Type hinting config.conf

[gexgd0419]

I tried to enable Pyright rules, and I saw many type checking errors related to config.conf.

Type-hinting config.conf is challenging. configobj.Section.__getitem__ can return many different types of values, such as a string, an integer, or a sub section. This makes static type checking difficult. For example, config.conf["general"]["loggingLevel"], how can a static type checker determine that config.conf["general"] is a section, and config.conf["general"]["loggingLevel"] is a string?

Worse, configobj currently lacks support for static type checking. Someone is working on this, though.

If you type-hint __getitem__ with every possible type, such as Section | str | int | ..., the type checker will still not able to determine the actual type, and will warn on operations not possible for some of the types. If you type hint with Any though, the type checker will allow anything, but then you lose the type checking ability, unless you explicitly mark the type at the caller's end, such as level: str = config.conf["general"]["loggingLevel"].

Type-checking for dictionaries can be achieved with TypedDict, which allows different keys to be mapped to different types of values.

from typing import TypedDict

class Conf(TypedDict, total=False):
    general: "GeneralConf"

class GeneralConf(TypedDict, total=False):
    language: str
    saveConfigurationOnExit: bool
    askToExit: bool
    playStartAndExitSounds: bool
    loggingLevel: str
    showWelcomeDialogAtStartup: bool
    preventDisplayTurningOff: bool

conf = Conf()
conf["general"] = GeneralConf()
conf["general"]["language"] = "Windows"  # pass
conf["general"]["askToExit"] = 1  # type checker error

It can work well with pure dictionaries, but config.conf is not just a dictionary, which can cause problems because TypedDict is treated as a dict at runtime.

print(type(conf))  # <class 'dict'>
print(type(conf["general"]))  # <class 'dict'>

class ConfigManager(Conf):
    pass  # defining member functions is not allowed here
    
mgr = ConfigManager()
print(type(mgr))  # <class 'dict'>

So Conf, GeneralConf and ConfigManager are not separate types at runtime. Also, defining extra member functions in a TypedDict is not allowed.

What are your ideas about how to type hint config?

[gexgd0419]

An alternative I can think of, is to use a bunch of overloads:

class GeneralConf:
    @overload
    def __getitem__(self, key: Literal[
        "language",
        "loggingLevel",
    ]) -> str: ...
    
    @overload
    def __getitem__(self, key: Literal[
        "saveConfigurationOnExit",
        "askToExit",
        "playStartAndExitSounds",
        "showWelcomeDialogAtStartup",
        "preventDisplayTurningOff"
    ]) -> bool: ...

    # actual implementation
    def __getitem__(self, key: str) -> Any:
        ...

which seemed to be working in VS Code. It can list all possible literals after typing the left bracket, just like a TypedDict does. And if the key matches one of the literals, VS Code can deduce its data type.