Encountered a weird behavior when working with variable with the same name as exception's alias. Observed behavior: - In case variable with the same name (e.g. 'e') already exists when any 'except Error as e' block is executed, the 'e' variable is removed after exception handling finishes (NameError: name 'e' is not defined) - Happens only in Python 3 - Code reproducing the issue included Expected behavior: - Variable is kept in its pre-exception state Proposed solution: - Store colliding variable into temporary variable, restore it afterwards
I don't believe this is a bug. 1) try/except does not introduce a new scope 2) 'except E as N' is just name assignment 3) except clauses are documented to always delete the exception alias at the end of the except block[0]. The example in the docs could be clarified to show that except E as N: foo is really more like except E: N = sys.exc_info()[1] try: foo finally: del N Note that 'as' assignment with the 'with' statement also overrides a local variable of the same name. The simplest way to avoid this issue is to use a different name for the exception alias. [0] https://docs.python.org/3/reference/compound_stmts.html#the-try-statement
So the exception is explicitly deleted when the `except` block is exited to prevent leaking memory from the traceback attached to the exception. Hence there's an implicit `del e` at the end of the `except` block which is what you're running up against. But I'm closing this as 'wont fix' because making this edge case work would be real troublesome and destroy performance whenever you bound the caught exception. Basically you would have to do the equivalent of: e = 42 try: 1/0 except ZeroDivisionError as _hidden_e: _overridden = False try: _old_e = e _overridden = True except NameError: pass e = _hidden_e # `except` block ... del e, _hidden_e if _hidden_flag: e = _old_e That's a lot of code to run on every `except` clause that just happens to shadow a previously existing variable name. Because we try and not make exceptions too expensive in order to make them usable for occasional control flow, I don't think we can afford to add all of this for this edge case.
