Outputting information from the program to the user

In python, use the print() function to output information to the user.

Here is a run-of-the-mill print statement:

print("Hello, World!")

When you run this line of code, the following is output to the console:

Hello, World!

The following two lines show that when you have two print statements, they get executed in order. 

print("Hello")
print("World")

The output shows that each print statement automatically takes you to the next line on the console.

Hello
World

You can put two or more strings in a print() statement, separated by commas. Usually, one or more of these string values comes from a variable. Note that the comma inside the quotes is part of the output, while the commas between strings are part of the syntax of the print() statement:

username = "Sam"
print("Hello,", username, "!")

The output shows that a space automatically is inserted between each parameter:

Hello, Sam !

Another way to build strings for output is to use the + operator for concatenation. Concatenation means putting two strings together by tacking the second one onto the end of the first one.

print("Hello," + username + "!")

The output below shows that when concatenating strings, a space does not automatically get inserted:

HelloSam!

Here are ways to put a space between two different strings:

print("Hello", username)
print("Hello " + username)

Use \n to insert a new line in the middle of a string:

print("Hello\nWorld")

Here's the output:

Hello
World

The backslash symbol \ in \n is called an escape character. It tells python that the letter following the backslash should be interpreted together with the \ as a pair with special meaning. Other examples include

  • \t for tab,
  • \" and \' to put a double quote or single quote into a string literal, and
  • \\ to insert a backslash.

For example, see if you can write a print() statement to produce the following tricky output:

"\n" is a newline,
while "\'" is a single quote.

Answer:

print("\"\\n\" is a newline,\nwhile \"\\\'\" is a single quote.")

A third way in python to combine multiple strings together and to include variable values within a string is a formatted string literal, commonly called an f-string. Create an f-string by prefixing the string with an f, then include any variable values within a pair of curly braces {}. For example:

print(f"Hello, {username}!")

outputs

Hello, Sam!

With f-strings, you can have rich control over the formatting. For example

import math
print(f"pi to 5 decimal places is {math.pi:.5f}, and e to 3 decimal places is {math.e:.3f}.")

outputs

pi to 5 decimal places is 3.14159, and e to 3 decimal places is 2.718.

Here's an online tutorial if you want to investigate more capabilities of f-strings: https://builtin.com/data-science/python-f-string

Finally, it sometimes causes us problems that a call to print() automatically includes a newline at the end of the output. Occasionally we want to suppress that. We do this by specifying in a second parameter to print() that the end should be an empty string "" rather than the default "\n". Here's an example showing how two calls to print() can output on a single line of the console:

print("Hello, world! ", end="")
print("It's a beautiful day! ")

Output:

Hello, world! It's a beautiful day!

Getting input from the user into the program

Use the input() function to ask the user of your program for a value. For example:

username = input("Enter your name: ")
print("Hello,", username)

Notice that the parameter within the parentheses of the input() function is the string that you want to display to the user as a message. That message, called a prompt, is output to the console when the input() line is executed. In other words, you don't need a separate print() statement. Make sure the prompt clearly communicates to the user what information you are asking them to enter. Once they type the value and hit the <return> key, the input() function returns the value they entered. In the above example, the user's response is stored in a variable called username. Here's a sample run of the above program:

Enter your name: Sam
Hello, Sam

Note that the input() function always returns a string (of type str). To clarify,

username = input("Enter your name: ")
print(type(username))

will output

<class 'str'>

But often we want the user to input a numerical value rather than a string (text) value. In this case we must cast the result to convert it to a numerical type. This is mandatory any time you want to do arithmetic with the number the user enters. For example:

user_age = input("Enter your age: ")
user_age = int(user_age)
print(f"Next year you will be {user_age + 1}.")

Sample run of the above program:

Enter your age: 19
Next year you will be 20.

Note that the casting can actually be done on the same line as the input. The input() function returns a string, which we can immediately cast using the int() function. The two lines below behave identically to the three lines of code just above.

user_age = int(input("Enter your age: "))
print(f"Next year you will be {user_age + 1}.")

Putting it all together

Here's a full sample program:

"""
Demo of creating and using variables and user input

File Name: variables_and_user_input.py
Date:
Course: COMP1351
Assignment: Preclass Assignment 2
Collaborators: 1351 Instructors
Internet Sources: None
"""

def main():
    print("Program that computes number of miles from steps walked")
    # Find out step count from user
    step_count = int(input("How many steps did you walk today? "))
    # On average there are 2250 steps per mile
    num_of_miles = step_count/2250
    # Report result to user
    print(f"You walked {num_of_miles:.2f} miles today.")


# Run the program:
if __name__ == '__main__':
    main()

Sample run from the above program:

Program that computes number of miles from steps walked
How many steps did you walk today? 10000
You walked 4.44 miles today.