Can't catch GitCommandError

[Nick Papior]

I am trying to do a cherry-pick, but abort when it fails:

import git
...
git = repo.git
try:
    git.cherry_pick(commit)
except git.GitCommandError as e:
    git.cherry_pick(abort=True)
...

but I get this:

    except git.GitCommandError as e:
   TypeError: catching classes that do not inherit from BaseException is not allowed

Now I can obviously do except Exception as e... But, I would like to be explicit to defer code-paths.

[Sebastian Thiel]

Thanks for reporting!

This seems fixable and testable, and even though I don't know what's the best way to fix this, somebody coming along will.
Thanks for helping.

[Eliah Kagan]

The problem is not with the GitCommandError exception, which works fine in except clauses and, like all exceptions in Python code, does derive ultimately from BaseException.

This is instead a bug in the code you showed. That code does not catch that exception, because that exception is not what really appears in the except clause. The problem is this assignment statement:

git = repo.git

Before that, git was the git module, whose GitCommandError attribute is an exception. After that, git is an instance of the Git class. All subsequent uses of git are as if repo.git had been used. So then you have:

except git.GitCommandError as e:

Because you reassigned git, this is like:

except repo.git.GitCommandError as e:

In most cases, a bug like this would result in an AttributeError. But the attributes of a Git instance, unless otherwise present, are dynamically generated functions that, when called, invoke the git executable to run git commands. For example, repo.git.gc is a function that, when called, runs a git gc command. Likewise, repo.git.GitCommandError is a function that, when called, runs a git GitCommandError command.

(Of course, the git executable has no GitCommandError subcommand. So calling it would fail, unless a GitCommandError alias has been defined, a git-GitCommandError script or other executable can be found in $PATH, or a future or alternative version of git is in use that has such a subcommand. Being open-ended in this way allows Git instances to be usable for all those things. That the git executable recognizes no GitCommandError subcommand is not relevant to what you are observing, though, since you you are attempting to catch repo.git.GitCommandError, not to call it. The key is that attempting to catch this function as though it were an exception type definitely fails: not only is it not an exception type, it is not even a type.)

One way to fix the bug in the code you showed would be to use a different variable name in the assignment, so that git still refers to the module that it referred to due to the import statement.

[Nick Papior]

Ha, so true!!! How did I miss that :) Thanks!

[Nick Papior]

Might I propose that https://gitpython.readthedocs.io/en/latest/tutorial.html#using-git-directly changes git = repo.git to something else? That was my inspiration ;)

[Sebastian Thiel]

Thanks so much for sharing, I wasn't aware!

Do you think you could give that change a try? It's in test/test_doc.py, and it's CI-tested so nothing can go wrong.
I would love it if that could be contributed.

[Nick Papior]

done!