[Python-Dev] Why is the return value of contains coerced to boolean, but that of lt and the like is not? (original) (raw)
Steven D'Aprano steve at pearwood.info
Mon Jul 15 05:47:56 CEST 2013
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On Mon, Jul 15, 2013 at 03:34:08PM +1200, Ben Hoyt wrote:
Thanks, Nick -- that's helpful info. Writing such a PEP is a nice idea, but I think it'd be beyond me (I'm not familiar enough with CPython internals, protocols, etc).
Can you explain what you mean by "symmetric protocol rather than the current only-controlled-by-the-container behaviour"?
Most operators can be controlled by either the left-hand or right-hand
operand. For example, x + y can end up calling either x.add(y) or
y._radd(x). The in operator is an exception, it only ever calls the
container:
x in y => y.contains(x)
but never x.contained_by(y)
-- Steven
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