fix: improve error messages in deep_iterable and deep_mapping validators

[JasonOA888]

Summary

Fixes #1245 (also referenced in #1206)

When inner validators fail in deep_iterable or deep_mapping, the error message now includes the index/key of the failing element instead of just the parent attribute name.

Problem

Before this fix:

@define
class A:
    x: list = field(
        validator=deep_iterable(
            member_validator=[instance_of(str), min_len(1)],
        ),
    )

A(["abc", ""])
# ValueError: Length of 'x' must be >= 1: 0

The error says 'x' but it's not x that is too short - it's one of its items.

Solution

After this fix:

A(["abc", ""])
# ValueError: Length of 'x[1]' must be >= 1: 0

Now the error clearly indicates which element failed validation.

Changes

• Modified _DeepIterable.__call__ to catch exceptions and rewrite the attribute name to include the index
• Modified _DeepMapping.__call__ to catch exceptions and rewrite the attribute name to include the key

Testing

from attr import define, field
from attr.validators import deep_iterable, instance_of, min_len

@define
class TestA:
    x: list = field(
        validator=deep_iterable(
            member_validator=[instance_of(str), min_len(1)],
        ),
    )

try:
    TestA(["abc", ""])
except ValueError as e:
    print(e)  # Length of 'x[1]' must be >= 1: 0