PyCharm vs. exceptiongroup

BayBE’s (de-)serialization machinery is build upon cattrs, which in turn relies on ExceptionGroups to report problems in a nicely structured format when using its detailed validation feature. However, ExceptionGroups were introduced in Python 3.11 and are therefore not usable with earlier Python versions. To enable the feature nevertheless, cattrs uses the exceptiongroup backport, which enables the same functionality by monkeypatching TracebackException and installing a special exception hook on sys.excepthook.

The Problem

The changes attempted by exceptiongroup will only be executed if no prior modifications have been made. However, PyCharm appears to make similar modifications for its own purposes, blocking those of exceptiongroup and thus preventing the exceptions from being properly thrown in detailed validation mode.

When to encounter

The chances of encountering this problem when interacting with BayBE are rather low as the (de-)serialization objects are usually created by BayBE itself under normal operation, so there is little risk of them being invalid in the first place. A potential situation where you might run into the problem is if you manually write a BayBE configuration and try to deserialize it into a Python BayBE object. This can happen, for example, while engineering the configuration for later API calls and testing it locally using PyCharm.

How to avoid

If you encounter the problem, you can use any of the following workarounds to circumvent the problem:

  • Run the code from the terminal instead of inside PyCharm

  • Change PyCharm’s run configuration from “Run with Python Console” to “Emulate terminal in output console”

  • Use Python version 3.11 or higher

  • Undo the monkeypatch applied by PyCharm by running the following code at the start of your script:

import sys
sys.excepthook = sys.__excepthook__

import exceptiongroup
from cattrs import ClassValidationError

try:
    <problematic code>
except ClassValidationError as e:
    raise ValueError("".join(exceptiongroup.format_exception(e)))