add pathlib.Path support

[gwenzek]

Add support for Path in Omegaconf.

Path type will never be automatically inferred when loading from YAML, so we will continue creating StringNode by default.
But if you use structured or _promote with a dataclass using Path some of StringNode can be converted to PathNode eg:

https://github.com/gwenzek/omegaconf/blob/1df8a05f9f740b7085fb65fe7a4d8b1e034f2ea6/tests/structured_conf/test_structured_config.py#L819-L833

This change is backward compatible, because code that currently use dataclass with Path fields currently see errors when running structured.

This should resolve #97

cc: @odelalleau @Mortimerp9

[lgtm-com]

This pull request introduces 1 alert when merging 1df8a05 into 5e73ee6 - view on LGTM.com

new alerts:

• 1 for Module is imported with 'import' and 'import from'

[Jasha10]

Closes #97.

[Jasha10]

Thanks very much for taking the time to create this PR, @gwenzek.
At the moment, I can't commit to a timeline for merging this, as the OmegaConf team currently has it's hands full.
Edit: we will review this shortly.

[Jasha10]

>>> OmegaConf.create({"foo": Path("bar")})
Traceback (most recent call last):
...
omegaconf.errors.UnsupportedValueType: Value 'PosixPath' is not a supported primitive type
    full_key: foo
    object_type=dict

I think we should support the above use-case. This would probably require updating the is_primitive_type function

[gwenzek]

good point, I've added a test case for this,
and indeed I had to modify is_primitive to allow this.

Note that I've extrcted the list of base types to a constant for cosmetic reasons.

[Jasha10]

Thanks @gwenzek. The implementation looks good to me.
I am planning to push some commits to this PR branch adding more tests.

[gwenzek]

I rebased to resolve the conflict with the new bytes node.

[Jasha10]

Demo of the API:

>>> from omegaconf import OmegaConf; from pathlib import Path
>>> OmegaConf.create({"foo": Path("hello.txt")})
{'foo': PosixPath('hello.txt')}

pathlib.Path can be used as a type annotation with structured configs too.
Paths are preserved through a round-trip to yaml:

>>> cfg = OmegaConf.create({"foo": Path("hello.txt")})
>>> print(OmegaConf.to_yaml(cfg))
foo: !!python/object/apply:pathlib.PosixPath
- hello.txt

>>> OmegaConf.create(OmegaConf.to_yaml(cfg))
{'foo': PosixPath('hello.txt')}

Notes:

• I've disabled automatic conversion from Path to str. This means assigning
  Path("foo.txt") to MyStructuredConf.string_field fails with a
  ValidationError. Conversion from string to path is implemented (assigning the
  string "foo.txt" to MyStructuredConf.path_field succeeds).
• Using the yaml tag !!python/object/apply:... means that the output of OmegaConf.to_yaml can be deserialized using vanilla pyyaml (i.e. without using OmegaConf):

  import yaml
  yaml_document = OmegaConf.to_yaml(OmegaConf.create({"foo": Path("hello.txt")}))
  assert OmegaConf.create(yaml_document) == yaml.unsafe_load(yaml_document)

cc @rsokl, @odelalleau

[jieru-hu]

looks great!

[gwenzek]

Thanks a lot @Jasha10 for adding the necessary tests and a updating the documentation.

[Mortimerp9]

Awesome! Thanks for getting this in Hydra!