Boolean values and expressions

In Python, there are several ways to express Boolean values; the Boolean constant False, 0, the Python type None and empty values (for example the empty list [] or the empty string "") are all considered False. The Boolean constant True and everything else is considered True.

<, <=, ==, >, >=

compares values:

>>> x = 3
>>> y = 3.0
>>> z = [3, 4, 5]
>>> x == y
True

Warning

However, you should never directly compare calculated floating point numbers:

>>> u = 0.6 * 7
>>> v = 0.7 * 6
>>> u == v
False
>>> u
4.2
>>> v
4.199999999999999

Instead, you can use math.isclose():

>>> import math
>>> math.isclose(u, v)
True

Alternatively, you can also use round():

>>> round(u, 2) == round(v, 2)
True

Warning

Integers smaller than -5 or larger than 256 are recreated each time, but integers in between are pre-instantiated by the interpreter and reused. Therefore, id() should not be used for comparisons of integers:

>>> x = 256
>>> y = 257
>>> id(x) == id(256)
True
>>> id(y) == id(257)
False

is, is not, in, not in

checks the identity:

>>> x is y
False
>>> x is not y
True
>>> x in z
True
>>> id(x)
4375911432
>>> id(y)
4367574480
>>> id(z[0])
4375911432

If x and z[0] have the same ID in memory, this means that we are referring to the same object in two places.

The is operator is mostly used for values that only exist once in memory, so-called singleton objects. For example, checking for None is the most common use of the is operator.

>>> x is None
False
>>> x is not None
True

The Python style guide in PEP 8 also recommends that you should check for identity with None and not for values, so never use x == None, but always use x is None instead.

and, not, or

are logical operators that we can use to link the above checks:

>>> x is y and x is z[0]
False
>>> x is y or x is z[0]
True
>>> x is y and not x is z[0]
False
>>> x is z[0] and not x is y
True