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bpo-39425: Document list.count() corner case #18130

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bpo-39425: Document list.count() corner case #18130

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vstinner commented Jan 22, 2020 •
edited by bedevere-bot

https://bugs.python.org/issue39425
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bpo-39425: Document list.count() corner case
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vstinner commented Jan 22, 2020

cc @corona10
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vstinner commented Jan 22, 2020

FYI CPython 3.7.2, CPython master branch, PyPy 7.1.1 and PyPy3 7.1.1-beta0 all give the same result:

>>> nan=float("nan"); ([nan]*5).count(nan), nan==nan
(5, False)
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Doc/library/stdtypes.rst
@@ -994,6 +994,11 @@ Notes:
without copying any data and with the returned index being relative to
the start of the sequence rather than the start of the slice.

(9)
In CPython, :meth:`tuple.count` and :meth:`list.count` consider that an

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I would suggest rephrasing to:

In CPython, tuple.count and list.count consider an element to be equal to x if it is identical to x (if element is x is true) skipping the call to element's __eq__() method.

or maybe even

skipping the result of element == x

for the last part.
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tim-one commented Jan 22, 2020

I don't think this is a good idea, at least not on its own. The behavior is all over the place. For example, off the top of my head, in list.remove() and list.index() too. And set membership testing. And using objects as dict keys. Documenting it only for .count() methods leaves a misleading impression that they're somehow special in this respect.
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vstinner commented Jan 22, 2020

I don't think this is a good idea, at least not on its own. The behavior is all over the place. For example, off the top of my head, in list.remove() and list.index() too. And set membership testing. And using objects as dict keys. Documenting it only for .count() methods leaves a misleading impression that they're somehow special in this respect.

Can we document it somewhere else in this case?
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tim-one commented Jan 22, 2020 •
edited

Can we document it somewhere else in this case?

For all I know, it already is 😉. Sorry, I'm not intimately familiar with the docs anymore.

Because PyObject_RichCompareBool() very deliberately does this, it's intended that "pointer equality implies object equality and not object inequality" be the ordinary behavior. That's why this behavior is "all over the place". It's really x == y and x != y that show oddball behaviors here.
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