About bool in Python

True+1+True=3. What bool values really are in Python.

About bool in Python

True + 1 + True = 3Permalink

Try this in Python's REPL:

True + 1 + True
# outputs: 3

What?! Yeah, it's surprising.

Bool values - True and False - can be used as numbers in Python. True is 1 and False is 0.

So we can do funny things like:

5 * False
# outputs: 0

sum([True, True, True, True, True])
# outputs: 5

True + 1 + False *10
# outputs: 2

Ok. I'm sure you get the point.

Why is that?Permalink

What if we ask for a type of True or False?

# outputs: <class 'bool'>

# outputs: <class 'bool'>

We can see that values True and False are of the type of the class bool. So why does it work like numbers too?

It is because class bool inherits from class int. The following code is in Python's source code:

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.

So bool values are backed by numbers in Python. It doesn't cause any problems most of the time, but sometimes we need to be careful.

x = True
isinstance(x, int)
# outputs: True

In the above code, we assign True to variable x. Then we check if x is an instance of class int. And it returns True. We already know that it is because bool inherits from int.

So any code that expects int will also work if it receives bool values instead.

By work, I mean it will not crash. But will it do what the user of that code is expecting? Possibly not, because bool values have a different purpose than int (number) values.

True and False are keywords in Python 3. True is always 1, and False is always 0.

# outputs 1
# outputs 0

We can also turn any int to bool:

# outputs True

# outputs True

# outputs True

# outputs False

bool(number) will output True for any number except 0. Keep this in mind. Every int number is True except for 0, which is False.

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