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.
----------
nosy: +steven.daprano
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<http://bugs.python.org/issue27605>
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