--- title: 2.9 Constants description: "A **constant** is a value that is intended to remain unchanged throughout the execution of a program. Unlike languages such as C, C++, or Java, **Python does not have built-in support for constant variables**. Any variable can be modified after…" url: "https://edcret.in/docs/python-programming/basic-refreshers-python/2-9-constants/" updated: "2026-07-29" category: Python Programming / Basic Refreshers --- # 2.9 Constants A **constant** is a value that is intended to remain unchanged throughout the execution of a program. Unlike languages such as C, C++, or Java, **Python does not have built-in support for constant variables**. Any variable can be modified after it is created. However, by convention, programmers write constant names using **uppercase letters** to indicate that their values should not be changed. --- ## Naming Convention Constants are usually written using uppercase letters with words separated by underscores. Examples: ``` PI = 3.14159 MAX_SIZE = 100 GRAVITY = 9.8 COMPANY_NAME = "Edcret" ``` > **Note:** This is only a naming convention. Python does not prevent these values from being modified. --- ## Example 1: Defining a Constant **main.py** ``` PI = 3.14159 print(PI) ``` ### Output ``` 3.14159 ``` ### Explanation - `PI` is intended to represent a constant value. - The variable name is written in uppercase. - This tells other programmers that the value should not be modified. --- ## Example 2: Using Constants in Calculations **main.py** ``` PI = 3.14159 radius = 5 area = PI * radius * radius print(area) ``` ### Output ``` 78.53975 ``` ### Explanation - `PI` stores the mathematical constant π. - `radius` stores the radius of the circle. - The formula `π × r²` is used to calculate the area. - Using a constant avoids writing the same value repeatedly. --- ## Example 3: Multiple Constants **main.py** ``` MAX_USERS = 100 MIN_PASSWORD_LENGTH = 8 print(MAX_USERS) print(MIN_PASSWORD_LENGTH) ``` ### Output ``` 100 8 ``` ### Explanation - Programs often contain values that remain fixed. - Giving these values meaningful constant names improves readability. - If a value ever changes, it only needs to be updated in one place. --- ## Example 4: Constants Can Still Be Modified **main.py** ``` PI = 3.14159 print(PI) PI = 3.14 print(PI) ``` ### Output ``` 3.14159 3.14 ``` ### Explanation - Python allows the value of `PI` to be changed. - Although this is valid syntax, it is considered poor programming practice. - Programmers are expected to treat uppercase variables as constants and avoid modifying them. --- ## Example 5: Constants Improve Readability **main.py** ``` DISCOUNT_PERCENT = 10 price = 500 discount = price * DISCOUNT_PERCENT / 100 print(discount) ``` ### Output ``` 50.0 ``` ### Explanation - `DISCOUNT_PERCENT` clearly describes the purpose of the value. - Using a named constant makes the code easier to understand than writing the number `10` directly. - If the discount changes, only the constant needs to be updated. --- ## Example 6: Avoiding Magic Numbers **main.py** ``` PASS_MARK = 40 marks = 55 print(marks >= PASS_MARK) ``` ### Output ``` True ``` ### Explanation - The value `40` has been stored in the constant `PASS_MARK`. - This avoids using unexplained numbers directly in the program. - Such unnamed values are often called **magic numbers**. - Replacing magic numbers with constants makes code more meaningful. --- ## Example 7: Constants Containing Strings **main.py** ``` COMPANY_NAME = "Edcret" print("Welcome to", COMPANY_NAME) ``` ### Output ``` Welcome to Edcret ``` ### Explanation - Constants are not limited to numeric values. - They can also store strings, Boolean values, or other objects. - Uppercase names indicate that the value should remain unchanged. --- ## Example 8: Constants Used Multiple Times **main.py** ``` TAX_RATE = 18 price1 = 100 price2 = 250 print(price1 * TAX_RATE / 100) print(price2 * TAX_RATE / 100) ``` ### Output ``` 18.0 45.0 ``` ### Explanation - The same constant is used in multiple calculations. - This avoids duplicating the value `18`. - Updating the tax rate requires changing only one line. --- # Common Mistakes ### 1. Writing Constants in Lowercase **Poor Practice** ``` pi = 3.14159 ``` **Better Practice** ``` PI = 3.14159 ``` **Reason** Uppercase names clearly indicate that the value is intended to be constant. --- ### 2. Modifying a Constant **Poor Practice** ``` MAX_SIZE = 100 MAX_SIZE = 200 ``` **Reason** Although Python allows this, constants should not be modified after they are defined. --- ### 3. Using Magic Numbers **Poor Practice** ``` radius = 5 area = 3.14159 * radius * radius ``` **Better Practice** ``` PI = 3.14159 radius = 5 area = PI * radius * radius ``` **Reason** Named constants make code easier to read and maintain. --- # Best Practices - Write constant names in uppercase. - Use underscores to separate words in constant names. - Avoid changing constant values after they are defined. - Replace unexplained numeric values (magic numbers) with meaningful constants. - Choose descriptive names that clearly indicate the purpose of the constant. --- # Key Points to Remember - A constant is a value that is intended to remain unchanged. - Python does not provide true constant variables. - Uppercase variable names are used by convention to represent constants. - Constants improve code readability and maintainability. - Constants help eliminate magic numbers from programs. - Although constants can be modified in Python, doing so is considered poor programming practice.