Structuring a non-dict type as TypedDict raises AttributeError

[bluetech]

• cattrs version: 24.1.0
• Python version: 3.11
• Operating System: Linux

Description

I am structuring a TypedDict type. The input is arbitrary (from JSON), and I want to offload the validation to cattrs.
The problem is, if the input is not a dict (in my case, not a JSON object), cattrs raises an AttributeError instead of the expected cattrs.errors.ClassValidationError.

from typing import TypedDict
import cattrs

class MyTypedDict(TypedDict):
    foo: int

cattrs.structure(42, MyTypedDict)

I would like to avoid catching AttributeError or adding my own pre-checks.

What I Did

Running the script above results in:

Traceback (most recent call last):
  File "x.py", line 8, in <module>
    cattrs.structure(42, MyTypedDict)
  File "venv/lib/python3.11/site-packages/cattrs/converters.py", line 544, in structure
    return self._structure_func.dispatch(cl)(obj, cl)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<cattrs generated structure __main__.MyTypedDict>", line 2, in structure_MyTypedDict
AttributeError: 'int' object has no attribute 'copy'

[Tinche]

Hm, agreed. Let me get this for the next version.

[Tinche]

I think I got this on main. Can you give it a try and let me know?

[bluetech]

Thanks! I will try it on my real example and let you know.

Regarding the fix, did you intentionally avoid an isinstance check? I wonder for example what happens if the input is a list (JSON array) -- a list does have a copy() method.

[Tinche]

Hm, interesting. My thinking is this: catching an AttributeError adds essentially no overhead, and a separate isinstance check would be felt, especially on small dicts. It's also a little unclear what to check against (collections.abc.Mapping presumably).

Let me see what happens if a list is passed in when I get to a computer.

[Tinche]

I think you're right, the error message kind of sucks when lists are involved.

Here's a new try: #617