Difference between facebook Hydra's + and ++ operators

Suppose we have the following simple hydra app:

├── config.yaml
└── my_app.py

# my_app.py
import hydra
from omegaconf import OmegaConf, DictConfig

@hydra.main(".", "config")
def app(cfg: DictConfig):
    print(OmegaConf.to_yaml(cfg))

if __name__ == "__main__":
    app()

# config.yaml
a:
  b: c

$ python my_app.py
a:
  b: c

Let's take a look at the four methods of "Modifying the Config Object" from the docs page you linked:

• Overriding a config value: foo.bar=value
• Appending a config value: +foo.bar=value
• Appending or overriding a config value: ++foo.bar=value
• Removing a config value: ~foo.bar, ~foo.bar=value

Overriding a config value:

$ python my_app.py a.b=y
a:
  b: y

$ python my_app.py a.x=y
Could not override 'a.x'.
To append to your config use +a.x=y
Key 'x' is not in struct
    full_key: a.x
    object_type=dict

Appending a config value:

$ python my_app.py +a.b=y
Could not append to config. An item is already at 'a.b'.
Either remove + prefix: 'a.b=y'
Or add a second + to add or override 'a.b': '++a.b=y'
...

$ python my_app.py +a.x=y
a:
  b: c
  x: y

Appending or overriding a config value:

$ python my_app.py ++a.b=y
a:
  b: y

$ python my_app.py ++a.x=y
a:
  b: c
  x: y

Removing a config value:

$ python my_app.py ~a.b
a: {}

$ python my_app.py ~a.b=c
a: {}

$ python my_app.py ~a.b=y
Could not delete from config. The value of 'a.b' is c and not y.
...

$ python my_app.py ~a.x
Could not delete from config. 'a.x' does not exist.
...

$ python my_app.py ~a.x=y
Could not delete from config. 'a.x' does not exist.

To summarize:

• Overriding a config value (foo.bar=value) works only if the given key is already present in the config
• Appending a config value (+foo.bar=value) works only if the given key not yet present in the config
• Appending or overriding a config value (++foo.bar=value) works whether or not the key is present in the config
• Removing a config value (~foo.bar, ~foo.bar=value) works only if the given key or key-value pair is in the config.