Structure when input is already partially structured

• cattrs version: 0.9.0
• Python version: 3.7.5
• Operating System: Mac OSX

Description

I am attempting to structure a blob of data into a recursive structure, where part of the blob has already been structured. When it reaches a key that already has a structured value, it errors out. After reading through the documentation it's unclear how to get around this issue.

What I would expect to happen is if a given key already has (valid) structured data, it includes it as is, rather than attempting to (and failing to) structure it.

I'm not sure if I am missing an obvious workaround (converter or hook?), or if this would require a PR to run a check of some sort before attempting to structure a sub-element.

What I Did

This is a minimal example:

from attr import dataclass
from cattr import structure, unstructure

@dataclass(auto_attribs=True)
class B:
    name: str


@dataclass(auto_attribs=True)
class A:
    name: str
    subber: B


dx = {
    "name": "alphas",
    "subber": {
        "name": "subclass"
    }
}

a = structure(dx, A)
print(a)
b = structure(dx.get("subber"), B)
print(b)
dy = {
    "name": "tau",
    "subber": unstructure(b)
}
a2 = structure(dy, A)
print(a2)
dz = {
    "name": "omega",
    "subber": b
}
a3 = structure(dz, A)

The above produces the following output:

A(name='alphas', subber=B(name='subclass'))
B(name='subclass')
A(name='tau', subber=B(name='subclass'))
Traceback (most recent call last):
  File "/Users/nford/Library/Preferences/IntelliJIdea2019.2/scratches/dc.py", line 46, in <module>
    a3 = structure(dz, A)
  File "/Users/nford/echelon/rolevp/echelon/.venv/lib/python3.7/site-packages/cattr/converters.py", line 178, in structure
    return self._structure_func.dispatch(cl)(obj, cl)
  File "/Users/nford/echelon/rolevp/echelon/.venv/lib/python3.7/site-packages/cattr/converters.py", line 298, in structure_attrs_fromdict
    conv_obj[name] = dispatch(type_)(val, type_)
  File "/Users/nford/echelon/rolevp/echelon/.venv/lib/python3.7/site-packages/cattr/converters.py", line 285, in structure_attrs_fromdict
    conv_obj = obj.copy()  # Dict of converted parameters.
AttributeError: 'B' object has no attribute 'copy'

But what I am hoping to achieve is:

A(name='alpha', subber=B(name='subclass'))
B(name='subclass')
A(name='tau', subber=B(name='subclass'))
A(name='omega', subber=B(name='subclass'))

Start with the structure entry point and structure_attrs_fromdict in cattr/converters.py, using the minimal example to reproduce the failure when subber is already a B instance. Trace how nested values are dispatched and verify that the completed behavior preserves valid structured values while retaining existing dictionary handling.