How to create a constant in Python?

Constants and variables are used to store data values ​​in programming. A variable usually refers to a value that can change over time. A constant is a type of variable whose value cannot be changed during program execution.

There are only six built-in constants available in Python, they are False, True, None, Not Implemented, Ellipsis(...) and __debug__. Apart from these constants, Python does not have any built-in data types to store constant values.

Example

The following demonstrates an example of constants -

False = 100

Output

SyntaxError: cannot assign to False 

False is a built-in constant in Python, used to store the Boolean value false. Assigning any value to it is illegal and will raise a SyntaxError.

But in the PEP 8 standard, constants are in uppercase. This helps the user know that it is a constant value. If we encounter any all-caps variables, by convention rather than rule, we should not change their values. Let's look at an example.

Example

π is a mathematical constant, approximately equal to 3.14159. Let us declare the value of the constant π in Python.

# declare constants
PI = 3.14159
print(PI)

Output

3.14

In the example above, the mathematical constant pi is declared using all uppercase letters.

Example

As mentioned in the Constants section of PEP 8, we should use uppercase letters and underscores to separate words.

# declare constants
LUMINOUS_EFFICACY = 683
VALUE_A = 100
COLOR = 'RED'

print(LUMINOUS_EFFICACY)
print(VALUE_A)
print(COLOR)

Output

683
100
RED

As we can see, constants are also created exactly like variables. Both variables and constants follow similar naming rules, the only difference is that constants only use uppercase letters.

Example

Normally, in Python, constants are declared in modules. Let's take an example and create constants.

Declare constants in a separate file and name the file with a .py extension.

Constants.py file

# declare constants

SPEED_OF_LIGHT_IN_VACUUM = 299792458
PI = 3.141592653589793
LUMINOUS_EFFICACY = 683
VALUE = 20

Example.py file

import Constants

print(Constants.VALUE)
print(Constants.SPEED_OF_LIGHT_IN_VACUUM)
print(Constants.PI)

Output

20
299792458
3.141592653589793

In the above example, we created the Constants.py file, called the Constants module. Then, we declared some constant values. After that, we create another python file which is an Example.py file and in this file we import the Constant module using the import keyword. Finally, access the constant value.

The purpose of using uppercase letters is to indicate that the current name is considered a constant. But it doesn't actually prevent the reallocation of constant values.

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