Python:Logical Operators

Logical operators in Python are those that link logical statements together and return True or False depending upon the nature of the logical operator and the value of the components. Many times, logical operators are used to link together the results of several relational operators.

Logical Operators

There are three main logical operators in Python:

Name	Example in code	Text description
NOT	not A	True if A is False; False if A is True
AND	A and B	True if both A and B are true
OR	A or B	True if either A or B (or both!) are true

There is also an exclusive or, XOR, which is True if one and only one component is True. Python lacks a command for this; instead, you can define one with:

def xor(a, b):
    return (a or b) and not (a and b)

or, for an arbitrary number of inputs,

def xor(*vals):
    out = False
    for val in vals:
        if val and not out:
            out = True
        elif val and out:
            return False
        
    return out

The following table shows the results of four different pairs of input values to the logical operators above:

A	B	not A	not B	A and B	A or B	xor(A, B)
False	False	True	True	False	False	False
False	True	True	False	False	True	True
True	False	False	True	False	True	True
True	True	False	False	True	True	False

Common Errors

The most common errors with logical operators involve the order of operations. In the absence of parenthesis, any NOT operations go first, followed by AND, then finally OR. The following two statements both evaluate to True because the and happens first:

True or False and False
False and False or True

On the other hand, the following two statements evaluate to False because the or is forced to happen first:

(True or False) and False
False and (False or True)

If there are multiple statements of the same kind, they evaluate from left to right.

Questions

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