Is False == 0 and True == 1 an implementation detail or is it guaranteed by the language?

In Python 2.x this is not guaranteed as it is possible for True and False to be reassigned. However, even if this happens, boolean True and boolean False are still properly returned for comparisons.

In Python 3.x True and False are keywords and will always be equal to 1 and 0.

Under normal circumstances in Python 2, and always in Python 3:

False object is of type bool which is a subclass of int:

object
   |
 int
   |
 bool

It is the only reason why in your example, ['zero', 'one'][False] does work. It would not work with an object which is not a subclass of integer, because list indexing only works with integers, or objects that define a __index__ method (thanks mark-dickinson).

Edit:

It is true of the current python version, and of that of Python 3. The docs for python 2 and the docs for Python 3 both say:

There are two types of integers: [...] Integers (int) [...] Booleans (bool)

and in the boolean subsection:

Booleans: These represent the truth values False and True [...] Boolean values behave like the values 0 and 1, respectively, in almost all contexts, the exception being that when converted to a string, the strings "False" or "True" are returned, respectively.

There is also, for Python 2:

In numeric contexts (for example when used as the argument to an arithmetic operator), they [False and True] behave like the integers 0 and 1, respectively.

So booleans are explicitly considered as integers in Python 2 and 3.

So you're safe until Python 4 comes along. ;-)

Link to the PEP discussing the new bool type in Python 2.3: http://www.python.org/dev/peps/pep-0285/.

When converting a bool to an int, the integer value is always 0 or 1, but when converting an int to a bool, the boolean value is True for all integers except 0.

>>> int(False)
0
>>> int(True)
1
>>> bool(5)
True
>>> bool(-5)
True
>>> bool(0)
False

In Python 2.x, it is not guaranteed at all:

>>> False = 5
>>> 0 == False
False

So it could change. In Python 3.x, True, False, and None are reserved words, so the above code would not work.

In general, with booleans you should assume that while False will always have an integer value of 0 (so long as you don't change it, as above), True could have any other value. I wouldn't necessarily rely on any guarantee that True==1, but on Python 3.x, this will always be the case, no matter what.