[Python-Dev] Informal educator feedback on PEP 572 (was Re: 2018 Python Language Summit coverage, last part) (original) (raw)

Ivan Pozdeev vano at mail.mipt.ru
Mon Jun 25 11:01:54 EDT 2018

On 25.06.2018 14:44, Nick Coghlan wrote:

On 25 June 2018 at 02:24, Guido van Rossum <guido at python.org> wrote:

A quick follow-up: PEP 572 currently has two ideas: (a) introduce := for inline assignment, (b) when := is used in a comprehension, set the scope for the target as if the assignment occurred outside any comprehensions. It seems we have more support for (a) than for (b) -- at least Nick and Greg seem to be +0 or better for (a) Right, the proposed blunt solution to "Should I use 'NAME = EXPR' or 'NAME := EXPR'?" bothers me a bit, but it's the implementation implications of parent local scoping that I fear will create a semantic tar pit we can't get out of later.

but -1 for (b). IIRC (b) originated with Tim. But his essay on the topic, included as Appendix A (https://www.python.org/dev/peps/pep-0572/#appendix-a-tim-peters-s-findings) does not even mention comprehensions. However, he did post his motivation for (b) on python-ideas, IIRC a bit before PyCon; and the main text of the PEP gives a strong motivation (https://www.python.org/dev/peps/pep-0572/#scope-of-the-target). Nevertheless, maybe we should compromise and drop (b)? Unfortunately, I think the key rationale for (b) is that if you don't do something along those lines, then there's a different strange scoping discrepancy that arises between the non-comprehension forms of container displays and the comprehension forms: (NAME := EXPR,) # Binds a local tuple(NAME := EXPR for _ in range(1)) # Doesn't bind a local [NAME := EXPR] # Binds a local [NAME := EXPR for _ in range(1)] # Doesn't bind a local list(NAME := EXPR for _ in range(1)) # Doesn't bind a local {NAME := EXPR} # Binds a local {NAME := EXPR for _ in range(1)} # Doesn't bind a local set(NAME := EXPR for _ in range(1)) # Doesn't bind a local {NAME := EXPR : EXPR2} # Binds a local {NAME := EXPR : EXPR2 for _ in range(1)} # Doesn't bind a local set((NAME := EXPR, EXPR2) for _ in range(1)) # Doesn't bind a local Those scoping inconsistencies aren't new, but provoking them currently involves either class scopes, or messing about with locals().

I've got an idea about this.

The fact is, assignments don't make much sense in an arbitrary part of a comprehension: `for' variables are assigned every iteration, so when the result is returned, only the final value will be seen. (And if you need a value every iteration, just go the explicit way and add it to the returned tuple.)

Contrary to that, the "feeder" expression is only evaluated once at the start -- there, assignments do make sense. Effectively, it's equivalent to an additional line:

seq = range(calculate_b() as bottom, calculate_t() as top) results = [calculate_r(bottom,r,top) for r in seq]

So, I suggest to evaluate the "feeder" expression in a local scope but expressions that are evaluated every iteration in a private scope.

-- Regards, Ivan

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