How can I access fixture values inside `pytest_exception_interact` and replace the original exception with a customized one?

General Discussions
1

I’m using pytest-vcr to test HTTP requests and want to customize exception messages when a request doesn’t match a recorded cassette.

Right now, when a request fails to match, VCR.py raises:

vcr.errors.CannotOverwriteExistingCassetteException

I want to use the pytest_exception_interact hook to replace this with a more informative exception that includes fixture information, like the cassette requests and match properties.

I tried something like this in conftest.py:

@pytest.hookimpl(hookwrapper=True)
def pytest_exception_interact(node, call, report):
    excinfo = call.excinfo
    if report.when == "call" and isinstance(excinfo.value, CannotOverwriteExistingCassetteException):
        if hasattr(node, "__vcr_fixtures"):
            cassette = node.__vcr_fixtures.get("vcr_cassette")
            vcr = node.__vcr_fixtures.get("vcr")
            if cassette and vcr:
                raise MyCustomVCRException(
                    f"Cassette Requests: {cassette.requests}"
                ) from excinfo.value

Problem: I’m unsure how to reliably access the vcr_cassette and vcr fixtures from the test node inside the hook, and how to modify the report so pytest shows my custom exception instead of the default one.

2

In pytest_exception_interact, the test node has a funcargs dictionary with all resolved fixture values.

You can access vcr_cassette and vcr directly from there:

import pytest
from vcr.errors import CannotOverwriteExistingCassetteException

class MyCustomVCRException(Exception):
    pass

@pytest.hookimpl(hookwrapper=True)
def pytest_exception_interact(node, call, report):
    excinfo = call.excinfo
    if report.when == "call" and isinstance(excinfo.value, CannotOverwriteExistingCassetteException):
        vcr_cassette = node.funcargs.get("vcr_cassette")
        vcr_instance = node.funcargs.get("vcr")
        if vcr_cassette and vcr_instance:
            # raise custom exception while preserving original traceback
            raise MyCustomVCRException(
                f"Custom VCR Error: cassette {vcr_cassette} "
                f"with requests {vcr_cassette.requests}"
            ) from excinfo.value
    yield

Using funcargs is reliable because pytest resolves fixtures before the test runs.

3

If you don’t want to replace the exception completely, you can wrap it with additional info and keep the original traceback:

@pytest.hookimpl(hookwrapper=True)
def pytest_exception_interact(node, call, report):
    excinfo = call.excinfo
    yield
    if report.when == "call" and isinstance(excinfo.value, CannotOverwriteExistingCassetteException):
        vcr_cassette = node.funcargs.get("vcr_cassette")
        extra_msg = f"\nCassette requests: {vcr_cassette.requests if vcr_cassette else 'N/A'}"
        report.longrepr = f"{excinfo.getrepr()} {extra_msg}"

This way, your pytest output includes the cassette details without changing the exception type.

4

Sometimes accessing fixtures in the hook feels fragile, especially if the fixture hasn’t been requested by the test.

A simpler pattern is to wrap the test itself:


@pytest.fixture
def vcr_safe(vcr_cassette, vcr):
    try:
        yield
    except CannotOverwriteExistingCassetteException as e:
        raise MyCustomVCRException(
            f"Cassette {vcr_cassette} failed. Requests: {vcr_cassette.requests}"
        ) from e

Then in your test:

def test_my_api(vcr_safe, vcr_cassette, vcr):
    # your HTTP requests here

This avoids the need to deal with pytest_exception_interact and guarantees fixture access.