Wrong ternary suggested

[sami-evangelista]

Bug description

# pylint: disable=missing-module-docstring,invalid-name

a = True
b = False
c = True
x = (a and b) or c
print(x)  #  x == True
print(b if a else c)  #  ternary suggested == False

Configuration

No response

Command used

pylint test.py

Pylint output

************* Module test
test.py:6:0: R1706: Consider using ternary (b if a else c) (consider-using-ternary)

Expected behavior

Success.

Pylint version

pylint 2.11.1
astroid 2.8.0
Python 3.9.7 (default, Aug 30 2021, 00:00:00) 
[GCC 11.2.1 20210728 (Red Hat 11.2.1-1)]

OS / Environment

Fedora 34

Additional dependencies

No response

[areveny]

This is a continuation of #1559 and #2058 which concern the message consider-using-ternary and the behavior on statements like a and b or c where b might be False.

#1559 made the check emit simplify-boolean-expression if b is falsey, suggesting simplifying to c.

#2058 tries to infer the value of b if it is an expression. However, I think the logic is a bit faulty.

        if inferred_truth_value in (None, astroid.Uninferable):
            truth_boolean_value = True
        else:
            truth_boolean_value = truth_value.bool_value()

        if truth_boolean_value is False:
            message = 'simplify-boolean-expression'
            suggestion = false_value.as_string()
        else:
            message = 'consider-using-ternary'
            suggestion = '{truth} if {cond} else {false}'.format(
                truth=truth_value.as_string(),
                cond=cond.as_string(),
                false=false_value.as_string()
            )

If truth_value is any variable, then truth_boolean_value.bool_value() is Uninferable. This means we always suggest consider-using-ternary unless truth_value is a falsey literal. The integration test on this ticket only tests the case where truth_value is truthy, and even then it doesn't suggest consider-using-ternary because truth_boolean_value is True, but because it is Uninferable.

My fix is to change truth_boolean_value = truth_value.bool_value() to truth_boolean_value = inferred_truth_value.bool_value(). I think this was the original intention. This will try to infer the boolean value of b, and sees that it is False, and suggests simplify-boolean-expression instead.