Add a test using dynamic subclasses and bump min python to 3.9

Python 3.8 doesn't have the dict union operator | and you can't make union types using Union[type1, type2] so I bumped the minimum python version to 3.9

pyproject.toml

@@ -8,7 +8,7 @@ license = "Apache-2.0"
 repository = "https://github.com/ecmwf/conflator/"
 
 [tool.poetry.dependencies]
-python = ">3.8"
+python = ">3.9"
 pydantic = "^2.5.3"
 argparse = "^1.4.0"
 rich-argparse = "^1.4.0"

tests/test_subclasses.py

@@ -0,0 +1,99 @@
+from unittest.mock import patch
+import pytest
+from conflator import CLIArg, Conflator, EnvVar, ConfigModel
+from annotated_types import Annotated
+from typing import Literal, Union
+from pydantic import Field, ConfigDict
+import yaml
+
+from dataclasses import dataclass, field
+
+class Action(ConfigModel):
+    model_config = ConfigDict(extra='forbid')
+    name: str
+
+class Source(Action):
+    "Produces messages"
+    name: Literal["Source"]
+    start_from: str
+
+class Process(Action):
+    "Processes messages"
+    name: Literal["Process"]
+    should_flub_widgets: bool
+
+class Sink(Action):
+    "Consumes messages"
+    name: Literal["Sink"]
+    emit_aviso_notification: bool
+
+@dataclass
+class Subclasses:
+    """Get all the subclasses for given class
+    returns {name : class}
+    deduplicated by name
+    cached
+    """
+
+    cache: dict = field(default_factory=dict)
+
+    def get(self, target):
+        try:
+            return self.cache[target.__name__]
+        except KeyError:
+            # Deduplicate classes by __name__
+            deduped = list({subcls.__name__: subcls for subcls in target.__subclasses__()}.values())
+            subclasses = {target.__name__: target} | {k: v for subcls in deduped for k, v in self.get(subcls).items()}
+            subclasses[target.__name__] = target
+            self.cache[target.__name__] = subclasses
+            return subclasses
+
+# Get all the subclasses of  Action and remove Action itself
+action_subclasses = tuple(set(Subclasses().get(Action).values()) - set([Action,]))
+
+# Constuct a union type out of the subclasses
+# Field(discriminator="name") tells pydantic to look at the name 
+# field to decide what type of the union to attempt to parse
+action_subclasses_union = Annotated[
+    Union[action_subclasses],
+    Field(discriminator="name")]
+
+class Config(ConfigModel):
+    actions: list[action_subclasses_union] = Field(discriminator='name')
+
+
+def test_subclasses():
+    config = yaml.safe_load("""
+actions:
+    - name: "Source"
+      start_from: 01031997
+    #   should_flub_widgets: True # extra key
+
+    - name: Process
+      should_flub_widgets: True
+
+    - name: Sink
+      emit_aviso_notification: False
+""")
+    conflator = Conflator("test-app", Config, cli = False, **config)
+    config = conflator.load()
+
+    assert config.actions[0].name == "Source"
+
+
+def test_subclasses_extra_key():
+    with pytest.raises(SystemExit):
+        config = yaml.safe_load("""
+    actions:
+        - name: "Source"
+          start_from: 01031997
+          should_flub_widgets: True # extra key
+
+        - name: Process
+          should_flub_widgets: True
+
+        - name: Sink
+          emit_aviso_notification: False
+    """)
+        conflator = Conflator("test-app", Config, cli = False, **config)
+        config = conflator.load()

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-env_list = py{38,39,310,311}
+env_list = py{39,310,311}
 minversion = 4.12.1
 
 [testenv]