[New-bugs-announce] [issue32856] Optimize the `for y in [x]` idiom in comprehensions

[Serhiy Storchaka]

New submission from Serhiy Storchaka <storchaka+cpython at gmail.com>:

There were a number of discussions about adding new syntax for temporary variables in comprehensions. The last was started yesterday on Python-Ideas (https://mail.python.org/pipermail/python-ideas/2018-February/048971.html). The problem is that all syntaxes proposed before are ugly. There are common solutions of this problem (calculating common subexpression only once): using internal comprehension or generator, or refactoring the inner expression as a local function where local variables can be used. For example [f(x) + g(f(x)) for x in range(10)] can be rewritten as

    f_samples = (f(x) for x in range(10))
    [y+g(y) for y in f_samples]

or

    def func(x):
        y = f(x)
        return y + g(y)
    [func(x) for x in range(10)]

Stephan Houben suggested other idea (https://mail.python.org/pipermail/python-ideas/2018-February/048971.html): perform an assignment by iterating a one-element list.

    [y + g(y) for x in range(10) for y in [f(x)]]

I never seen this idiom before, but seems it is well known for some other developers, and it looks less clumsy than other solutions with current syntax. Its advantage over hypothetical syntax ideas is that it is an existing syntax. Its disadvantage over hypothetical syntax ideas is that iterating a one-element list is slightly slower that a simple assignment.

The proposed PR makes `for y in [f(x)]` in comprehensions as fast as just an assignment `y = f(x)`. This will make this idiom more preferable for performance reasons. Other existing solutions, iterating an inner generator and calling a local function in a loop, have an overhead.

----------
components: Interpreter Core
messages: 312228
nosy: benjamin.peterson, brett.cannon, ncoghlan, serhiy.storchaka, yselivanov
priority: normal
severity: normal
status: open
title: Optimize the `for y in [x]` idiom in comprehensions
type: performance
versions: Python 3.8

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32856>
_______________________________________