4.9. User Input

print works fine to display messages on the screen that never change. If we want to display different messages, we could try adding a variable:

Example

Run the code below, then change the value of name to make the program greet you by name!

To print a greeting for a specific user, print("Hello Dave!") only works if Dave is the actual user. To greet someone else, we could change the string inside the () to be 'Hello Sarah!' or 'Hello Elastigirl!' or any other name we need.

Similarly, to make the statement print("Hello, " + name + "!") work for different users, we need to go into the code and change the string we assign to name.

However, what if we do not know the name of the user beforehand?

It would be great if we could ask the user of our program to enter a name, save that string, and then print a personalized greeting.

Of course, Python gives us a way to do this!

4.9.1. Requesting Data

To personalize the greeting, we need input from the user. This involves displaying a prompt on the screen (e.g. Please enter a number:), and then saving the response from the user. Whatever information the user enters can be stored in a variable.

Python has a built-in function to collect information from a user. As you might expect, it is called input.

4.9.1.1. Syntax

variable_name = input('User prompt...')

The keyword input tells Python to display the prompt in the console. It then waits for the user to type in some information. Once the user taps Enter or Return, the data is collected and stored in the variable.

Note

There is a lot going on here behind the scenes, but for now you should follow this bit of wisdom:

I turn the key, and it goes.

Most of us do not need to know exactly how cars, phones, or microwave ovens work. We just know enough to use them in our day to day lives. Similarly, we do not need to understand how input collects data from the console. We just need to know that it does if we use the correct syntax.

4.9.1.2. Try It!

In the editor above:

  1. Replace line 2 with name = input('Please enter your name: ').
  2. Run the program again. You should see the text Please enter your name: appear in the black output box (the console).
  3. In the console, type in a name, tap Enter, and examine the result.
  4. Run the program several more times and enter different names.

Try adding another + name term inside the print statement and see what happens. Next, add code to prompt the user for a second name. Store the response in other_name, then print both names using print.

4.9.2. Using the Collected Data

After collecting a name, the program does not actually DO anything with the information. If we want to use the data, we need to tell Python what to do with the name variable.

By storing the user’s name inside the variable called name, we gain the ability to hold onto the data and use it when we want.

Try It

Write a program that requests a user’s first and last name, then prints an output that looks like:

First name: Elite
Last name: Coder
Last, First: Coder, Elite

4.9.3. Critical input Detail

There is one very important quirk about the input function that we need to remember. Given print(7 + 2), the output would be 9.

Now explore the following code, which prompts the user for two numbers and then prints their sum:

Do you see the output you expected?

If we enter 7 and 2, we may expect an output of 9. The result printed is 72. What gives?!?!?

The quirk with the input function is that it treats all entries as strings, so numbers get concatenated rather than added. Concatenation means that the second string gets attached to the end of the first.

Just like "ABC" + "def" outputs as ABCdef, "7" + "2" outputs as the string 72.

Python treats input data as strings!

If we want our program to perform math operations on the entered numbers, we must use type conversion to change the string values into numbers.

Try It

  1. In the print statement, use int() to convert num_1 and num_2 from strings to integers. Run the program and examine the result.

  2. Instead of collecting num_1 as a string and then converting it later, we can do this in a single step. In line 1, place input("Enter a number: ") inside the int() function like this:

    int(input("Enter a number: ")).

  3. Repeat step 2 for num_2.

  4. Remove the int functions from the print statement. Run the program and examine the result.

  5. What happens if you enter Hi or 4.33 instead of a whole number?

4.9.4. Check Your Understanding

Question

What is printed when the following program runs?

1
2
3
4
user_age = input("Please enter your age: ")
# The user enters 25.

print(type(user_age))
  1. <class 'str'>
  2. <class 'int'>
  3. user_age
  4. 25

Question

Assume you want the user to enter a decimal value, like 4.33. Which of the following statements would throw an error after the user taps Return?

  1. input('Enter a decimal value: ')
  2. float(input('Enter a decimal value: '))
  3. int(input('Enter a decimal value: '))