How to raise a DeprecationWarning when catching an exception with python?

Question

I wrote a library that sometimes raises exceptions. There is an exception that I want to deprecate, and I would like to advise people to stop catching them, and provide advises in the warning message. But how to make an exception emit a DeprecationWarning when catched?

library code

import warnings

class MyException(ValueError):
    ...
    warnings.warn(
        "MyException is deprecated and will soon be replaced by `ValueError`.",
        DeprecationWarning,
        stacklevel=2,
    )
    ...

def something():
    raise MyException()

user code

try:
    mylib.something()
except MyException: # <-- raise a DeprecationWarning here
    pass

How can I modify MyException to achieve this?

You've edited your question to make it so there's no clear transition path. `except MyException` is deprecated, but there's no good alternative for users to use instead. — user2357112, Sep 14 '20 at 21:49
Related: [module `__getattr__`](https://stackoverflow.com/questions/2447353/getattr-on-a-module). — wim, Sep 14 '20 at 22:01
The question is, if catching this exception is deprecated, what else should your users do? — a_guest, Sep 17 '20 at 07:24

user2357112 · Answer 1 · 2020-09-14T22:13:18.613

10

You can't. None of the logic that occurs in except MyException is customizable. Particularly, it completely ignores things like __instancecheck__ or __subclasscheck__, so you can't hook into the process of determining whether an exception matches an exception class.

The closest you can get is having the warning happen when a user tries to access your exception class with from yourmodule import MyException or yourmodule.MyException. You can do that with a module __getattr__:

class MyException(ValueError):
    ...

# access _MyException instead of MyException to avoid warning
# useful if other submodules of a package need to use this exception
# also use _MyException within this file - module __getattr__ won't apply.
_MyException = MyException
del MyException

def __getattr__(name):
    if name == 'MyException':
        # issue warning
        return _MyException
    raise AttributeError

edited Sep 14 '20 at 22:13

answered Sep 14 '20 at 22:00

user2357112

260,549
28
431
505

Won't work as written. `__getattr__` is only invoked if the name is not found by the normal means. – wim Sep 14 '20 at 22:05
@wim: Oh, whoops - I meant to have a `del MyException` there, but I forgot to write it. Fixed now. – user2357112 Sep 14 '20 at 22:06
2

+1 This is the correct way, and deprecating names is actually the first [rationale given in the PEP](https://www.python.org/dev/peps/pep-0562/#rationale) for the module `__getattr__` feature. – wim Sep 14 '20 at 22:13

score -1 · Answer 2 · answered Sep 11 '20 at 08:27

-1

Try using this:

import warnings


class MyOtherException(Exception):
    pass


class MyException(MyOtherException):
    def __init__(self):
        warnings.warn(
            "MyException is deprecated and will soon be replaced by `MyOtherException`.",
            DeprecationWarning,
            stacklevel=2,
        )


if __name__ == "__main__":
    try:
        mylib.something()
    except Exception:
        raise MyException()

answered Sep 11 '20 at 08:27

theNishant

663
5
15

I can only edit the `library code` and not the `user code`, so I cannot assume anything going in the except block. No guarantee that the user will raise a `MyException` – azmeuk Sep 11 '20 at 12:44
This is missing the point. User code might catch `MyException` class without ever instantiating one. – wim Sep 14 '20 at 22:08

How to raise a DeprecationWarning when catching an exception with python?

library code

user code

2 Answers2