlyIfPrevious tests, these tests are limited to the
+specifics of the ChangedOnly implementation.
+"""
+
+from dataclasses import dataclass
+from unittest.mock import MagicMock
+
+import pytest
+
+from frequenz.channels.experimental import ChangedOnly, OnlyIfPrevious
+
+
+@dataclass(frozen=True, kw_only=True)
+class EqualityTestCase:
+    """Test case for testing ChangedOnly behavior with tricky equality cases."""
+
+    title: str
+    first_value: object
+    second_value: object
+    expected_second_result: bool
+
+
+EQUALITY_TEST_CASES = [
+    # Python's equality weirdness cases
+    EqualityTestCase(
+        title="Integer equals float",
+        first_value=1,
+        second_value=1.0,
+        expected_second_result=False,
+    ),
+    EqualityTestCase(
+        title="Boolean equals integer",
+        first_value=True,
+        second_value=1,
+        expected_second_result=False,
+    ),
+    EqualityTestCase(
+        title="Boolean equals float",
+        first_value=True,
+        second_value=1.0,
+        expected_second_result=False,
+    ),
+    EqualityTestCase(
+        title="False equals zero",
+        first_value=False,
+        second_value=0,
+        expected_second_result=False,
+    ),
+    EqualityTestCase(
+        title="Zero equals False",
+        first_value=0,
+        second_value=False,
+        expected_second_result=False,
+    ),
+    # Edge cases that should be different
+    EqualityTestCase(
+        title="NaN is never equal to NaN",
+        first_value=float("nan"),
+        second_value=float("nan"),
+        expected_second_result=True,
+    ),
+    EqualityTestCase(
+        title="Different list instances with same content",
+        first_value=[1],
+        second_value=[1],
+        expected_second_result=False,
+    ),
+]
+
+
+def test_changed_only_inheritance() -> None:
+    """Test that ChangedOnly is properly inheriting from OnlyIfPrevious."""
+    changed_only = ChangedOnly()
+    assert isinstance(changed_only, OnlyIfPrevious)
+
+
+def test_changed_only_predicate_implementation() -> None:
+    """Test that ChangedOnly properly implements the inequality predicate."""
+    # Create mock objects that we can control the equality comparison for
+    old = MagicMock()
+    new = MagicMock()
+
+    # Set up the inequality comparison
+    # mypy doesn't understand mocking __ne__ very well
+    old.__ne__.return_value = True  # type: ignore[attr-defined]
+
+    changed_only = ChangedOnly()
+    # Skip the first message as it's handled by first_is_true
+    changed_only(old)
+    changed_only(new)
+
+    # Verify that __ne__ was called with the correct argument
+    old.__ne__.assert_called_once_with(new)  # type: ignore[attr-defined]
+
+
+@pytest.mark.parametrize(
+    "test_case",
+    EQUALITY_TEST_CASES,
+    ids=lambda test_case: test_case.title,
+)
+def test_changed_only_equality_cases(test_case: EqualityTestCase) -> None:
+    """Test ChangedOnly behavior with Python's tricky equality cases.
+
+    Args:
+        test_case: The test case containing the input values and expected result.
+    """
+    changed_only = ChangedOnly()
+    assert changed_only(test_case.first_value) is True  # First is always True
+    assert changed_only(test_case.second_value) is test_case.expected_second_result
+
+
+def test_changed_only_representation() -> None:
+    """Test the string representation of ChangedOnly."""
+    changed_only = ChangedOnly()
+    assert str(changed_only) == "ChangedOnly"
+    assert repr(changed_only) == "ChangedOnly(first_is_true=True)"