[Python-3000] self-contained exceptions

[Phillip J. Eby]

At 12:05 PM 1/10/2007 -0500, Greg Falcon wrote:
>I'm coming from a philosophy, though, that believes any time a
>syntactic construct creates a local binding and a new suite to use it
>in, the binding shouldn't escape that suite.  Yes, I'm really talking
>about 'for', and I know this is probably a controversial view.  But
>don't the same arguments apply?  (It's probably an error to use the
>variable outside the suite, and if you really need to, you could be
>explicit and store another reference to it in a separate local.)

Breaking out of a loop with a found value is way too common a use case to 
want to delete the iteration variable.


>I'd also like to add that this "foo=none; del foo" approach to fake
>block-scopes is clever, but probably deserves a bit of extra error
>checking as well.  Specifically, Python should generate a
>SyntaxWarning whenever one of these block-scope constructs binds to a
>local that was previously assigned to.  I'm worried about this mistake
>going unnoticed:
>
>re = get_resource()
>...
>try:
>   something()
>except RareException as re:
>   ... # handle it
>...
>re.use_resource()
>
>Which *appears* to work fine most of the time, but when the
>RareException is raised and handled, suddenly the use_resource() line
>generates a surprising UnboundLocalError.  These are the sort of
>things that should be caught ahead of time if possible.

We could go even further, and say it's a syntax error to bind or refer to 
<name> outside the except suite where it's bound, unless it's being used in 
another except suite.

Or, we could look at it this way: "except <expr> as <tmpvar>" and "with 
<expr> as <tmpvar> (,<tmpvar>)*", and say that a tmpvar is a normal local 
variable, *except* that it may only be used within the block where it is 
defined, or in another block that uses it as a <tmpvar>.

So, you can do as many "as" bindings of the same name as you want, but you 
can't use that name for any "non-as" use.

We could also, for that matter, prohibit:

    with x as y:
        with z as y:

since when the inner block exits, y will no longer be bound.

But I'm not sure whether we really care about any of these points.