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User Input and Conditionals

In the last lesson we learned how to compare values. In this lesson we will expand that knowledge and learn how to influence programs decision to do something depending on conditions. We will also cover how to get input from the user.

Lesson Overview​

In this lesson you will learn:

  • How to receive an input from user
  • How to make program respond to conditions
  • How to use if-elif-else to branch your program

User Input​

Our program would not be very useful if it cannot take data from the user. In Python we do this using input() function. input() takes any string as a user prompt (what will be displayed to the user while waiting for input)

Python Sandbox


input() always returns a string
input() function always returnes a string. This is something you need to watch for when you ask the user to enter some data. If you need numbers (integer or float) you will need to type cast it to some other type.
If you cannot remember how to do this, check out previus Core Datatypes - Type Casting lesson.


Conditionals - What Are They?​


Conditionals is a name for a group of keywords that branch the program logic depending on condition.
In Python we have:


    

  • if
    • 

  • elif
    • 

  • else
    • 



We use these keywords to make our program do something if one condition is True and something else if its False.


We start by using if keyword, followed by a condition and end it with colon (:).
Then we must indent (4 spaces or 1 tab) the next line.


Let's see a simple example by running the code below. Feel free to change variable a to see how the program reacts.


Python Sandbox


In this basic example, our program will execute its task depending on conditions. In the current example, we ask Python to compare value in variable a to int 5. If they are equal, program executes one branch and if its not, program executes another branch.


Indentation as Code Section
Unlike other programming languages that use curly braces to mark blocks (sections) of code, in Python we use indentation.
This is extremely important to learn as Python will raise IndentationError if you do it wrong.


In Python, we also have elif keyword which means else-if. Its used to check more than one condition. Let's see an example:


Python Sandbox


In this example, we check the value of variable a. Then based on conditions set, our program will execute specific branch.


We use this branching logic to make our program do varius things based on these conditions. Each condition we have always evaluates to boolean - being True or False. We can also use logical operators (and, or, not) to chain multiple conditions for Python to check.


Python Sandbox


Assigment​


Now that you know how to get user input and branch your program using conditional logic (if, elif, else) the code can move away from hardcoded values and become interactive.


Goal: use input() to collect user choices, cast strings to numbers, and use conditional logic to apply discounts and stock checks.


    

  1. Open main.py file in our simple-python-shop project directory.
    2. 

  2. Add a new variable item_stock and set it to some integer
    3. 

  3. Interactive input:

      

    • Ask the user for desired item_quantity using input(). Don't forget to cast the string to integer !
      • 

    

    4. 

  4. Stock & Availability Check

      

    • If item_quantity is less or equal to 0 print an error message: Invalid quantity ordered
      • 

    • If item_quantity is greater then item_stock print Sorry, we do not have enough stock.
      • 

    

    5. 

  5. Dynamic discount (conditionals)

      

    • Calculate the total (you should have this from the last lesson)
      • 

    • If total is over 100 apply a 10% discount (total * 0.90) and print: Discount applied: 10%
      • 

    • Otherwise print: No discount applied
      • 

    

    6. 

  6. Receipt Output

      

    • Output the final receipt showing item name, quantity, applied discount state, and final amount due.
      • 

    

    7. 

  7. Commit and push your updated code to Github.
    8. 



Deepen Your Knowledge​



What's Next​


Now that our programs can take user input, calculate things and evaluate what to do on those calculations, we can jump into loops. These help us to run a peace of code multiple times without us repeating the code.