[New-bugs-announce] [issue45735] Promise that the long-time truth that `args=list` works

[Tim Peters]

New submission from Tim Peters <tim at python.org>:

A number of contexts allow specifying a tuple of arguments to be passed later to a function. The Thread constructor is a fine example, and happened to come up (again! for me) here today:

https://stackoverflow.com/questions/69858950/why-do-we-have-to-add-comma-in-args-in-python-multithreading/69859068

This often confuses especially newbies, because the function they intend to parallelize often takes only a single argument, and Python's syntax for a 1-element tuple actually _requires_ parentheses in the context of an argument list, with a naked trailing comma:

t = threading.Thread(target=access, args=(thread_number,))

It "looks weird" to people.

I'm not suggesting to change that, but instead to officially bless the workaround I've seen very often in real code: use a list instead.

t = threading.Thread(target=access, args=[thread_number])

Nobody scratches their head over what that means.

CPython's implementations typically couldn't care less what kind of sequence is used, and none that I'm aware of verify that it's specifically a tuple. The implementations just go on to do some simple variation of

    self.target(*self.args)

Tuple or list makes no real difference. I'm not really keen to immortalize the "any sequence type whatsoever that just happens to work" implementation behavior, but am keen to promise that a list specifically will work. A lot of code already relies on it.

----------
assignee: docs at python
components: Documentation
keywords: easy
messages: 405846
nosy: docs at python, tim.peters
priority: low
severity: normal
stage: needs patch
status: open
title: Promise that the long-time truth that `args=list` works
type: behavior
versions: Python 3.11

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45735>
_______________________________________