Provide method to get more concise error message

[virtuald]

Currently if you have a deeply nested data structure, when a validation error occurs then a user sees a deeply nested exception that displays several exceptions and their tracebacks, which isn't particularly easy to see what's incorrect. It would be useful for ValidationError to get a flatten method that converts the many nested exceptions into something more readable.

I put this together really quickly (so it might not be ideal yet), flatten could be _convert_validation_error?

import typing

from validobj import errors
import validobj.validation

T = typing.TypeVar("T")


class ValidationError(Exception):
    pass


def _convert_validation_error(fname: str, ve: errors.ValidationError) -> ValidationError:
    locs = []
    msg = []

    e = ve
    while e is not None:

        if isinstance(e, errors.WrongFieldError):
            locs.append(f".{e.wrong_field}")
        elif isinstance(e, errors.WrongListItemError):
            locs.append(f"[{e.wrong_index}]")
        else:
            msg.append(str(e))

        e = e.__cause__

    loc = "".join(locs)
    if loc.startswith("."):
        loc = loc[1:]

    vmsg = f"{fname}: {loc}:\n  {'\n  '.join(msg)}"
    return ValidationError(vmsg)


def parse_input(value: typing.Any, spec: typing.Type[T], fname: str) -> T:
    try:
        return validobj.validation.parse_input(value, spec)
    except errors.ValidationError as ve:
        raise _convert_validation_error(fname, ve) from None

The idea is you'd get a shorter more useful error message like this:

util.ValidationError: downloaded.yml: functions.downloaded_fn:
  Expecting value to be a dict, compatible with 'FunctionData', not 'NoneType'

It would be nice if something like this was in the library instead of needing to duplicate the same logic across a bunch of different programs. In particular, this would be useful for CLIs that read configuration files using validobj.

[Zaharid]

This looks similar to the YAML example here

https://validobj.readthedocs.io/en/latest/examples.html#yaml-line-numbers

and it is the kind of thing I write to use this library. But that comes with slight variations depending on the context: Do you add some context information like line numbers? Do you use some terminal formatting library to add nice colors (e.g. for the wrong_field)? Do you serialize the error for logging?. So it seems to me this belongs in an example or recipe. As your example shows, it is possible to support having nice error messages with the existing API.