Show param values in validation error messages

[sscherfke]

Feature Request

This originates from fastapi/fastapi#473:

The error messages created by the parameter validation are like "msg": "value is not a valid integer".

It would be helpful for debugging if they contained the actual value of that parmeter: "msg": f"value '{param}' is not a valid integer" -> "msg": "value 'spam' is not a valid integer"

[samuelcolvin]

I disagree. What if the value is a string of 5000 characters? Should we show the entire thing in the error message.

However I think we could do with improving the data available about errors, for example we should make sure the provided value is always available in ctx and that all error types are unique, so completely custom logic and error message translations work properly.

[sscherfke]

If the string is too long, we might abbreviate it like this?

str_val = str(value)
if len(str_val) > 15:
    str_val = f'{str_val[:8]}...{str_val[-4:]}'

But I would be hapy with anything that improves the error message (and thus, debugging). :)

[samuelcolvin]

debugging would be fixed by adding the original value to ctx which would then be shown in the str() method of ValidationError and therefore python's standard error printing.

[pawelad]

Can I somehow help with this?

[JosXa]

Mentioning #1938 for another use-case/problem that arises from this issue.

[JosXa]

@Fireclunge sounds like we got a PR on our hands 😜

[tek08]

Hallo. If this is actually of interest to have in Pydantic, I'm (somewhat) happy to make the PR. Many unit tests to update. Before I go on what could be a fool's errand and continue working on tweaking each and every unit test, would there be objection to surfacing more info in ctx for each ValidationError?

[johtso]

Just to check.. I'm currently struggling with the fact that I don't get a traceback to the offending line of code when there's an exception in custom validation, just the validation error that includes the original exception. Would this possibly help with that?

[tek08]

Nope -- My in-progress change's only delta is to include the offending value for ValidationErrors.