Schema if_key_missing default values

[GevatterGaul]

In the standard schema the value for if_key_missing defaults to NoDefault. This overrides the if_missing setting for the included validators. So when a validator has an if_missing setting, this will not be evaluated, as the schema will raise an Invalid exception before.

This behaviour is not documented and the comment to the if_key_missing value in the code suggests that it is also not intended. (schema.py line 52)

I suggest changing the default behaviour for if_key_missing in a way that the if_missing value of the validator can be evaluated. On the other hand, if this behaviour is intended we could document it and save other people the effort of looking it up in the code.

[lambacck]

Can you provide a concrete example? I read the code here starting on line 176 as
going with the if_missing value on the validator first and then taking the value of if_key_missing which is then validated using the validator. The code does exactly what is stated on line 52.