[Python-Dev] Re: Why does "except Ex as x" not restore the previous value of x?

Nov. 17, 2020

      On 17Nov2020 09:55, Mark Shannon <mark@hotpy.org> wrote:
...
I'm wondering why
```
x = "value"
try:
   1/0
except Exception as x:
   pass
```
does not restore "value" to x after
the `except` block.
Because the except is not a new scope. So it is the same "x".

Here:

    https://docs.python.org/3/reference/compound_stmts.html#try

it says:

    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 means the exception must be assigned to a different name to be 
    able to refer to it after the except clause. Exceptions are cleared 
    because with the traceback attached to them, they form a reference 
    cycle with the stack frame, keeping all locals in that frame alive 
    until the next garbage collection occurs.
...
Here's an example of restoring the value of the variable after the 
`except` block:
...
...
...
def f(x):
...     try:
...         1/0
...     except Exception as x:
...         pass
...     return x
...
f("hi")
'hi'
In the Python 3.8.5 I don't see this:

    Python 3.8.5 (default, Jul 21 2020, 10:48:26)
    [Clang 11.0.3 (clang-1103.0.32.62)] on darwin
    Type "help", "copyright", "credits" or "license" for more information.
    >>> def f(x):
    ...   try:
    ...     1/0
    ...   except Exception as x:
    ...     pass
    ...   return x
    ...
    >>> f(3)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<stdin>", line 6, in f
    UnboundLocalError: local variable 'x' referenced before assignment

and the same outside a function.

Cheers,
Cameron Simpson <cs@cskk.id.au>