Control Flow Branching

Learning objective: By the end of this lesson, students will be able to use if...else statements to implement control flow in Python.

Indentation

Before we practice more control flow, it’s essential to know that Python uses indentation to define blocks of code - not curly braces.

In JavaScript, we used indentation for readability purposes; however, proper indentation is mandatory in Python!

Branching with the if statement

Like in JavaScript, we can run one of several code paths depending upon the result of conditional expression(s).

Single path if statement:

floor = "sticky"
walls = "clean"

# notice the colon ':' after the conditional expression
# it indicates the start of the if block
if floor == "sticky":
    print("Clean the floor! It's sticky!")
    # this is where you would add more lines of code related to the condition
    # each line must be indented to be part of the 'if' block

# unindented code indicates that we have returned to normal code execution
print("This will always print, it's not inside of an if block!")

# parentheses are not required around the conditional expression
if walls == "sticky":
    print("Clean the walls! They're sticky!")

🧠 A quick note on code style - we follow the PEP8 Style Guide for Python Code as closely as possible for all our work in Python. In this convention we use 4-space indentation.

Dual path if...else statement:

val = 3

# if path
if val == 1:
    print('val is one')
# else path
else:
    print('val is not one')
    # prints: val is not one
    # since val is not 1, the else path is taken

Multi-path if...elif...else statement:

val = 3

if val == 1:
    print('val is one')
elif val == 2:
    print('val is two')
elif val == 3:
    print('val is three')
    # prints: val is three
    # since val is 3, this elif path is taken
else:
    print('not one, two, or three')

The elif is not a typo! It is Python’s syntax for else if

else is always optional, just like it is in JavaScript.

🎓 You Do

In your control_flow.py file, add the following code that accepts text input from the user:

color = input('Enter "green", "yellow", "red": ').lower()
print(f'The user entered {color}')

Below that code, write an if...elif...else statement that prints out one of the following messages:

• If green is entered, print the message Go!
• If yellow is entered, print the message Slow Down!
• If red is entered, print the message Stop!
• If anything else is entered, print the message Bogus!

Use the following command to run this code in your terminal:

python3 control_flow.py