False positive ``expression-not-assigned`` when calling functions with no return inside ternary expression

[da-dada]

Bug description

def f1():
    pass


def f2():
    pass


test = True
f1() if test else f2()

Configuration

No response

Command used

pylint test.py

Pylint output

test.py:9: [W0106(expression-not-assigned), ] Expression "f1() if test else f2()" is assigned to nothing

Expected behavior

no warning (according to documentation of pylint)

Pylint version

2.16.0.b1

OS / Environment

windows

[mbyrnepr2]

Thanks for the report @da-dada. This one looks like expected behaviour. Please see the doc for some examples of why this message is triggered: https://pylint.pycqa.org/en/latest/user_guide/messages/warning/expression-not-assigned.html

[mbyrnepr2]

Re-opening. Indeed the checker states: Used when an expression that is not a function call is assigned to nothing.

[nickdrozd]

I think the expression in question is the whole conditional expression f1() if test else f2(), which isn't itself a function call. And the value produced by the conditional expression isn't assigned to anything.

[mbyrnepr2]

If it is re-written as the following, the message goes away; so I think it makes sense for consistency to consider the one-line version a false positive:

if test:
    f1()
else:
    f2()

[nickdrozd]

Another way to handle the warning is to explicitly ignore the value:

_ = f1() if test else f2()

This acknowledges that a value is being produced and not used.

Switching to an if / else block would be the better choice if the purpose of calls is to execute side effects. It's not wrong to use a conditional expression for side effects, but it is a little irregular.

I guess the question is: did they forget to use the value produced by the conditional expression, or did they just want to do a branch as a one-liner?

[da-dada]

to get it into perspective I use it to call functions from a library (wxpython) underneath

wx.CallAfter(self.Layout) if evt_size else self.Layout()

and they don't give any value back !

discarding a one-liner just to please the linter does not really sound pythonic: I would say it starts to smell..

[Pierre-Sassoulas]

We did not detect this false positive in our primer that include pandas, django, numpy, sentry, flask, psychopg, coveragepy, pytest or pygame ..among others. So I would say that calling function conditionally in one liner is not wrong, but pretty unusual for python codebase.