Short-Circuit in Python's Compound Conditional Expressions

This article explains how Python's short-circuit evaluation in compound conditional expressions enhances efficiency by stopping the evaluation as soon as the outcome is determined.

Alexander S. Ricciardi

February 25th, 2024

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To understand the concept of short-circuiting in compound conditional expressions in Python, it is important to be familiar with the logical operators and and or. The table below summarizes the logical outcomes for these operators.

Table 1

The ‘and’ and ‘or’ Operators

A	B	A  AND B	A OR B
False	False	False	False
False	True	False	True
True	False	False	True
True	True	True	True

Note: From Module 3: Understanding Python decision control structure, ITS320: Basic Programming, by Colorado State University Global, 2024. Modified 2024, February 25.

In Python, short-circuiting in the context of compounded conditional expressions is when the interpreter stops evaluating a logical expression as soon as the logical expression outcome is determined (Severance, 2016).

In other words, when in the process of reading a logical expression, if the interpreter can determine the outcome of the expression before reaching the end of it, it would stop reading the expression.Note: the interpreter reads from left to right.

This occurs when using the operators and and or in an expression. This is called a short-circuit boolean evaluation. (Hrehirchuk et al, 2024)

For example:

When using the and operator:

a = 1
b = 2
c = 3
d = 4
if a < b and a > c and a < d:
              #--- Do something

Here the short-circuiting happens when the Python interpreter stops evaluating the logical expression a < b and a > c and a < d at step a > c because a > c returns False. Consequently, the expression a < b and a > c and a < d is False, it does not matter if the expression 

a < d returns False or True.

When using the or operator:

a = 1
b = 2
c = 3
d = 4

 if a > b or a < c or a > d:
            #--- Do something

Here the short-circuiting happens when the Python interpreter stops evaluating the logical expression a > b or a < c or a > d at step a < c because a < c returns True. Consequently, the expression a > b or a < c or a > d is True, it does not matter if the expression 

a > d returns False or True.

When using a combination of and and or logical operators, the and operator has precedent over the or operator. This is similar to the arithmetic operator precedence between '+' and '*', where '*' has precedence over '+'.The table below depicts the logical operators' precedence utilizing parentheses.

Table 2

Precedence of Logical Operators

A or B and C	means	A or (B and C)
A and B or C and D	means	(A and B) or (C and D)
A and B and C or D	means	((A and B) and C) or D
!A and B or C	means	((!A) and B) and C

Note: from Chapter 40 Boolean Expressions and Short-Circuit Operators - Precedence of Logical Operators, by Kjell, n.d. Modified 2024, February 25.

In conclusion, short-circuiting occurs when the logical operators and and or determine when the Python interpreter stops evaluating an expression once the outcome is clear. For example, when the operator and is used it stops the evaluation at the first False and when the operator or is used it stops at the first True, this enhances efficiency. Therefore, understanding short-circuit evaluation in Python is crucial for writing efficient and effective conditional expressions.