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When storing a bool in memcached through python-memcached I noticed that it's returned as an integer. Checking the code of the library showed me that there is a place where isinstance(val, int) is checked to flag the value as an integer.

So I tested it in the python shell and noticed the following:

>>> isinstance(True, int)
True
>>> issubclass(bool, int)
True

But why exactly is bool a subclass of int?

It kind of makes sense because a boolean basically is an int which can just take two values but it needs much less operations/space than an actual integer (no arithmetics, only a single bit of storage space)....

ThiefMaster
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    Here's [Alex Martelli's take on a related question](http://stackoverflow.com/questions/3174392/is-it-pythonic-to-use-bools-as-ints/3175293#3175293). – Sven Marnach Nov 17 '11 at 14:48
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    It's worth noting that since in Python, everything is an object, with the overhead that employs, it's pretty much pointless to try to save space by making `bool`s smaller. If you cared about memory use, you'd be using a different language to begin with. – kindall Nov 17 '11 at 17:35
  • Also there's only one copy of `True` and `False` in the entire process, so saving a few bytes on those two specific objects would have basically no impact on anything. – user2357112 Jan 25 '23 at 19:05

3 Answers3

109

From a comment on http://www.peterbe.com/plog/bool-is-int

It is perfectly logical, if you were around when the bool type was added to python (sometime around 2.2 or 2.3).

Prior to introduction of an actual bool type, 0 and 1 were the official representation for truth value, similar to C89. To avoid unnecessarily breaking non-ideal but working code, the new bool type needed to work just like 0 and 1. This goes beyond merely truth value, but all integral operations. No one would recommend using a boolean result in a numeric context, nor would most people recommend testing equality to determine truth value, no one wanted to find out the hard way just how much existing code is that way. Thus the decision to make True and False masquerade as 1 and 0, respectively. This is merely a historical artifact of the linguistic evolution.

Credit goes to dman13 for this nice explanation.

Polynomial
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    Note that this might be historically true, but idiomatically you see a lot of `sum([f(value) for value in values])` where `f(x)` is some sort of filter function and you need to see how many values pass the filter. – Adam Smith Dec 29 '15 at 00:48
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    Personally I would rather write `sum(1 for value in values if f(value))`, but I've actually seen respected people advocate in favor of numerical operations on bools. – Marius Gedminas Jun 10 '16 at 07:28
32

See PEP 285 -- Adding a bool type. Relevent passage:

6) Should bool inherit from int?

=> Yes.

In an ideal world, bool might be better implemented as a separate integer type that knows how to perform mixed-mode arithmetic. However, inheriting bool from int eases the implementation enormously (in part since all C code that calls PyInt_Check() will continue to work -- this returns true for subclasses of int).

Steven Rumbalski
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0

Can also use help to check the Bool's value in Console:

help(True)

help(True)
Help on bool object:
class bool(int)
 |  bool(x) -> bool
 |  
 |  Returns True when the argument x is true, False otherwise.
 |  The builtins True and False are the only two instances of the class bool.
 |  The class bool is a subclass of the class int, and cannot be subclassed.
 |  
 |  Method resolution order:
 |      bool
 |      int
 |      object
 |

help(False)

help(False)
Help on bool object:
class bool(int)
 |  bool(x) -> bool
 |  
 |  Returns True when the argument x is true, False otherwise.
 |  The builtins True and False are the only two instances of the class bool.
 |  The class bool is a subclass of the class int, and cannot be subclassed.
 |  
 |  Method resolution order:
 |      bool
 |      int
 |      object
aircraft
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