Simplify API to allow granular reader operation

In response to [^1].

Previously it was hard to operate the reader classes from calling
code that separates resource management ("open" / "close") and the
actual polling of data ("read_one"). This exposes new APIs to make
it easier to use reader classes in such a granular way.

Implementation notes:

- Reinstating a type hint for __enter__ of "-> Self" will become
possible in Python 3.11.

- "start" / "stop" were discussed in relation to the Sensor being
operated by a SensorReader, but these concepts don't make much sense
for the MessageReader class, so I've used "open" / "close" instead.

[^1]: #32 (comment)

src/pms/core/reader.py

@@ -50,7 +50,7 @@ def hexdump(self, line: Optional[int] = None) -> str:
         return f"{offset:08x}: {hex}  {dump}"
 
 
-class Reader(AbstractContextManager):
+class Reader:
     @abstractmethod
     def __call__(self, *, raw: Optional[bool]) -> Iterator[Union[RawData, ObsData]]:
         """
@@ -60,6 +60,21 @@ def __call__(self, *, raw: Optional[bool]) -> Iterator[Union[RawData, ObsData]]:
         """
         ...
 
+    @abstractmethod
+    def open(self) -> None:
+        ...
+
+    @abstractmethod
+    def close(self) -> None:
+        ...
+
+    def __enter__(self):
+        self.open()
+        return self
+
+    def __exit__(self, exception_type, exception_value, traceback) -> None:
+        self.close()
+
 
 class SensorReader(Reader):
     """Read sensor messages from serial port
@@ -119,7 +134,7 @@ def _pre_heat(self):
         # only pre-heat the firs time
         self.pre_heat = 0
 
-    def __enter__(self) -> "SensorReader":
+    def open(self) -> None:
         """Open serial port and sensor setup"""
         if not self.serial.is_open:
             logger.debug(f"open {self.serial.port}")
@@ -143,9 +158,7 @@ def __enter__(self) -> "SensorReader":
             logger.error(f"Sensor is not {self.sensor.name}")
             raise UnableToRead("Sensor failed validation")
 
-        return self
-
-    def __exit__(self, exception_type, exception_value, traceback) -> None:
+    def close(self) -> None:
         """Put sensor to sleep and close serial port"""
         logger.debug(f"sleep {self.sensor}")
         buffer = self._cmd("sleep")
@@ -188,14 +201,13 @@ def __init__(self, path: Path, sensor: Sensor, samples: Optional[int] = None) ->
         self.sensor = sensor
         self.samples = samples
 
-    def __enter__(self) -> "MessageReader":
+    def open(self) -> None:
         logger.debug(f"open {self.path}")
         self.csv = self.path.open()
         reader = DictReader(self.csv)
         self.data = (row for row in reader if row["sensor"] == self.sensor.name)
-        return self
 
-    def __exit__(self, exception_type, exception_value, traceback) -> None:
+    def close(self) -> None:
         logger.debug(f"close {self.path}")
         self.csv.close()
 

tests/core/test_reader.py

@@ -12,12 +12,12 @@ def __init__(self, raise_on_enter=False):
     def __call__(self):
         raise NotImplemented
 
-    def __enter__(self):
+    def open(self):
         if self.raise_on_enter:
             raise reader.UnableToRead()
         self.entered = True
 
-    def __exit__(self, *_args):
+    def close(self):
         self.exited = True