[Python-ideas] If branch merging

[Andrew Barnert]

On Jun 9, 2015, at 16:46, Nick Coghlan <ncoghlan at gmail.com> wrote:
> 
> However if name bindings *didn't* leak out of their containing expression by default, and while/if/elif code generation instead gained machinery to retrieve the name bindings for any named subexpressions in the condition, that would eliminate most of the potentially bizarre edge cases.

I don't think here's any consistent way to define "containing expression" that makes any sense for while/if statements. But "containing _statement_", that's easy.

In addition to the function local scope that exists today, add a statement local scope. Only an as-binding expression creates a new statement-local binding, and it does so in the smallest containing statement (so, e.g., in a while statement's condition, it's the whole while statement, suite and else suite as well as the rest of the condition). These bindings shadow outer as-bindings and function-locals. Assignments inside a statement that as-binds the variable change the statement-local variable, rather than creating a function-local. Two as-bindings within the same statement are treated like an as-binding followed by assignment in the normal (possibly implementation-dependent) evaluation order (which should rarely be relevant, unless you're deliberately writing pathological code). Of course this is much more complex than Python's current rules. But it's not that hard to reason about. In particular, even in silly cases akin to "x[(a.b as b)] = b" and "x[b] = (a.b as b)", either it does what you'd naively expect or raises an UnboundLocalError; it never uses any outer value of b. And, I think, in all of the cases you actually want people to use, it means what you want it to. It even handles cases where you put multiple as bindings for the same name in different subexpressions of an expression in the same part of a statement.

Now, for implementation: any statement that contains an as expression anywhere is compiled to a function definition and a call to that function. The only trick is that any free variables have to be compiled as nonlocals in the inner function and as captured locals in the real function. (This trick doesn't have to apply to lambdas or comprehensions, because they can't have assignment statements inside them, but a while statement can.) I believe this scales to nested statements with as-bindings, and to as-bindings inside explicit local functions and vice-versa.

The question is, is that the behavior you'd intuitively want, or is escaping to the rest of the smallest statement sometimes unacceptable, or are the rules about assignments inside a controlled suite wrong in some case?