Bug description

Cannot pass a boolean to the save_last parameter of the ModelCheckpoint callback using LightningCLI.

All parameters work fine except for save_last. I think jsonargparse is having trouble with the validation of the annotation of save_last which is currently Optional[Literal[True, False, 'link']].

Ideally, this should work like any other boolean flag e.g., like --my_model_checkpoint.verbose=false.

I have already forked the project and proposed a solution together with tests in the relevant directory. I have readiness to submit a PR if you think this might be useful.

What version are you seeing the problem on?

master

How to reproduce the bug

import inspect
import jsonargparse
from lightning.pytorch.callbacks import ModelCheckpoint

val = 'true'
annot = inspect.signature(ModelCheckpoint).parameters["save_last"].annotation
parser = jsonargparse.ArgumentParser()
parser.add_argument("--a", type=annot)
args = parser.parse_args(["--a", val])

Error messages and logs

error: Parser key "a":
  Does not validate against any of the Union subtypes
  Subtypes: (typing.Literal[True, False, 'link'], <class 'NoneType'>)
  Errors:
    - Expected a typing.Literal[True, False, 'link']
    - Expected a <class 'NoneType'>. Got value: True
  Given value type: <class 'str'>
  Given value: true

Environment

More info

The solution is to change the typing annotation of the save_last parameter in the constructor of ModelCheckpoint.

I have made a draft PR and added a test to check that the bug is fixed here fore reference.

The tests are passing: