Control Flow

In This Chapter

• How if / elif / else works and how Python evaluates conditions
• How for and while loops work
• What break, continue, and pass do
• How range() behaves
• The for / else pattern

if / elif / else

score = 85

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

Python evaluates conditions top to bottom and stops at the first True branch. Only one branch runs.

Truthiness — Python treats these as False: 0, "", [], {}, None. Everything else is True.

name = ""
if name:
    print("has name")
else:
    print("empty")   # prints this

for Loops

for iterates over any iterable — list, string, range, dict, etc.

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

Use enumerate() when you need both index and value:

for i, fruit in enumerate(fruits):
    print(i, fruit)
# 0 apple
# 1 banana
# 2 cherry

Use zip() to loop over two sequences together:

names = ["Alice", "Bob"]
scores = [90, 85]
for name, score in zip(names, scores):
    print(name, score)

range()

range(stop), range(start, stop), range(start, stop, step):

for i in range(5):       # 0 1 2 3 4
for i in range(1, 6):    # 1 2 3 4 5
for i in range(0, 10, 2): # 0 2 4 6 8
for i in range(5, 0, -1): # 5 4 3 2 1

range is lazy — it generates numbers on demand, not all at once.

while Loops

Runs as long as the condition is True:

n = 10
total = 0
while n > 0:
    total += n
    n -= 1
print(total)  # 55

Always make sure the condition eventually becomes False, or you get an infinite loop.

break, continue, pass

# break — exit the loop immediately
for i in range(10):
    if i == 5:
        break
    print(i)   # prints 0 1 2 3 4

# continue — skip the rest of this iteration
for i in range(5):
    if i == 2:
        continue
    print(i)   # prints 0 1 3 4

# pass — do nothing (placeholder)
for i in range(5):
    pass   # valid empty loop

for / else

The else block on a loop runs if the loop completed without hitting break:

def find_prime(nums):
    for n in nums:
        for i in range(2, n):
            if n % i == 0:
                break
        else:
            print(f"{n} is prime")

find_prime([4, 5, 6, 7])
# 5 is prime
# 7 is prime

This pattern avoids a flag variable.

Key Questions

Q: What is the difference between break and continue?

break exits the loop entirely — no more iterations run. continue skips the rest of the current iteration and moves to the next one. The loop itself keeps going after continue.

Q: What values are falsy in Python?

False, None, 0, 0.0, "" (empty string), [] (empty list), {} (empty dict/set), () (empty tuple). Any object with __bool__ returning False or __len__ returning 0 is also falsy.

Q: What does for / else do?

The else clause on a for or while loop runs only if the loop finished normally — i.e., without hitting a break. It's commonly used to signal "nothing was found" after a search loop, avoiding a separate boolean flag.

Q: What is the difference between range(5) and list(range(5))?

range(5) is a lazy range object — it doesn't store all values in memory, just computes them on demand. list(range(5)) materializes all values into a list [0, 1, 2, 3, 4]. For loops, use range directly; only convert to a list if you need random access or slicing.

Q: How do you iterate over a dictionary?

for key in d iterates over keys. for key, value in d.items() iterates over key-value pairs. for value in d.values() iterates over values only. In Python 3.7+, dictionaries maintain insertion order.