Idiomatic way to collect & report multiple exceptions in Python

Question

What have people used to catch, log, and report multiple data validation errors at once in Python?

I’m building an application in Python 3 that first validates input data and then processes it. Reporting errors in the first step is part of the intended functionality of the program, so I don’t want my validator to give up on the first exception. In particular, the data are tabular and I want be able to return — rather than raise — an exception for each line of the table that does not validate.

A forum discussion from a couple of years ago contemplates multiple solutions, including the following, which seems the cleanest to me:

errors = []
for item in data:
    try:
        process(item)
    except ValidationError as e:
        errors.append(e)
if errors:
    raise MultipleValidationErrors(errors)

where the MultipleValidationErrors class would have an appropriate __str__ method to list useful information about all the ValidationErrors in it.

Others recommend using the traceback module, but since the exceptions I want to catch are data validation errors rather than program errors, that seems inappropriate. Getting the logging module involved might be appropriate, though.

Asked By: wkschwartz

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Answer 1

I’ve used this idiom in both C++ and Python. It’s the cleanest solution that I know of when what you want is an exception, rather than a log message. The downside to it is that the combined exception takes up linear space in general, which can be problematic when processing large datasets with many errors.

Answered By: Fred Foo

Answer 2

I follow the list of errors approach but contain it in an object like this:

class Multi_Error(Exception):
    def __init__(self, errors: list[Exception]) -> None:
        self.errors = errors
        super().__init__(self.errors)

    def __str__(self) -> str:
        return "n".join([str(x) for x in self.errors])

Answered By: Sean D

Idiomatic way to collect & report multiple exceptions in Python

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