[issue4617] SyntaxError when free variable name is also an exception target

[Guido van Rossum]

Guido van Rossum <guido at python.org> added the comment:

On Mon, Feb 22, 2010 at 6:51 PM, Jeremy Hylton <report at bugs.python.org> wrote:
> There's no reason we couldn't revise the language spec to explain that
> except clauses and comprehensions are block statements, i.e.
> statements that introduce a new block.

However (even apart from the below example) it would be tough to
implement cleanly in CPython.

> For the except case, there would be some weird effects.
>
> y = 10
> try:
>  ...
> except SomeError as err:
>  y = 12
> print y  # prints 10
>
> In the example above, y would be a local variable in the scope of the
> except handler that shadows the local variable in the block that
> contains the try/except.  It might be confusing that you couldn't
> assign to a local variable in the except handler without using a
> nonlocal statement.

Yeah, there are all sorts of problems with less-conspicuous nested
scopes like this, for a language that defaults to local assignment
like Python. Hence the horrible hacks.

>> As long as we don't have nested blocks, I think it's okay to see the limitation on (implicit or explicit) "del" of a cell variable as a compiler deficiency and fix that deficiency.
>
> The general request here is to remove all the SyntaxErrors about
> deleting cell variables, right?  Instead, you'd get a NameError at
> runtime saying that the variable is currently undefined.  You'd want
> that change regardless of whether we change the language as described
> above.

Yeah, if we could kill those SyntaxErrors we can leave the rest as is.

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<http://bugs.python.org/issue4617>
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