New submission from Giacomo Alzetta:
Currently the documentation for set (at:
http://docs.python.org/2/library/stdtypes.html#set) does not mention which
operand is preferred when performing the usual binary operations.
For example the following sample code doesn't have a defined result according
to the documentation:
class MyObj:
def __init__(self, key, value):
self.key = key
self.value = value
def __key(self):
return self.key
def __hash__(self):
return hash(self.__key())
def __eq__(self, other):
return type(self) is type(other) and self.__key() == other.__key()
a = {MyObj(1, 'a')}
b = {MyObj(1, 'b')}
print((a & b).pop().value) # 'a' or 'b'?
As far as I can tell currently there is no rule about this. Intersection
prefers the second operand, while union prefers the first. These are the only
operations where this is important since they include common elements.
I believe that this kind of information ought to be included explicitly near
such operations. At the very least, if the behaviour should really be somehow
undefined, it should be stated there so that people don't rely on such
behaviour.
The same should be stated for |= and &=, since the first will not modify common
elements while the latter will.
PS: I can't find any resource about the formatting of issues (e.g. if and which
syntax is used for code blocks. Sorry for that.
----------
assignee: docs@python
components: Documentation
messages: 213305
nosy: Giacomo.Alzetta, docs@python
priority: normal
severity: normal
status: open
title: Which operand is preferred by set operations? Missing information in the
documentation
type: enhancement
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue20902>
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