When structuring, how to forbid type casting to base classes?

[philip-bl]

When structuring, how to forbid type casting to base classes?

[Tinche]

Hello!

You can override the default hooks for those types to be anything you like. Cattrs is based on function composition to this is how problems are usually solved.

A simple solution would be like this:

from cattrs import Converter

c = Converter()


def structure_int(val, _) -> int:
    if not isinstance(val, int):
        raise ValueError("Not an int")
    return val


c.register_structure_hook(int, structure_int)

c.structure("1", int)  # Error

You can get fancier by making use of the fact structure hooks receive their target type as their second argument, like this:

c = Converter()


def validate_type(val, type):
    if not isinstance(val, type):
        raise ValueError(f"Not a {type}")
    return val


c.register_structure_hook(int, validate_type)
c.register_structure_hook(float, validate_type)
c.register_structure_hook(str, validate_type)
c.register_structure_hook(bool, validate_type)

c.structure("1", bool)

These rules propagate to collections, attrs classes etc.

[Tinche]

I'm planning a strategy to make this easier, probably in the next version.