Using Types of Members of Generic Containers

[hmc-cs-mdrissi]

I currently have code that uses a generic config like class that is processed by various other classes. Sometimes those other classes there types depend on what types contained by config class. Something like,

class Config(Generic[T1, T2]):
  foo: dict[str, T1]
  bar: list[T1]
  baz: T2
  # ... (more shared non-generic elements)

ConfigT = TypeVar('ConfigT', bound=Config)

class Processor(Generic[ConfigT]):
  def __init__(self, config: ConfigT):
    self._config = config

  def process_foo(self) -> list[XYZ]:
    return list(self._config.foo.values())

The main issue is filling in XYZ. One option I tried was making processor generic on T1, T2 too, but #548 I think is a blocker to write ConfigT[T1, T2]. Are there any alternative ways to have classes operate on top of generic classes like this? For now I'm just falling back to XYZ is Any. The processor class mainly cares for the inner types of foo/bar/baz/etc because of relationships they hold. Since same typevar T1 appears multiple times in Config class so even though it may know little about T1 it would ideally type check things like

x = self.process_foo()
x.extend(self._config.bar)

is good but

x = self.process_foo()
x.append(self._config.baz)

is an error.

[erictraut]

How about this?

class Processor(Generic[T1, T2]):
    def __init__(self, config: Config[T1, T2]):
        self._config = config

    def process_foo(self) -> list[T1]:
        return list(self._config.foo.values())

[hmc-cs-mdrissi]

That looks like a good trade off and works better than making ConfigT generic. The one issue it has is I also have subclasses of Processor that may only work for certain configs (something like),

class ConfigX(Config):
  ...

class ProcessorX(Processor[ConfigX]):
  ...

but I can handle that by having subclasses override the type of config: ... A liskov error, but I think liskov error and proper checking of T1/T2 is better than T1/T2 being uncheckable. It also pushes me hard to try to minimize logic specific to config subclasses.