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gh-112331: Fix reference manual description of attribute lookup mechanics #112375

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Merged
merged 7 commits into from
Nov 25, 2023
Merged

gh-112331: Fix reference manual description of attribute lookup mechanics #112375

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07f968d
gh-112331: Fix reference manual description of attribute lookup mecha…
rhettinger Nov 24, 2023
43e3a2c
Add ~object links
rhettinger Nov 24, 2023
cf24384
Put the ~object link only in the first reference.
rhettinger Nov 24, 2023
3bace81
Missing verb
rhettinger Nov 24, 2023
6a4be55
Update Doc/reference/expressions.rst
rhettinger Nov 25, 2023
b8f4457
Update Doc/reference/expressions.rst
rhettinger Nov 25, 2023
3ebd693
Merge branch 'main' into ref_attribute_reference
rhettinger Nov 25, 2023
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18 changes: 12 additions & 6 deletions Doc/reference/expressions.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -823,12 +823,18 @@ An attribute reference is a primary followed by a period and a name:

The primary must evaluate to an object of a type that supports attribute
references, which most objects do. This object is then asked to produce the
attribute whose name is the identifier. This production can be customized by
overriding the :meth:`__getattr__` method. If this attribute is not available,
the exception :exc:`AttributeError` is raised. Otherwise, the type and value of
the object produced is determined by the object. Multiple evaluations of the
same attribute reference may yield different objects.

attribute whose name is the identifier. The type and value produced is
determined by the object. Multiple evaluations of the same attribute
reference may yield different objects.

This production can be customized by overriding the
:meth:`~object.__getattribute__` method or the :meth:`~object.__getattr__`
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Suggested change
:meth:`~object.__getattribute__` method or the :meth:`~object.__getattr__`
:meth:`~object.__getattribute__` method and/or the :meth:`~object.__getattr__`

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I think "or" is fine. The "and/or" is correct but looks weird and doesn't read well.

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I disagree that it looks weird, but won't insist

method. The :meth:`!__getattribute__` method is called first and either
returns a value or raises :exc:`AttributeError` if the attribute is not
available.
Comment on lines +832 to +834
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Of course, it is possible to create badly behaved objects that raise other exceptions from __getattribute__:

>>> class Foo:
...     def __getattribute__(self, attr):
...         if attr == 'foo':
...             raise TypeError('no')
...         return object.__getattribute__(self, attr)
...
>>> f = Foo()
>>> f.a = 1
>>> f.a
1
>>> f.b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    f.b
  File "<stdin>", line 5, in __getattribute__
    return object.__getattribute__(self, attr)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Foo' object has no attribute 'b'
>>> f.foo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    f.foo
  File "<stdin>", line 4, in __getattribute__
    raise TypeError('no')
TypeError: no

Does that need to be mentioned here? Or is it too obscure?

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@rhettinger rhettinger Nov 25, 2023 •
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No need. Any code can raise a weird error at any time but we don't need to mention that in the docs in the thousands of places where this could occur:

This wording corresponds to what the code actually does:

def getattr_hook(obj, name):
    "Emulate slot_tp_getattr_hook() in Objects/typeobject.c"
    try:
        return obj.__getattribute__(name)
    except AttributeError:
        if not hasattr(type(obj), '__getattr__'):
            raise
    return type(obj).__getattr__(obj, name)


If an :exc:`AttributeError` is raised and the object has a :meth:`!__getattr__`
method, that method is called as a fallback.

.. _subscriptions:

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