Add default value to source classes init params

[hramezani]

This makes source classes inheritable in custom source classes. So, the custom source class doesn't need to override the __init__ to provide a fallback value from the config.

Selected Reviewer: @dmontagu

[hramezani]

please review

[dmontagu]

Suggested change

	env_file_sentinel: DotenvType = Path('')
	ENV_FILE_SENTINEL: DotenvType = Path('')

I generally prefer to have module-level constants be all-caps (unless they are type aliases, like the one just above this line, in which case PascalCase is fine). You can ignore this suggestion if preferred, I just think it makes it easier to tell when skimming through code that the variable was a module-level constant since I can't immediately see where it was defined.

Also, can we add documentation of how this sentinel works? I don't understand what the meaning is vs. using None — I think it should be easy to determine that just from reading source code, and since I can't tell what it's doing by looking at the diff in this PR, I think it would be good to add a comment either in the docstring of DotEnvSettingsSource.__init__ or right under this line defining env_file_sentinel explaining what the distinction is between providing the value None and the value env_file_sentinel.

[hramezani]

I've renamed it to ENV_FILE_SENTINEL and added some comments on top of it.
Please read it and let me know who do you feel.

Note: we have a test_env_file_override_none for this behavior.

pydantic-settings/tests/test_settings.py

Lines 809 to 820 in 3e93d17

	def test_env_file_override_none(tmp_path):
	p = tmp_path / '.env'
	p.write_text(test_env_file)
	class Settings(BaseSettings):
	a: Optional[str] = None
	model_config = ConfigDict(env_file=p)
	s = Settings(_env_file=None)
	assert s.a is None

[dmontagu]

please update

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b08fa5b) 97.41% compared to head (5784d22) 97.41%.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #82   +/-   ##
=======================================
  Coverage   97.41%   97.41%           
=======================================
  Files           5        5           
  Lines         309      309           
  Branches       67       67           
=======================================
  Hits          301      301           
  Misses          6        6           
  Partials        2        2           

Impacted Files	Coverage Δ
pydantic_settings/main.py	100.00% <100.00%> (ø)
pydantic_settings/sources.py	97.68% <100.00%> (+<0.01%)	⬆️

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[hramezani]

please review

[dmontagu]

Can we make this a bit more clear of the exact behavior so I know what will happen without needing to check the logic (right now it says "special meaning", but not what that special meaning is)? Here's an attempt — I haven't checked the logic closely, can you confirm if this is correct?

Suggested change

	# This is used as default value for `_env_file` in `Settings` class and
	# `env_file` in `DotEnvSettingsSource`. Then passing `None` as `_env_file`
	# during initialization of `Settings` has a special meaning and it will
	# override the `env_file` specified in config.
	ENV_FILE_SENTINEL: DotenvType = Path('')
	# This is used as default value for `_env_file` in `Settings` class and
	# `env_file` in `DotEnvSettingsSource`. When this value is used, environment
	# variables will be read from the `model_config`-specified file. Using this
	# as the default instead of `None` allows us to use `None` to mean that environment
	# variables should not be read from any env file.
	ENV_FILE_SENTINEL: DotenvType = Path('')

Another approach we could take is putting this in the pydantic_settings.main.BaseSettings.__init__ docstring, and then we could shorten/simplify the above comment:

    def __init__(
        __pydantic_self__,
        _case_sensitive: bool | None = None,
        _env_prefix: str | None = None,
        _env_file: DotenvType | None = env_file_sentinel,
        _env_file_encoding: str | None = None,
        _env_nested_delimiter: str | None = None,
        _secrets_dir: str | Path | None = None,
        **values: Any,
    ) -> None:
        """
        TODO: Properly document all additional kwargs here.

        When `_env_file == ENV_FILE_SENTINEL`, the `model_config`-specified env file will be used.
        When `_env_file is None`, environment variables will not be read from any env file.
        """

I do think we need to document all these keyword arguments eventually, but even if you don't want to do that now, I think it's okay to put a placeholder docstring in like this and it would address my concern.

[dmontagu]

Oh I just realized that this is documented in the pydantic_settings.main.BaseSettings docstring. I think we can just augment that docstring as part of this PR if you don't mind. I would change the current value:

        _env_file: The env file(s) to load settings values from. Defaults to `Path('')`.

to

        _env_file: The env file(s) to load settings values from. Defaults to `Path('')`, which
        means that the value from `model_config['env_file']` should be used. You can also pass
        `None` to indicate that environment variables should not be loaded from an env file.

or similar

[dmontagu]

If we change the BaseSettings docstring, I think we can change the comment here to just

Suggested change

	# This is used as default value for `_env_file` in `Settings` class and
	# `env_file` in `DotEnvSettingsSource`. Then passing `None` as `_env_file`
	# during initialization of `Settings` has a special meaning and it will
	# override the `env_file` specified in config.
	ENV_FILE_SENTINEL: DotenvType = Path('')
	# This is used as default value for `_env_file` in the `BaseSettings` class and
	# `env_file` in `DotEnvSettingsSource` so the default can be distinguished from `None`.
	# See the docstring of `BaseSettings` for more details.
	ENV_FILE_SENTINEL: DotenvType = Path('')