<var> is None vs. <var> == None
tjreedy at udel.edu
Mon Jan 26 05:07:44 CET 2009
>> And, just for completeness, the "is" test is canonical precisely because
>> the interpreter guarantees there is only ever one object of type None,
>> so an identity test is always appropriate. Even the copy module doesn't
>> create copies ...
> Does the interpreter guarantee the same for False and True bools?
Yes. Bool(x) should return one of the two existing instances.
In 2.x, the *names* 'True' and 'False' can be rebound because bool is
new and people write
False,True = 0,1
to make code back compatible.
In 3.0, the names are keywords, just like 'None' and cannot be rebound,
so x is True is guaranteed to answer whether x *is* the true object.
Back before rebinding 'None' was prohibited, 'is None' was not
completely guaranteed either (absent reading the rest of a file to be
sure no rebinding would be done).
More information about the Python-list