Python 3 UnboundLocalError not detected when variable referenced after shadowed by exception variable

[jayvdb]

For

def a():
    e = 1
    try:
        raise RuntimeError('forced error')
    except Exception as e:
        print(e)
    print(e)

a()

In both Python 2 and 3, exception variable e shadows the local variable e, and is not being reported.

In python3, e is unbound after the exception block, and a UnboundLocalError occurs when the local variable e is referenced because it was disappeared.

[jayvdb]

Note this was raised a lot at #59 , but the issue of shadowing wasnt considered.

The following does not raise an error UnboundLocalError on Python 3 ; instead e is 1 after the exception block.

def a():
    e = 1

    try:
        pass
    except Exception as e:
        print(e)

    print(e)

a()

As we can not know whether the try block will pass or which exception might fail, we can not know which local variables were discarded.

Thus the shadowing actually results in very non-deterministic behaviour, and should be considered a bug.

[jayvdb]

Note solving this, especially with python2 support, may depend on solving at least parts of #236 first, which puts it in the moderately hard bucket.
While shadowing is problematic, it is relatively common on python 2 to start with e = None followed by multiple try blocks to re-use the same exception name e and then some reporting to occur at the end if any of them assigned a value to e.
Solving it only for python 3 is quite a bit easier.