AssertionError Examples

Short, runnable programs that show python assertionerror in action. Read the explanation, press Run, then change the code and try your own ideas. New to the topic? Start with the Python AssertionError lesson.

1A guard inside a function

An assert documents what a function assumes about its own inputs and fails loudly during development.

A guard inside a function

def percent_of(part, whole):
    assert whole != 0, "whole must not be 0"
    return round(part / whole * 100, 1)

print(percent_of(45, 60))
try:
    percent_of(5, 0)
except AssertionError as err:
    print("AssertionError:", err)

Expected output75.0
AssertionError: whole must not be 0

2Checking types with isinstance

isinstance in an assert catches a wrong type early, before it causes a confusing error further down.

Checking types with isinstance

def total_price(prices):
    assert isinstance(prices, list), f"expected a list, got {type(prices).__name__}"
    return sum(prices)

print(total_price([4, 6.5]))
try:
    total_price("4,6.5")
except AssertionError as err:
    print("AssertionError:", err)

Expected output10.5
AssertionError: expected a list, got str

3Float comparison with isclose

math.isclose compares within a small relative tolerance, which is what you want after floating-point arithmetic.

Float comparison with isclose

import math

prices = [19.99, 5.01, 0.10]
total = sum(prices)
print(total == 25.1, math.isclose(total, 25.1))
assert math.isclose(total, 25.1, abs_tol=1e-9)
print("total is correct")

Expected outputFalse True
total is correct

4Assert as a tiny test

Before learning a test framework, a few asserts with known answers already catch many bugs.

Assert as a tiny test

def initials(name):
    return "".join(part[0].upper() for part in name.split())

assert initials("asha rao") == "AR"
assert initials("Kiran") == "K"
assert initials("  maria  de souza ") == "MDS"
print("initials() passed 3 checks")

Expected outputinitials() passed 3 checks

5Checking an invariant after a calculation

The assert verifies your own logic: a shuffled bill split must still add up to the total.

Checking an invariant after a calculation

def split_bill(total_cents, people):
    base, extra = divmod(total_cents, people)
    shares = [base + (1 if i < extra else 0) for i in range(people)]
    assert sum(shares) == total_cents, "shares must add up to the bill"
    return shares

print(split_bill(1000, 3))
print(split_bill(999, 3))

Expected output[334, 333, 333]
[333, 333, 333]

Practise next

Python AssertionError exercises → Back to the lesson

Python Booleans examples Python Exceptions: try, except, else and finally examples Python ValueError examples Python Testing examples