assertIn & assertNotIn

assertIn & assertNotIn

assertIn(a, b) checks that element a is found in sequence b, while assertNotIn(a, b) checks that it is not. Perfect for testing membership in lists, strings, sets, and dictionaries.

Basic Usage

import unittest

class TestMembership(unittest.TestCase):
    def test_user_in_list(self):
        users = ["alice", "bob", "charlie"]
        self.assertIn("alice", users)

    def test_invalid_email_not_in_valid(self):
        valid_emails = ["a@test.com", "b@test.com"]
        self.assertNotIn("hacker@evil.com", valid_emails)

    def test_char_in_string(self):
        self.assertIn("P", "Python")

    def test_key_in_dict(self):
        config = {"debug": True, "version": "1.0"}
        self.assertIn("debug", config)

    def test_element_not_in_set(self):
        allowed = {1, 2, 3}
        self.assertNotIn(5, allowed)

With Custom Messages

def test_item_in_cart(self):
    cart = get_cart(user_id=1)
    self.assertIn(
        "laptop",
        [item.name for item in cart.items],
        "Laptop should be in the cart after adding it"
    )

Works With

• Lists: assertIn("x", ["x", "y"])
• Tuples: assertIn(1, (1, 2, 3))
• Sets: assertIn("a", {"a", "b"})
• Strings: assertIn("py", "python")
• Dictionaries: checks keys — assertIn("k", {"k": 1})

Key Takeaway

Use assertIn and assertNotIn to test membership in any iterable or collection. They work with lists, strings, sets, tuples, and dict keys.