[issue27605] Inconsistent calls to __eq__ from built-in __contains__

[Steven D'Aprano]

Steven D'Aprano added the comment:

I'm reluctant to call this a behaviour bug, because I'm reluctant to guarantee the *precise* behaviour when the classes are different.

I haven't checked the source of dict.__contains__, but by experimentation it seems that:

    needle in {key: value}

calls key == needle, rather than needle == key as the docs would imply. This seems to apply going all the way back to at least CPython2.4.

On the other hand, Jython2.5 and IronPython2.6 seem to call needle == key as Vasily expected.

But the docs are clearly incomplete, as they suggest a linear search through all the keys of the dict (or set), which is wrong. Only if needle hashes to the same value as key will == be called.

I suggest updating the docs to make it clear that the sample code shown is only the *approximate* behaviour, which may call == in either direction.

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nosy: +steven.daprano

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27605>
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