Overriding a config's argument makes everything else fall back to the base class

[carmocca]

🐛 Bug report

To reproduce

repro.py:

from dataclasses import dataclass
from typing import Optional

@dataclass
class Thing:
    foo: int = 1
    bar: int = 2

def fn(thing: Optional[Thing] = None):
    thing = Thing() if thing is None else thing
    print(thing)

from jsonargparse import CLI
CLI(fn)

repro.yaml:

thing:
  foo: 123
  bar: 321

python repro.py --config repro.yaml --thing.foo 9

Expected behavior

The above prints

Thing(foo=9, bar=2)

but I expected

Thing(foo=9, bar=321)

Context

A more real world example is that you have a trainer_config.yaml and you want to only modify the --train.max_epochs value without having everything else fall back to dataclass defaults

Environment

Version 4.275

[carmocca]

Given that

class Thing:
    def __init__(self, foo=1, bar=2):
        print(locals())

from jsonargparse import CLI
CLI(Thing)

foo: 123
bar: 321

python repro.py --config repro.yaml --foo 9

Works as expected, I would expect these two approaches to produce the same result

[mauvilsa]

Thank you for reporting! This is fixed with #471.