Example:

>>> err = None
>>> try: raise ValueError
... except ValueError as err: pass
... 
>>> err
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'err' is not defined

It is expected that either the variable will have the catched value, or save the old value, or the compiler will raise an error. But it didn't even warns.