Real Python: Python Constants: Improve Your Code's Maintainability

In programming, the term constant refers to names representing values that don’t change during a program’s execution. Constants are a fundamental concept in programming, and Python developers use them in many cases. However, Python doesn’t have a dedicated syntax for defining constants. In practice, Python constants are just variables that never change.

To prevent programmers from reassigning a name that’s supposed to hold a constant, the Python community has adopted a naming convention: use uppercase letters. For every Pythonista, it’s essential to know what constants are, as well as why and when to use them.

In this tutorial, you’ll learn how to:

• Properly define constants in Python
• Identify some built-in constants
• Use constants to improve your code’s readability, reusability, and maintainability
• Apply different approaches to organize and manage constants in a project
• Use several techniques to make constants strictly constant in Python

By learning to define and use constants, you’ll dramatically improve your code’s readability, maintainability, and reusability.

To learn the most from this tutorial, you’ll need basic knowledge of Python variables, functions, modules, packages, and namespaces. You’ll also need to know the basics of object-oriented programming in Python.

Understanding Constants and Variables

Variables and constants are two historical and fundamental concepts in computer programming. Most programming languages use these concepts to manipulate data and work in an effective and logical fashion.

Variables and constants will probably be present in each project, app, library, or other piece of code that you’ll ever write. The question is: what are variables and constants in practice?

What Variables Are

In math, a variable is defined as a symbol that refers to a value or quantity that can change over time. In programming, a variable is also a symbol or name typically associated with a memory address containing a value, object, or piece of data. Like in math, the content of a programming variable can change during the execution of the code that defines it.

Variables typically have a descriptive name that’s somehow associated with a target value or object. This target value can be of any data type. So, you can use variables to represent numbers, strings, sequences, custom objects, and more.

You can perform two main operations on a variable:

1. Access its value
2. Assign it a new value

In most programming languages, you can access the value associated with a variable by citing the variable’s name in your code. To assign a new value to a given variable, you’ll use an assignment statement, which often consists of the variable’s name, an assignment operator, and the desired value.

In practice, you’ll find many examples of magnitudes, data, and objects that you can represent as variables. A few examples include temperature, speed, time, and length. Other examples of data that you can treat as variables include the number of registered users in a web app, the number of active characters in a video game, and the number of miles covered by a runner.

What Constants Are

Math also has the concept of constants. The term refers to a value or quantity that never changes. In programming, constants refer to names associated with values that never change during a program’s execution.

Just like variables, programming constants consist of two things: a name and an associated value. The name will clearly describe what the constant is all about. The value is the concrete expression of the constant itself.

Like with variables, the value associated with a given constant can be of any of data type. So, you can define integer constants, floating-point constants, character constants, string constants, and more.

After you’ve defined a constant, it’ll only allow you to perform a single operation on it. You can only access the constant’s value but not change it over time. This is different from a variable, which allows you to access its value, but also reassign it.

You’ll use constants to represent values that won’t change. You’ll find lots of these values in your day-to-day programming. A few examples include the speed of light, the number of minutes in an hour, and the name of a project’s root folder.

Why Use Constants

In most programming languages, constants protect you from accidentally changing their values somewhere in the code when you’re coding at two in the morning, causing unexpected and hard-to-debug errors. Constants also help you make your code more readable and maintainable.