On Wed, Aug 8, 2012 at 10:11 AM, Michael Foord fuzzyman@gmail.com wrote:
True and False are singletons, so if you want to check precisely for True and False then an identity check seems appropriate.
Technically speaking, True and False are not singletons - they are each an instance of the bool class and a singleton is the *only* instance of a class. I am not sure what the right term is for bool instances behavior, but the key feature here is that if x is known to be bool, x == True is equivalent to x is True. This property is not unique to bool an singletons like None. For example, in CPython if x is an int, x == 0 is equivalent to x is 0 because a few small integers are preallocated. However, in the case of int, this property is clearly an implementation detail and alternative implementations should be free to vary the number of preallocated integers or not preallocate at all.
With respect to bool, an argument can be made that it having only two preallocated instances is a language feature rather than an implementation detail and user can rely on the fact that True, bool(1) and say not [] all return the same instance. If this is so, I think this fact should be mentioned somewhere, but probably not in PEP 8.