False positive unnecessary-dunder-call for __iadd__

[sam-s]

Bug description

the following code generates an unnecessary-dunder-call message

"https://github.com/pylint-dev/pylint/issues/8978"

class Incrementable:
    "class with an incrementable field"
    def __init__(self, a):
        self.a = a
    def __iadd__(self, other):
        if isinstance(other, Incrementable):
            self.a += other.a
            return self
        return NotImplemented

d = {}
d.setdefault("a", Incrementable(6)).__iadd__(Incrementable(7))

I see 2 work-arounds, both pollute the namespace:

1. define an intermediate variable:

x = d.setdefault("a", Incrementable(6))
x += Incrementable(7)

2. Import operator:

import operator
operator.iadd(d.setdefault("a", Incrementable(6)), Incrementable(7))

Using defaultdict is even worse - I will have to convert it back to the ordinary dict for serialization...

Configuration

No response

Command used

pylint z.py

Pylint output

************* Module z
z.py:14:0: C2801: Unnecessarily calls dunder method __iadd__. Use += operator. (unnecessary-dunder-call)

Expected behavior

no diagnostics - or better one.
since += requires an extra variable, it is not an acceptable recommendation.

Pylint version

pylint 2.17.5
astroid 2.15.6
Python 3.10.12 (main, Jun 11 2023, 05:26:28) [GCC 11.4.0]

OS / Environment

windows, linux

Additional dependencies

No response

[Pierre-Sassoulas]

What about adding the __add__ function directly in Incrementable ?

"""https://github.com/pylint-dev/pylint/issues/8978"""

class Incrementable:
    """class with an incrementable field"""
    def __init__(self, a):
        self.a = a

    def __add__(self, other):
        return self.__iadd__(other)

    def __iadd__(self, other):
        if isinstance(other, Incrementable):
            self.a += other.a
            return self
        return NotImplemented

d = {}
d.setdefault("a", Incrementable(6) + Incrementable(7))

[nickdrozd]

Following the suggestion given by the warning results in code that fails to parse:

d.setdefault("a", Incrementable(6)) += Incrementable(7)  # Use += operator

E0001: Parsing failed: ''function call' is an illegal expression for augmented assignment (<unknown>, line 14)' (syntax-error)

[Pierre-Sassoulas]

Yes, but I think the message should be modified and still raised (in order to make the code more pythonic like in my proposed snippet). Something like 'create an __add__ method in 'Increment' and use it'

[sam-s]

What about adding the __add__ function directly in Incrementable ?

"""https://github.com/pylint-dev/pylint/issues/8978"""

class Incrementable:
    """class with an incrementable field"""
    def __init__(self, a):
        self.a = a

    def __add__(self, other):
        return self.__iadd__(other)

    def __iadd__(self, other):
        if isinstance(other, Incrementable):
            self.a += other.a
            return self
        return NotImplemented

d = {}
d.setdefault("a", Incrementable(6) + Incrementable(7))

this does not modify d["a"] if it is already present, i.e.

d = {"a":Incrementable(2)}
d.setdefault("a", Incrementable(6) + Incrementable(7))

is not the same as

d = {"a":Incrementable(2)}
d.setdefault("a", Incrementable(6)).__iadd__(Incrementable(7))

IOW, this code is NOT equivalent to the code that I posted in all cases.