Control flow: if, for, while — Python Module 4 | LeadAfrik Public Economics Hub

Conditional logic, loops, and the comparison operators that drive them. Python's syntax is clean enough that control flow rarely surprises — but the truthiness rules catch every beginner at least once.

if / elif / else

python

rate = 0.115
if rate > 0.13:
    stance = 'tight'
elif rate > 0.10:
    stance = 'neutral'
else:
    stance = 'loose'

for loops — iterate over anything

Python's for loop is a 'for-each' over an iterable. There is no 'for i = 0; i < n; i++' style — you iterate over a sequence directly.

python

rates = [0.10, 0.115, 0.125, 0.13, 0.135]

# Most common: iterate values
for r in rates:
    print(f'{r:.2%}')

# When you need the index too
for i, r in enumerate(rates):
    print(f'Year {i+1}: {r:.2%}')

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

while loops — when you don't know how many iterations

python

balance = 10000
rate = 0.10
years = 0
while balance < 20000:
    balance *= (1 + rate)
    years += 1
print(f'Doubled in {years} years')   # 8 years (the rule of 72 in code)

Truthiness — the rules that catch beginners

Python evaluates these as False: False, None, 0, 0.0, empty string '', empty list [], empty dict {}, empty tuple (). Everything else is True.

python

if not rates:           # True if the list is empty
    print('No data')
else:
    print(f'{len(rates)} rates loaded')

break, continue, else

break exits the loop. continue skips to the next iteration. The little-known else clause on a for loop runs only if the loop completed without break — useful for search-and-act patterns.

Prefer comprehensions to loops

If your loop is just building a list/dict/set, use a comprehension (covered in module 6). They are more idiomatic, more readable, and slightly faster.

Exercise

Sum all the rates in a list using a for loop (without using sum()).