PY Python

Python: Control Flow

What you will learn

How to make decisions with if/elif/else, and how to repeat actions with for and while loops.

Conditionals

score = 85
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "F"
print(grade)  # B

Python uses indentation (4 spaces) to define blocks — no braces. The elif is short for "else if".

for loops

for fruit in ["apple", "banana", "cherry"]:
    print(f"I like {fruit}s")

for i in range(5):
    print(i)  # 0, 1, 2, 3, 4

range(n) generates 0, 1, ..., n-1. range(start, stop, step) gives more control.

while loops

count = 0
while count < 3:
    print(count)
    count += 1  # 0, 1, 2

Common mistakes

  • Forgetting the colon at the end of if, for, while.
  • Mixing spaces and tabs (use 4 spaces consistently).
  • Infinite loops: make sure your while condition eventually becomes False.

Quick check below!

Next: Functions