Make FormulaEngine instances composable, instead of FormulaReceivers (#327)

This PR changes how formulas are composed -
  - receivers from formulas are no longer composable.
- formula composition is now done by composing FormulaEngine instances.
- Automatic formulas from the logical meter and *pools, are now
properties, and return `FormulaEngine` instances, which can be composed
further, or can provide a receiver to fetch values.

  ``` python
grid_power_receiver =
microgrid.logical_meter().grid_power.new_receiver()

  self._inverter_power = (
      microgrid.logical_meter().pv_power
      + microgrid.logical_meter().battery_power
  ).build("inverter_power")

  inverter_power_receiver = self._inverter_power.new_receiver()
  ```

The type hint issues in the formula engine are also resolved.

Author: sahas-subramanian-frequenz

RELEASE_NOTES.md

@@ -6,6 +6,24 @@
 
 ## Upgrading
 
++ Formulas composition has changed (#327) -
+  - receivers from formulas are no longer composable.
+  - formula composition is now done by composing FormulaEngine instances.
+  - Automatic formulas from the logical meter and *pools, are now
+    properties, and return `FormulaEngine` instances, which can be
+    composed further, or can provide a receiver to fetch values.
+
+  ``` python
+  grid_power_receiver = microgrid.logical_meter().grid_power.new_receiver()
+
+  self._inverter_power = (
+      microgrid.logical_meter().pv_power
+      + microgrid.logical_meter().battery_power
+  ).build("inverter_power")
+
+  inverter_power_receiver = self._inverter_power.new_receiver()
+  ```
+
 * Update BatteryStatus to mark battery with unknown capacity as not working (#263)
 * The channels dependency was updated to v0.14.0 (#292)
 * Some properties for `PowerDistributingActor` results were renamed to be more consistent between `Success` and `PartialFailure`:

examples/power_distribution.py

@@ -230,7 +230,7 @@ async def run() -> None:
 
     client_actor = DataCollectingActor(
         request_channel=power_dist_req_chan.new_sender(),
-        active_power_data=await logical_meter.grid_power(),
+        active_power_data=logical_meter.grid_power.new_receiver(),
     )
 
     await actor.run(service_actor, client_actor, power_distributor)

pyproject.toml

@@ -134,12 +134,6 @@ src_paths = ["src", "examples", "tests"]
 asyncio_mode = "auto"
 required_plugins = [ "pytest-asyncio", "pytest-mock" ]
 
-[tool.mypy]
-# This is a temporary hack, for details see:
-# * https://github.com/frequenz-floss/frequenz-sdk-python/pull/219
-# * https://github.com/frequenz-floss/frequenz-sdk-python/issues/226
-exclude = ["src/frequenz/sdk/timeseries/_formula_engine/"]
-
 [[tool.mypy.overrides]]
 module = [
     "grpc.aio",

src/frequenz/sdk/timeseries/_formula_engine/__init__.py

@@ -2,24 +2,13 @@
 # Copyright © 2023 Frequenz Energy-as-a-Service GmbH
 
 """A formula engine for applying formulas."""
-from ._formula_engine import (
-    FormulaEngine,
-    FormulaEngine3Phase,
-    FormulaReceiver,
-    FormulaReceiver3Phase,
-    _GenericEngine,
-    _GenericFormulaReceiver,
-)
+from ._formula_engine import FormulaEngine, FormulaEngine3Phase
 from ._formula_engine_pool import FormulaEnginePool
 from ._resampled_formula_builder import ResampledFormulaBuilder
 
 __all__ = [
     "FormulaEngine",
     "FormulaEngine3Phase",
-    "FormulaReceiver",
-    "FormulaReceiver3Phase",
     "FormulaEnginePool",
-    "_GenericEngine",
-    "_GenericFormulaReceiver",
     "ResampledFormulaBuilder",
 ]