When an exception "as" variable occurs, it deletes local variables with the same name. This is certainly surprising, and doesn't appear to be a documented behavior (but maybe I don't know where to look). The word "bug" comes to mind.  The following few lines of code illustrate it nicely:

def testme():
    err = Exception("nothing worked")
    try:
        raise ValueError("no value")
    except ValueError as err:
        pass
    print(err)

testme()

Hi Alan, this is documented at https://docs.python.org/3/reference/compound_stmts.html#the-try-statement:


> When an exception has been assigned using as target, it is cleared at the end of the except clause. This is as if
>
> except E as N:
>    foo
>
> was translated to
>
> except E as N:
>    try:
>        foo
>    finally:
>        del N
>

This is because the exception keeps a reference to the code frame that would make a cycle with the locals which would not be destroyed until the next garbage collection.

I think you will need to use a different name in the except clause.

Yes, this is a known issue.

See https://docs.python.org/3/reference/compound_stmts.html#the-try-statement, in particular the sentence "When an exception has been assigned using as target, it is cleared at the end of the except clause" and the following text.

Basically, at the end of the exception block python does a "del err" in order to prevent a long-lived reference cycle including the exception.

Thanks for your kind explanation. I may even vaguely remember seeing this sometime in the past. Thanks much for your time!