[Python-Dev] PEP 479 and asyncio (original) (raw)

Chris Angelico rosuav at gmail.com
Thu Nov 27 23:32:21 CET 2014

On Fri, Nov 28, 2014 at 8:54 AM, Victor Stinner <victor.stinner at gmail.com> wrote:

def returnvalue(value): if 0: yield raise Return(value)

This is one known significant backward-incompatibility issue with this PEP - it'll be difficult to make this work on Python 2.7, where "return value" would be a problem, and 3.7, where "raise StopIteration" would be a problem. At present, I don't know of a solution to this. In 3.x-only code, you could simply use 'return value' directly; in 2.7 code, StopIteration doesn't seem to even have a .value attribute (and .args[0] has to be used instead).

But I don't like the idea of a "from past" directive. It means backward-compatibility code has to be maintained through eternity (otherwise it just shifts the problem to "why did you remove my past directive, I want a from paster import division"), which means both the Python implementation code (every Python, not just CPython) needs to cope, and everyone who reads Python code needs to cope. For python-list, Stack Overflow, and other such coding help places, this means more questions to ask about a piece of code. For real-world usage, it means scanning back up to the top of the file every time you read something that's been affected by a past directive.

Plus, which future directives need past equivalents? Personally, I wouldn't bother making "from past import lack_of_with_statement", but your friend is wanting "division", and I'm sure "print_statement" would be wanted... and, this is the one that'd split everyone and put the sides to war: "bytes_literals". Personally, I would want python-dev to say "There will NEVER be a from past import bytes_literals directive", but there are going to be others who say "But my code would be so much cleaner AND faster if you do!", and IMO this is a good reason to avoid having any past directives at all.

ChrisA

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