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

Nov. 18, 2020

      On Tue, Nov 17, 2020 at 3:57 AM Mark Shannon <mark@hotpy.org> wrote:
...
On 17/11/2020 10:22 am, Cameron Simpson wrote:
...
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.
Sorry, I should have made it clearer.
I'm not asking what are the semantics of the current version of Python.
I'm asking why they are that way.
...
...
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.
But why have we chosen for it do this?
Because that solution didn't occur to anyone at the time?

Because we didn't realize anyone would want those semantics?

Because the current semantics evolved from previous semantics that didn't
erase the variable at the end of the except block?

Why do you need this question answered?
...
Wouldn't restoring the value of x be a superior option?
That depends. It would be more work to implement, which (at the time that
feature was designed, probably more so than today) was a concern.

But "superior" is rather subjective.

-- 
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-c...>