Python 3 removes name binding from outer scope

Mon Jul 24 21:41:54 EDT 2017

Howdy all,
How can I stop Python from deleting a name binding, when that name is
used for binding the exception that is caught? When did this change in
behaviour come into Python?
I am writing code to run on both Python 2 and Python 3::
 exc = None
 try:
 1/0
 text_template = "All fine!"
 except ZeroDivisionError as exc:
 text_template = "Got exception: {exc.__class__.__name__}"
 print(text_template.format(exc=exc))
Notice that `exc` is explicitly bound before the exception handling, so
Python knows it is a name in the outer scope.
On Python 2.7, this runs fine and the ‘exc’ name survives to be used in
the ‘format’ call::
 Got exception: ZeroDivisionError
Great, this is exactly what I want: The ‘except’ clause binds the name
and I can use that name in the rest of the function to refer to the
exception object.
On Python 3.5, the ‘format’ call fails because apparently the ‘exc’
binding is *deleted*::
 Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 NameError: name 'exc' is not defined
Why is the ‘exc’ binding deleted from the outer scope? How are we meant
to reliably preserve the name binding to use it *after* the ‘except’
clause?
When did this change come into Python, where is it documented?
Would I be right to report this as a bug in Python 3?
-- 
 \ “The cost of education is trivial compared to the cost of |
 `\ ignorance.” —Thomas Jefferson |
_o__) |
Ben Finney