Deprecate OmegaConf.is_none()

[odelalleau]

PROBLEM

With the change to OmegaConf.is_missing() in #545, the current behavior of OmegaConf.is_none() in OmegaConf 2.0 can be confusing (because it resolves interpolations, while OmegaConf.is_missing() doesn't).

In addition, OmegaConf.is_none() has potentially non-intuitive behavior in situations where the key can't be resolved (see below for details).

SOLUTION
OmegaConf.is_none() is being deprecated in OmegaConf 2.1, and will be removed in OmegaConf 2.2

MIGRATION INSTRUCTIONS

Please do not hesitate to open an issue if you run into problems that the instructions below do not address.

Scenario 1 -- Your code is checking a specific key:

OmegaConf.is_none(cfg, "foo")
OmegaConf.is_none(cfg, key)

It is recommended to change these to:

cfg.foo is None
cfg[key] is None

Note however that when using this new syntax, an exception may be raised when accessing the key, while OmegaConf.is_none() used to silence such exceptions. If you want to keep the exact (but possibly unintuitive) behavior from before, you can implement your own is_none() function as follows:

def is_none(obj: Container, key: Union[int, DictKeyType]) -> bool:
    try:
        return obj[key] is None
    except ConfigKeyError:
        return True  # `True` if the key does not exist in the config
    except (InterpolationResolutionError, MissingMandatoryValue):
        return False  # `False` if it is either missing ("???") or an invalid interpolation

Scenario 2 -- Your code is not providing a key:

OmegaConf.is_none(obj)

There is currently no plan to keep supporting this usage in OmegaConf 2.2. If you need it, please open an issue explaining why.

Additional context
See discussion in #545 (comment)

Dev note: at the same time, the internal Node._is_none() can be modified to not resolve interpolations.

[Jasha10]

=> let's deprecate OmegaConf.is_none(), instructing users to use cfg.foo is None instead.

I like this -- cfg.foo is None is more pythonic than OmegaConf.is_none(cfg, "foo").