How it works

This library works by mutating the default values in the underlying click context (click.Context.default_map) before the command is executed (see phha/click_config_file). It is essentially overwriting the default values that you specified in your source code. Then, the shell environment variables and CLI parameters are parsed by typer to override the values already set.

The @use_config decorator works by modifying your function's signature to include a config parameter with a sane default. The typer library then sees this extended signature and parses/generates the help text for the config parameter. Internally, the decorator then removes the config parameter from the arguments passed to the actual implementation that you wrote. Otherwise, your function would error with an unknown argument.

If you use the config parameter directly in your function, you must use is_eager=True in the parameter definition because that will cause it to be processed first. For example:

config: str = typer.Option("", is_eager=True, callback=...)

If you don't use is_eager, then your parameter values will depend on the order in which they were processed (read: unpredictably).