[Python-ideas] Saving state in list/generator comprehension (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Mon Jun 11 08:45:46 CEST 2012

On Mon, Jun 11, 2012 at 1:09 PM, Andrew Carter <acarter at cs.hmc.edu> wrote:

 I feel like there is a need from personal experience of mapping with state in a short concise way. However it is quite possible that it is just me, and I need to think about the problem differently, or perhaps live with 4 line functions that are only used once. As for the backwards compatibility I think was getting ahead of myself, I feel the with/as solution is quite clunky, but I couldn't come up with a more elegant solution that operated in a similar vein to how python feels as a language.

Part of how Python feels as a language is due to the fact that stateful operations cannot, in general, be expressed cleanly as expressions - you have to step up to a multi-statement procedural algorithm if your state can't be expressed cleanly through simple iteration.

I and others have put forward various proposals to change this over the years, but it's a complex problem that touches on the heart of the statement/expression dichotomy that Guido deliberately introduced when creating the language.

The mechanism I personally consider most promising is one that makes it easier to be explicit that a particular function is only used in the current statement (see PEP 403). It still feels like Python (i.e. no embedded assignments), but also clearly expresses when a function exists solely for code structure purposes, and has nothing to do with splitting out a component that will be used from multiple locations.

The current design proposal in PEP 403 is still quite flawed, though, and needs a substantial amount of work to be brought up to a standard where it makes a compelling case for a change to Python.

Cheers, Nick.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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