On 2 March 2018 at 23:26, Paul Moore <p.f.moore@gmail.com> wrote:
Adding "mustn't just be another way to spell name = expr" to the
requirements is an extra requirement - and arguably one that
contradicts the original requirement which was "find a way to allow
name = expr in expressions".

I consider allowing the semantic equivalent of "name = expr" as part of arbitrary expressions to be an anti-requirement, not a requirement, for three reasons:

1. It means that arbitrary statements may accidentally clobber local variables that we care about. We used to have this for comprehension iteration variables, and it was so widely deplored that we broke compatibility with the variable overwriting behaviour in Python 3.0.
2. It means that naming a subexpression may significantly increase the longevity of that reference (especially in generators and coroutines).

3. It means that naming a subexpression will define a new class or module attribute when used in a class or module scope

Folks do sometimes *ask* for these semantics, as it's the most obvious behaviour to ask for when it comes to naming subexpressions, but it would be a really bad idea to say "yes" to those requests.

Chris's PEP addresses all those potential problems:

1. Statement locals explicitly avoid overwriting regular function locals (although they may shadow them in the current PEP - the '.name' syntax avoids even that)
2. Statement local bindings are cleared after each statement, so they only significantly extend the reference lifespan when used in compound statement headers or directly in the same statement as a yield or await. Even in comprehensions they'll typically be cleared on the next loop iteration.
3. Statement locals aren't added to locals() or globals(), so they never show up as module or class attributes either

The eventual conclusion may still be "The proposal adds too much additional complexity without an adequate corresponding gain in expressiveness", but that complexity does have a solid rationale behind it. (Although it may be worth explicitly answering "Why not use the exact same semantics as 'name = expr'? in the PEP itself, since it's a reasonable question to ask)


Nick Coghlan   |   ncoghlan@gmail.com   |   Brisbane, Australia