Validation

cattrs has a detailed validation mode since version 22.1.0, and this mode is enabled by default. When running under detailed validation, the un/structuring hooks are slightly slower but produce more precise and exhaustive error messages.

Detailed Validation

New in version 22.1.0.

In detailed validation mode, any un/structuring errors will be grouped and raised together as a cattrs.BaseValidationError, which is a PEP 654 ExceptionGroup. ExceptionGroups are special exceptions which contain lists of other exceptions, which may themselves be other ExceptionGroups. In essence, ExceptionGroups are trees of exceptions.

When un/structuring a class, cattrs will gather any exceptions on a field-by-field basis and raise them as a cattrs.ClassValidationError, which is a subclass of BaseValidationError.

When structuring sequences and mappings, cattrs will gather any exceptions on a key- or index-basis and raise them as a cattrs.IterableValidationError, which is a subclass of BaseValidationError.

The exceptions will also have their __notes__ attributes set, as per PEP 678, showing the field, key or index for each inner exception.

A simple example involving a class containing a list and a dictionary:

@define
class Class:
    a_list: list[int]
    a_dict: dict[str, int]

>>> structure({"a_list": ["a"], "a_dict": {"str": "a"}}, Class)
  + Exception Group Traceback (most recent call last):
  |   File "<stdin>", line 1, in <module>
  |   File "/Users/tintvrtkovic/pg/cattrs/src/cattr/converters.py", line 276, in structure
  |     return self._structure_func.dispatch(cl)(obj, cl)
  |   File "<cattrs generated structure __main__.Class>", line 14, in structure_Class
  |     if errors: raise __c_cve('While structuring Class', errors, __cl)
  | cattrs.errors.ClassValidationError: While structuring Class
  +-+---------------- 1 ----------------
    | Exception Group Traceback (most recent call last):
    |   File "<cattrs generated structure __main__.Class>", line 5, in structure_Class
    |     res['a_list'] = __c_structure_a_list(o['a_list'], __c_type_a_list)
    |   File "/Users/tintvrtkovic/pg/cattrs/src/cattr/converters.py", line 457, in _structure_list
    |     raise IterableValidationError(
    | cattrs.errors.IterableValidationError: While structuring list[int]
    | Structuring class Class @ attribute a_list
    +-+---------------- 1 ----------------
      | Traceback (most recent call last):
      |   File "/Users/tintvrtkovic/pg/cattrs/src/cattr/converters.py", line 450, in _structure_list
      |     res.append(handler(e, elem_type))
      |   File "/Users/tintvrtkovic/pg/cattrs/src/cattr/converters.py", line 375, in _structure_call
      |     return cl(obj)
      | ValueError: invalid literal for int() with base 10: 'a'
      | Structuring list[int] @ index 0
      +------------------------------------
    +---------------- 2 ----------------
    | Exception Group Traceback (most recent call last):
    |   File "<cattrs generated structure __main__.Class>", line 10, in structure_Class
    |     res['a_dict'] = __c_structure_a_dict(o['a_dict'], __c_type_a_dict)
    |   File "", line 17, in structure_mapping
    | cattrs.errors.IterableValidationError: While structuring dict
    | Structuring class Class @ attribute a_dict
    +-+---------------- 1 ----------------
      | Traceback (most recent call last):
      |   File "", line 5, in structure_mapping
      | ValueError: invalid literal for int() with base 10: 'a'
      | Structuring mapping value @ key 'str'
      +------------------------------------

Transforming Exceptions into Error Messages

New in version 23.1.0.

ExceptionGroup stack traces are great while you’re developing, but sometimes a more compact representation of validation errors is better. cattrs provides a helper function, cattrs.transform_error(), which transforms validation errors into lists of error messages.

The example from the previous paragraph produces the following error messages:

>>> from cattrs import transform_error

>>> try:
...     structure({"a_list": ["a"], "a_dict": {"str": "a"}}, Class)
... except Exception as exc:
...     print(transform_error(exc))

[
    'invalid value for type, expected int @ $.a_list[0]',
    "invalid value for type, expected int @ $.a_dict['str']"
]

A small number of built-in exceptions are converted into error messages automatically. This can be further customized by providing cattrs.transform_error() with a function that it can use to turn individual, non-ExceptionGroup exceptions into error messages. A useful pattern is wrapping the default, cattrs.v.format_exception() function.

>>> from cattrs.v iomport format_exception

>>> def my_exception_formatter(exc: BaseException, type) -> str:
...     if isinstance(exc, MyInterestingException):
...         return "My error message"
...     return format_exception(exc, type)

>>> try:
...     structure(..., Class)
... except Exception as exc:
...     print(transform_error(exc, format_exception=my_exception_formatter))

If even more customization is required, cattrs.transform_error() can be copied over into your codebase and adjusted as needed.

Non-detailed Validation

Non-detailed validation can be enabled by initializing any of the converters with detailed_validation=False. In this mode, any errors during un/structuring will bubble up directly as soon as they happen.