A constant is a value that cannot be altered by the program during normal execution, i.e., the value is constant. When associated with an identifier, a constant is said to be “named,” although the terms “constant” and “named constant” are often used interchangeably. This is contrasted with a variable, which is an identifier with a value that can be changed during normal execution, i.e., the value is variable.
A constant is a data item whose value cannot change during the program’s execution. Thus, as its name implies – the value is constant.
A variable is a data item whose value can change during the program’s execution. Thus, as its name implies – the value can vary.
Constants are used in two ways. They are:
- literal constant
- defined constant
A literal constant is a value you type into your program wherever it is needed. Examples include the constants used for initializing a variable and constants used in lines of code:
21 12.34 'A' "Hello world!" false null
In addition to literal constants, most textbooks refer to symbolic constants or named constants as a constant represented by a name. Many programming languages use ALL CAPS to define named constants.
Technically, Python does not support named constants, meaning that it is possible (but never good practice) to change the value of a constant later. There are workarounds for creating constants in Python, but they are beyond the scope of a first-semester textbook.
Defining Constants and Variables
Named constants must be assigned a value when they are defined. Variables do not have to be assigned initial values. Variables once defined may be assigned a value within the instructions of the program.
- A data item whose value cannot change during the program’s execution.
- A data item whose value can change during the program’s execution.