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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


tip
In below assigment you are tasked with checking some rules and exiting application if any are broken.
You can use built-in module called sys which has some handy functionality to work with our operating system. For now, we are only interested in using its exit() function. It takes in int value to provide as exit code which other applications can use to interpret whether your program run successfully or failed and in what way.
It is common to use 0 as success and any other positive integer to signify a failure.
To use other modules, you use import keyword, followed by the name of the module. Usually, these are located at the top of the file, before writing your logic.
You can use it like this:
import sys # once at the top of the file
sys.exit(0) # success
sys.exit(1) # failed / error
We will learn more about built-in, custom and third-party modules (called libraries) later in the course.


Assigment​


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


    

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

  2. Instead of hardcoding values (booktore_name, book_title, book_quantity, book_price) we will ask the user to enter these values using input() function. Remember that input() always returns str so we must cast the string to desired type (book_quantity to int and book_price to float). Replace all of these variables hardcoded values to input() function, asking the user to enter the data.
    3. 

  3. Now the system must add some contraints, printing the error message and exiting if any of the rules are broken. Add these below each of the inputs, so that check runs as soon as user enters something. Rules are as follows:

      

    • bookstore_name must be between 3 and 20 characters long
      • 

    • book_name must be between 3 and 15 characters long
      • 

    • book_quantity must be positive integer (above 0)
      • 

    • book_price must be positive float (above 0.00)
      • 

    

    4. 

  4. System must now also calculate taxes that depend on the book_price, so make another variable book_tax and set it to 0. Then calculate the tax using below rules, assign it to book_tax and print out the tax applied.

      

    • If book_price is above 5 tax is 3% (0.03)
      • 

    • If book_price is above 10 tax is 5% (0.05)
      • 

    • If book_price is above 18 tax is 10% (0.1)
      • 

    

    5. 

  5. Display inventory listing like this:

    Welcome to WILLOW CREEK BOOKS inventory system
    - - - - - - - - - - - - - - - - - - - -
    CURRENT INVENTORY
    - - - - - - - - - - - - - - - - - - - -
    - Book: The Last Cartographer -> [price:$18.5, tax: 10%, copies: 3, available: True]
    - - - - - - - - - - - - - - - - - - - -
    Inventory total: $55.5 [VAT: $60.05]
    - - - - - - - - - - - - - - - - - - - -
    Thank you for using our inventory system.
    

    6. 

  6. Commit and push your changes.
    7. 



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.