[New-bugs-announce] [issue41706] docs: operator dunder (`__add__`, et al.) invocations described incorrectly

[William Chargin]

New submission from William Chargin <wchargin at gmail.com>:

The operator override dunder methods, like `__add__`, are described in
the “Data model” docs here:
<https://docs.python.org/3.10/reference/datamodel.html#object.__add__>

Those docs say:

> For instance, to evaluate the expression `x + y`, where `x` is an
> instance of a class that has an `__add__()` method, `x.__add__(y)` is
> called.

But this is not correct: `x + y` always uses `type(x).__add__`, which
may be different from `x.__add__` if `__add__` has been set directly on
the instance.

Demonstration:

```
class C:
    def __add__(self, other):
        return "from class"


c = C()
print(c + c)  # prints "from class"

c.__add__ = lambda other: "from instance"
print(c.__add__(c))  # prints "from instance"
print(type(c).__add__(c, c))  # prints "from class"

print(c + c)  # prints "from class"!
```

The same issue appears in the reversed operator dunder (`__radd__`,
et al.) docs below.

I have a patch that I can submit as a PR; just need a bpo number.

----------
assignee: docs at python
components: Documentation
messages: 376316
nosy: docs at python, wchargin
priority: normal
severity: normal
status: open
title: docs: operator dunder (`__add__`, et al.) invocations described incorrectly
type: enhancement
versions: Python 3.10, Python 3.6, Python 3.7, Python 3.8, Python 3.9

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41706>
_______________________________________