PEP 760 – No More Bare Excepts

PEPs
103

That has been discussed a few times and is even worse for backwards compatibility since it’s not a clear break and the locations where this is an issue are hard to spot.

I still think this PEP should go through while allowing a bare except when raise is used and think the community consequences is mostly by uninformed people who are just angry that they have to do a small amount of work, but I can’t exactly force anyone to do anything.

1 Like
104

The good purpose of PEP 760 could be successful if users refrain from subclassing BaseException to create their exceptions. There is no guarantee that they will stop doing so.

When I test a library in a sandbox, I am forced to use a bare except to log any unknown exceptions before reviewing the entire library. Forcing me to write except BaseException won’t stop users from subclassing BaseException, nor will it help ensure that interrupts like ^C work properly.

105

code. edited.

106

For the record, there are many cases of bare except in the Python standard library: Code search results · GitHub

Even if the PEP is no longer proposed, if you (anybody, not just the PEP authors) believe bare excepts are bad, perhaps replacing those 150+ bare excepts with the appropriate exceptions would make sense. (I don’t think it’s worth the time, but maybe you do.)

2 Likes
107

A PR replacing except with except BaseException will probably be rejected, because changing code style only isn’t allowed. And when narrowing it, you need to be careful to not introduce bugs.

1 Like
108

You prompted me to look at a few, and I found a bug: test_traceback's PurePythonExceptionFormattingMixin does not fail correctly · Issue #125514 · python/cpython · GitHub

9 Likes
109

I’ve created a follow-up discussion thread here.

4 Likes
110

I have not read the whole discussion and does not know whether this argument was already mentioned, there is a case in which the bare except cannot be simply replaced by except BaseException.

If the code can be called very late in the interpreter shutdown process, when the builtins module has already be cleared. Then built-in BaseException can not be found. sys.stderr is already cleared as well, so most such errors are unnoticed.

The common workaround is to save a reference to BaseException in function defaults, for example:

def _cleanup(BaseException=BaseException):
    try:
        ...
    except BaseException:
        ...

But if the function is the part of the public interface, this needs rewriting more code.

4 Likes