Is False == 0 and True == 1 an Implementation Detail or Is It Guaranteed by the Language

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. ;-)

Is it guaranteed that False is 0 and True is 1?

It is part of the language specification, so any Python implementation should implement the booleans as equivalent to the integers.

Booleans

These represent the truth values False and True. The two objects representing the values False and True are the only Boolean objects. The Boolean type is a subtype of plain integers, and 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.

Why True value returns True with membership in function in python

True is implictly converted to a number, 1. (False would be converted to 0) The in operator checks if any element is equal to the left operand, and True is equal to the first element, 1.

>>> True == 1
True

Why can I use True or False as indexes of a list?

Since bool is a subclass of int that's why True acts like 1 and False like 0

Python bool + bool returns value 2

input() in Python 3 returns a string. No data conversions are performed. So the inputs you get are, literally, the strings "True" and/or "False". Then bool() applied to a non-empty string returns True.

>>> bool("True")
True
>>> bool("False")
True
>>> bool("just about anything")
True
>>> bool("") # except an empty string
False

That's why you always get 2 no matter what you enter. Both inputs become True, which equals 1 in a numeric context.

>>> True == 1
True

While people will yell at you if you do this, eval() is the most convenient (although dangerous!) way to evaluate a string as if it were Python code:

>>> eval("True")
True
>>> eval("False")
False

It's dangerous because eval() will evaluate any Python expression, so if you don't trust your input it can trick your program into doing just about anything.

How can you add and multiple to True or False?

The answer is 5.25 because in Python, True is equal to 1 while False is equal to 0.

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