[python] How do I get the opposite (negation) of a Boolean in Python?

For the following sample:

def fuctionName(int, bool):
    if int in range(...):
        if bool == True:
            return False
        else:
            return True

Is there any way to skip the second if-statement? Just to tell the computer to return the opposite of the boolean bool?

Python has a "not" operator, right? Is it not just "not"? As in,

  return not bool

The accepted answer here is the most correct for the given scenario.

It made me wonder though about simply inverting a boolean value in general. It turns out the accepted solution here works as one liner, and there's another one-liner that works as well. Assuming you have a variable "n" that you know is a boolean, the easiest ways to invert it are:

n = n is False

which was my original solution, and then the accepted answer from this question:

n = not n

The latter IS more clear, but I wondered about performance and hucked it through timeit - and it turns out at n = not n is also the FASTER way to invert the boolean value.

The not operator (logical negation)

Probably the best way is using the operator not:

>>> value = True
>>> not value
False

>>> value = False
>>> not value
True

So instead of your code:

if bool == True:
    return False
else:
    return True

You could use:

return not bool

The logical negation as function

There are also two functions in the operator module operator.not_ and it's alias operator.__not__ in case you need it as function instead of as operator:

>>> import operator
>>> operator.not_(False)
True
>>> operator.not_(True)
False

These can be useful if you want to use a function that requires a predicate-function or a callback.

For example map or filter:

>>> lst = [True, False, True, False]
>>> list(map(operator.not_, lst))
[False, True, False, True]

>>> lst = [True, False, True, False]
>>> list(filter(operator.not_, lst))
[False, False]

Of course the same could also be achieved with an equivalent lambda function:

>>> my_not_function = lambda item: not item

>>> list(map(my_not_function, lst))
[False, True, False, True]

Do not use the bitwise invert operator ~ on booleans

One might be tempted to use the bitwise invert operator ~ or the equivalent operator function operator.inv (or one of the other 3 aliases there). But because bool is a subclass of int the result could be unexpected because it doesn't return the "inverse boolean", it returns the "inverse integer":

>>> ~True
-2
>>> ~False
-1

That's because True is equivalent to 1 and False to 0 and bitwise inversion operates on the bitwise representation of the integers 1 and 0.

So these cannot be used to "negate" a bool.

Negation with NumPy arrays (and subclasses)

If you're dealing with NumPy arrays (or subclasses like pandas.Series or pandas.DataFrame) containing booleans you can actually use the bitwise inverse operator (~) to negate all booleans in an array:

>>> import numpy as np
>>> arr = np.array([True, False, True, False])
>>> ~arr
array([False,  True, False,  True])

Or the equivalent NumPy function:

>>> np.bitwise_not(arr)
array([False,  True, False,  True])

You cannot use the not operator or the operator.not function on NumPy arrays because these require that these return a single bool (not an array of booleans), however NumPy also contains a logical not function that works element-wise:

>>> np.logical_not(arr)
array([False,  True, False,  True])

That can also be applied to non-boolean arrays:

>>> arr = np.array([0, 1, 2, 0])
>>> np.logical_not(arr)
array([ True, False, False,  True])

Customizing your own classes

not works by calling bool on the value and negate the result. In the simplest case the truth value will just call __bool__ on the object.

So by implementing __bool__ (or __nonzero__ in Python 2) you can customize the truth value and thus the result of not:

class Test(object):
    def __init__(self, value):
        self._value = value

    def __bool__(self):
        print('__bool__ called on {!r}'.format(self))
        return bool(self._value)

    __nonzero__ = __bool__  # Python 2 compatibility

    def __repr__(self):
        return '{self.__class__.__name__}({self._value!r})'.format(self=self)

I added a print statement so you can verify that it really calls the method:

>>> a = Test(10)
>>> not a
__bool__ called on Test(10)
False

Likewise you could implement the __invert__ method to implement the behavior when ~ is applied:

class Test(object):
    def __init__(self, value):
        self._value = value

    def __invert__(self):
        print('__invert__ called on {!r}'.format(self))
        return not self._value

    def __repr__(self):
        return '{self.__class__.__name__}({self._value!r})'.format(self=self)

Again with a print call to see that it is actually called:

>>> a = Test(True)
>>> ~a
__invert__ called on Test(True)
False

>>> a = Test(False)
>>> ~a
__invert__ called on Test(False)
True

However implementing __invert__ like that could be confusing because it's behavior is different from "normal" Python behavior. If you ever do that clearly document it and make sure that it has a pretty good (and common) use-case.

You can just compare the boolean array. For example

X = [True, False, True]

then

Y = X == False

would give you

Y = [False, True, False]

If you are trying to implement a toggle, so that anytime you re-run a persistent code its being negated, you can achieve that as following:

try:
    toggle = not toggle
except NameError:
    toggle = True

Running this code will first set the toggle to True and anytime this snippet ist called, toggle will be negated.

Another way to achieve the same outcome, which I found useful for a pandas dataframe.

As suggested below by mousetail:

bool(1 - False)

bool(1 - True)