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

However, you should never compare calculated floating point numbers with each other:

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

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