Input

Now comes some real fun with input! Input allows us to prompt the user to give us some sort of data to use in our program. Now our programs can be interactive! Interactive = more fun for sure. :)

A quick example:

name = input("What's your name, little buddy?")
print(f"Hi there {name}!")

Let's see how this works.

$ python3 greetings.py
What's your name, little buddy?

At this point the program pauses for the user to input something.

What's your name, little buddy?Ginger
Hi there Ginger!

That input line looks a little awkward since Python prints the prompt exactly as we type it, so let's add a space after the question mark.

name = input("What's your name, little buddy? ")
print(f"Hi there {name}!")

$ python3 greetings.py
What's your name, little buddy? Ginger
Hi there Ginger!

Nice!

Let's try out adding a couple of numbers together in a program called add.py:

number1 = input("What is the first number you would like to add? ")
number2 = input("What is the second number you would like to add? ")

sum = number1 + number2

print(f"The sum is {sum}")

$ python3 add.py
What is the first number you would like to add? 4
What is the second number you would like to add? 4
The sum is 44

Well, that's now what we were going for. Why? If we think back to when we were looking at adding things together in the REPL, adding numbers gave us the sum of the numbers, but adding strings concatenated—smashed together—the strings. Could hat be what's happening here?

We can use the type function in Python to investigate. Let's change add.py momentarily.

number1 = input("What is the first number you would like to add? ")
number2 = input("What is the second number you would like to add? ")

print(type(number1))
print(type(number2))

$ python3 add.py
What is the first number you would like to add? 4
What is the second number you would like to add? 4
<class 'str'>
<class 'str'>

They're strings! Data given to the input function are strings by default. We can convert them to floating point numbers using float().

number1 = input("What is the first number you would like to add? ")
number2 = input("What is the second number you would like to add? ")

sum = float(number1) + float(number2)

print(f'The sum is {sum}')

$ python3 add.py
What is the first number you would like to add? 4
What is the second number you would like to add? 4
The sum is 8.0

Neat! We could also convert to an integer using float() while we're assigning the input to a variable.

number1 = float(input("What is the first number you would like to add? "))
number2 = float(input("What is the second number you would like to add? "))

sum = number1 + number2

print(f'The sum is {sum}')

Input Exercises

Party Grocery List Creator

Make a program party_shopping.py that does the following:

  1. Prompts the user for a number of guests

  2. Prints the number of packages of veggie dogs needed (there are 10 to a pack)

  3. Prints the number of packages of hot dog buns needed (there are 8 to a pack)

  4. Prints the number of fun pack chip boxes needed (there are 32 to a pack)

  5. Prints the number of soda 6-packs needed (there are 6 to a pack)

Terminal Madlibs

Get input of nouns, verbs, etc. and print a story.

Interactive Difference

Create a program difference.py to take input for two numbers and print the positive difference.

Target Retirement Date

Create a program retirement.py that does the following:

  1. Prompts the user for the amount of money they need at retirement

  2. Prompts the user for the amount of money they can save each month

  3. Calculates the number of months needed to hit the target amount of money based on an an average return on investment of 9%, and an average yearly inflation of 3%.