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.

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

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

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.

References:

Colorado State University Global (2024). Module 3: Understanding Python decision control structure, ITS320: Basic Programming. https://csuglobal.instructure.com/courses/88479/pages/module-3-overview?module_item_id=4620787

Hrehirchuk, M., Chalmers, E., Curtis, C., & Perri, P. (2024, January 30). 5.11 Short-circuit evaluation. Foun-dations of Python programming: Functions first. Runestone Acade-my. https://runestone.academy/ns/books/published/foppff/conditionals_short-circuit-evaluation.htmlLinks to an external site.

Kjell, B.(n.d.). Chapter 40 Boolean expressions and short-circuit operators. Precedence of logi-cal operators. Introduction to computer science using Java. Central Connecticut State Universi-ty. https://chortle.ccsu.edu/java5/Notes/chap40/ch40_16.htmlLinks to an external site.

Severance, C. (2016, July 5). 4.8. Short-circuit evaluation of logical expressions._ Python for every-body-interactive_. Runestone Academy. https://runestone.academy/ns/books/published/py4e-int/conditional/shortCircuit.html

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Originally published at Short-Circuit in Python’s Compound Conditional Expressions - Medium on August 19, 2024.