Logical Operators

There are 3 logical operators in Python: and, or, and not. The meaning of these operators is similar to their meaning in English.

3 < 4 and 4 > 1

It returns True

x % 2 == 0 or x % 4 == 0

It returns True if either of the conditions is true, i.e. if the number x is divisible by 2 or 4.

The not operator negates a boolean expression, so,

not (a > b )

It returns True if a > b is false i.e. if a is less than or equal to b.

Generally, the operands of the logical operators should be boolean expressions, but Python is not very strict. Any nonzero number is interpreted as True.

23 and True

It returns True.

While processing a logical expression such as x >= 2 and (x/y) > 2, Python evaluates the expression from left to right. Because of the definition of and, if x is less than 2, the expression x >= 2 is False and so the whole expression is False regardless of whether (x/y) > 2 evaluates to True or False.

So, when there is nothing to be profitable by evaluating the rest of a logical expression, it stops its evaluation and does not do the computations in the rest of the logical expression. When the evaluation of a logical expression stops because the overall value is already known, it is called short-circuiting the evaluation.

Short-circuiting helps in creating a guard. For eg,

if y != 0 and x/y:
    print("Correct")

Here, y != 0 acts as a guard to ensure that we only execute (x/y) if y is non-zero because division by 0 would give an error.