Conditionals (if / elif / else)

Why this matters

Programs have to make decisions: pass or fail, discount or full price, even or odd. That is branching, and in Python you write it with if, elif, and else. This is the code version of the diamond you saw in a flowchart.

The idea

A comparison operator asks a yes/no question and answers with True or False:

if runs its indented block only when the condition is True. else covers the other case. Use elif ("else if") to test more conditions in order. Python takes the first branch whose condition is true and skips the rest.

if condition:
    # runs when condition is True
elif other_condition:
    # runs when the first was False and this is True
else:
    # runs when none above were True

Picture it

flowchart TD
  A([Start: score]) --> B{score >= 90?}
  B -- Yes --> C[grade = A]
  B -- No --> D{score >= 50?}
  D -- Yes --> E[grade = B]
  D -- No --> F[grade = C]
  C --> G([print grade])
  E --> G
  F --> G

Worked example

x = 70
if x >= 90:
    result = 'Grade is A'
elif x >= 50:
    result = 'Grade is B'
else:
    result = 'Grade is C'
print(result)   # Grade is B

x >= 90 is False, so Python checks the next condition. x >= 50 is True, so it takes that branch and ignores else.

Your turn

Predict which branch a program takes, then write functions that classify their input with if / elif / else.

Recap

• Comparison operators give True or False.
• if runs when its condition is true; else is the fallback.
• elif adds more cases; the first true branch wins.