Implement Mapping for GroupingLatestValueCache

Signed-off-by: Sahas Subramanian <[email protected]>

Author: shsms

src/frequenz/channels/experimental/_grouping_latest_value_cache.py

@@ -10,12 +10,12 @@
 takes a [Receiver][frequenz.channels.Receiver] and a `key` function as arguments and
 stores the latest value received by that receiver for each key separately.
 
-As soon as a value is received for a `key`, the
-[`has_value`][frequenz.channels.experimental.GroupingLatestValueCache.has_value] method
-returns `True` for that `key`, and the [`get`][frequenz.channels.LatestValueCache.get]
-method for that `key` returns the latest value received.  The `get` method will raise an
-exception if called before any messages have been received from the receiver for a given
-`key`.
+The `GroupingLatestValueCache` implements the [`Mapping`][collections.abc.Mapping]
+interface, so it can be used like a dictionary.  In addition, it provides a
+[has_value][frequenz.channels.experimental.GroupingLatestValueCache.has_value] method to
+check if a value has been received for a specific key, and a
+[clear][frequenz.channels.experimental.GroupingLatestValueCache.clear] method to clear
+the cached value for a specific key.
 
 Example:
 ```python
@@ -39,20 +39,21 @@
 
 import asyncio
 import typing
-from collections.abc import Set
+from collections.abc import Iterator, KeysView, Mapping
+
+from typing_extensions import override
 
 from .._receiver import Receiver
 
 T_co = typing.TypeVar("T_co", covariant=True)
+T = typing.TypeVar("T")
 HashableT = typing.TypeVar("HashableT", bound=typing.Hashable)
 
 
-class GroupingLatestValueCache(typing.Generic[T_co, HashableT]):
-    """A cache that stores the latest value in a receiver.
-
-    It provides a way to look up the latest value in a stream without any delay,
-    as long as there has been one value received.
-    """
+class GroupingLatestValueCache(
+    typing.Generic[T_co, HashableT], Mapping[HashableT, T_co]
+):
+    """A cache that stores the latest value in a receiver, grouped by key."""
 
     def __init__(
         self,
@@ -84,32 +85,65 @@ def unique_id(self) -> str:
         """The unique identifier of this instance."""
         return self._unique_id
 
-    def keys(self) -> Set[HashableT]:
+    @override
+    def keys(self) -> KeysView[HashableT]:
         """Return the set of keys for which values have been received.
 
         If no key function is provided, this will return an empty set.
         """
         return self._latest_value_by_key.keys()
 
-    def get(self, key: HashableT) -> T_co:
-        """Return the latest value that has been received.
+    @typing.overload
+    def get(self, key: HashableT, default: None = None) -> T_co | None:
+        """Return the latest value that has been received for a specific key."""
 
-        This raises a `ValueError` if no value has been received yet. Use `has_value` to
-        check whether a value has been received yet, before trying to access the value,
-        to avoid the exception.
+    # MyPy passes this overload as a valid signature, but pylint does not like it.
+    @typing.overload
+    def get(  # pylint: disable=signature-differs
+        self, key: HashableT, default: T
+    ) -> T_co | T:
+        """Return the latest value that has been received for a specific key."""
+
+    @override
+    def get(self, key: HashableT, default: T | None = None) -> T_co | T | None:
+        """Return the latest value that has been received.
 
         Args:
             key: An optional key to retrieve the latest value for that key. If not
                 provided, it retrieves the latest value received overall.
+            default: The default value to return if no value has been received yet for
+                the specified key. If not provided, it defaults to `None`.
 
         Returns:
             The latest value that has been received.
+        """
+        return self._latest_value_by_key.get(key, default)
+
+    @override
+    def __iter__(self) -> Iterator[HashableT]:
+        """Return an iterator over the keys for which values have been received."""
+        return iter(self._latest_value_by_key)
+
+    @override
+    def __len__(self) -> int:
+        """Return the number of keys for which values have been received."""
+        return len(self._latest_value_by_key)
+
+    @override
+    def __getitem__(self, key: HashableT) -> T_co:
+        """Return the latest value that has been received for a specific key.
+
+        Args:
+            key: The key to retrieve the latest value for.
+
+        Returns:
+            The latest value that has been received for that key.
 
         Raises:
-            ValueError: If no value has been received yet.
+            KeyError: If no value has been received yet for that key.
         """
         if key not in self._latest_value_by_key:
-            raise ValueError(f"No value received for key: {key!r}")
+            raise KeyError(f"No value received for key: {key!r}")
         return self._latest_value_by_key[key]
 
     def has_value(self, key: HashableT) -> bool:

tests/test_grouping_latest_value_cache_integration.py

@@ -22,8 +22,7 @@ async def test_latest_value_cache_key() -> None:
     sender = channel.new_sender()
 
     assert not cache.has_value(5)
-    with pytest.raises(ValueError, match="No value received for key: 0"):
-        cache.get(0)
+    assert cache.get(0) is None
 
     assert cache.keys() == set()
 
@@ -41,8 +40,7 @@ async def test_latest_value_cache_key() -> None:
 
     assert cache.keys() == {5, 6}
 
-    with pytest.raises(ValueError, match="No value received for key: 7"):
-        cache.get(7)
+    assert cache.get(7, default=(7, "default")) == (7, "default")
 
     await sender.send((12, "d"))
     await asyncio.sleep(0)
@@ -65,6 +63,5 @@ async def test_latest_value_cache_key() -> None:
     assert not cache.has_value(5)
     assert cache.has_value(6)
 
-    with pytest.raises(ValueError, match="No value received for key: 5"):
-        assert cache.get(5)
+    assert cache.get(5) is None
     assert cache.keys() == {6, 12}