[Feature] Throw error when structuring dict w/ extra keys

[alanhdu]

• cattrs version: 1.0.0
• Python version: Python 3.6
• Operating System: Linux (Fedora)

Description

Right now, cattr.structure will silently ignore extra keys when structuring into an attrs object:

In [1]: import attr
   ...: import cattr
   ...: 
   ...: @attr.s
   ...: class A:
   ...:     a: int = attr.ib()
   ...: 
   ...: cattr.structure({"a": 1, "b": 2}, A)
Out[1]: A(a=1)

I assume this is intended behavior (since I believe this changed since cattrs 0.9), but it'd be nice to have an option to throw an error on extra keys instead.

In our particular use-case, we are using cattrs and attrs to verify some configuration files we are using, some of which have default keys:

@attr.s
class Config:
    some_key: int = attr.ib(default=0)

b/c of this "silently allow extra keys" behavior, cattr.structure doesn't catch typos (e.g. some_key vs sme_key), because it will just ignore the incorrectly-typed key and use the default value for the substitute.

[alanhdu]

If this is something that you'd be open to (either as the default behavior, or as a flag on structure), I'd be happy to dig through the code and submit a PR for the implementation.

[Tinche]

Hello,

I'm not really open to changing the default behavior since that would make it very hard to implement backwards compatibility for a lot of use cases cattrs is used for.

I would be open to including this as an option. This should be a new parameter to cattr.gen.make_dict_structure_fn (forbid_extra_fields I guess?).

[asford]

Would this solution allow us to set this as the "standard" behavior of a GenConverter instance? It'd be ideal to be able to provide a "strict" vs "lenient" converter over the same types without needing pre-register custom structure/unstructure functions.

[Tinche]

Implementing the functionality in make_dict_structure_fn is step 1. Modifying the GenConverter to pass that option to make_dict_structure_fn whenever a structuring function is generated would be step 2.

[bkurtz]

Another +1 for this feature. I'm open to doing some work on a PR if no one else has one open already.

[Tinche]

@bkurtz Sure, go for it (minding my previous comments). Since there is interest for this I might do this myself in a week if no one else steps up.