The input Function

The input() function is a very useful tool for making your programs interactive.

Here is an example usage of this function:

color = input("What is your favorite color? ")

Let’s break down this function similarly to how we broke down the print function.

The first 3 elements of the input function are the same as the print function:

  1. input this is the part of the function that says what we want to do.
  2. ( ... ) the parentheses are indicating that this function takes an argument (see part 3).
  3. "What is your favorite color? " this is what we call the argument. This is the prompt that we want to display to the user.
    1. Note: don’t forget your prompt! If you put nothing here, you will get confusing behavior.
  4. NEW color = this is a variable that saves the return value. We’ll break this part down more after we run the function to see what it actually does.

The General Form

We can write a general form of how to use this function as:

variable = input(prompt)

# TODO: run this code!
# When you run this code, there will be a text box where you can type.
# To give answer, type in your color, then press the return key.
color = input("What is your favorite color? ")
print("You answered: " + color)


# TODO: Run this code!
# This code demonstrates the confusing behavior of input if you
# forget to put a prompt in the parentheses

mystery = input() # I've forgotten a prompt!

# Notice that when no prompt is given, 
# the code just "hangs" (keeps running), waiting for the user 
# to type a word, without telling the user what to do!

The Return Value

Let’s dive deeper into part 4 of this function — the part where we save the return value.

return value: a value given back to the program (“returned”) by a function.

If you want to use a return value, it must always be saved in a variable.

Let’s look at what happens when we run the following code behind the scenes:

color = input("What is your favorite color? ")
print("You answered: " + color)
  1. The input function gets executed, printing “What is your favorite color? “.
  2. The user types an answer to this question, then presses return.
  3. Pressing return indicates to the program that whatever the user typed should be the return value.
  4. The return value gets stored in the variable color.
  5. The print function concatenates the value of the variable color to the end of the string “You answered: “ then prints out the entire string.

For example, if the user answers “purple”:

  1. The input function gets executed, printing “What is your favorite color? “.
  2. The user types “purple”, then presses return.
  3. Pressing return indicates to the program that “purple” is the return value.
  4. The string “purple” gets stored in the variable color.
  5. The print function concatenates the value of the variable color (“purple”) to the end of the string “You answered: “ then prints out the entire string (“You answered: purple”).

Notice that the return value has a type: “purple” is a string.

All return values have types.

The input function always always always returns a string. No matter what. ALWAYS.

# TODO: run this code. Answer the question with a number.
age = input("How old are you? ")
print(type(age))


# TODO: Add code to this snippet so that the type of 
# the variable age becomes an integer, then run the cell. 
age = input("How old are you? ")

# add code here to change the value stored in 
# age to an integer


print(type(age))

Type Casting + input

When we are using the input function, we frequently use type casting to help convert the user’s answer to the type that we actually want so that, for instance, we can do math with the user’s answer.

# TODO: run this code. What error do you get?
# Fix the error by adding the appropriate type casting function
temp = input("What temperature is it outside (celsius)? ")  

# add code here to convert temp to a number

kelvin = temp + 273.15

# notice the cast of the variable kelvin to a string — necessary so that
# we can concatenate its value with other strings!
print("It's " + str(kelvin) + " degrees kelvin!")

Danger Zone! (forgetting to save return values)

If you happen to write code that doesn’t save the return value, you will not be able to access that information:

input("How old are you? ")
# what is this variable? where did it come from? Produces a NameError
print(age)

A more devious example:

age = "27"
input("How old are you? ")
print(age)  # No error! but NEVER the user's answer to the question

If the user answer’s “16”, the output of the above code will still be “27”!