The documentation for the `str` class[^1] says:

> If neither encoding nor errors is given, str(object) returns object.__str__()

This led our students[^2] to try the following code:

    >>> class Test:
    ...     def __str__(self):
    ...         return 'hi'
    ... 
    >>> test = Test()
    >>> test.__str__ = lambda self: 'bye'
    >>> str(test)

The expected result was calling `test.__str__()`, but instead the interpreter output was `'hi'`.

The docs for special method lookup[^3] do say that instance attributes are ignored:

> For custom classes, implicit invocations of special methods are only guaranteed to work correctly if defined on an object’s type, not in the object’s instance dictionary.

This makes sense, and explains the observed behavior, but the `str` class docs seem to be inconsistent with this. Perhaps it would be more clear if the `str` documentation mentioned that it called `type(object).__str__()` instead of `object.__str__()`.

[^1]: https://docs.python.org/3/library/stdtypes.html#str
[^2]: CS 61A at UC Berkeley, https://cs61a.org
[^3]: https://docs.python.org/3/reference/datamodel.html#special-method-lookup

You mean `type(object).__str__(object)` instead of `type(object).__str__()`, obviously.

New changeset bb86d1d9fbd1888524e04475383f4ea764277f67 by Vanshaj Singhania in branch 'main':
bpo-47007: [doc] `str` special method lookup (GH-31863)
https://github.com/python/cpython/commit/bb86d1d9fbd1888524e04475383f4ea764277f67

Thanks for the report and fix!

New changeset f502dadb332f911fa3b6c531bbc5065795cca242 by Miss Islington (bot) in branch '3.10':
bpo-47007: [doc] `str` special method lookup (GH-31863)
https://github.com/python/cpython/commit/f502dadb332f911fa3b6c531bbc5065795cca242

New changeset c0063bdc7b5fed98c6799f3da0a954a775e3d89e by Miss Islington (bot) in branch '3.9':
bpo-47007: [doc] `str` special method lookup (GH-31863)
https://github.com/python/cpython/commit/c0063bdc7b5fed98c6799f3da0a954a775e3d89e