Len and In

Lesson

Len and In

len returns the number of elements in a list:

tasks = ["email", "report", "review"]
print(len(tasks))

Output:

An empty list is written [] and has length 0.

The in operator checks whether a value is an element of a list. It produces a boolean:

print("report" in tasks)
print("meeting" in tasks)

Output:

True
False

not in is the opposite check: "meeting" not in tasks is True.

len also works on strings — len("Ada") is 3. in on strings is covered in the strings section.

Exercise

Define a function named is_registered with two parameters: attendees (a list) and name. It returns True when name is in attendees.
Define a function named seats_left with two parameters: attendees and capacity. It returns how many seats remain: capacity minus the number of attendees.

seats_left(["Ada", "Grace"], 10) returns 8.

Run your code to see the output, then press Submit.

Tests

import unittest


class TestLenAndIn(unittest.TestCase):
    def test_registered_name_is_found(self):
        self.assertIs(is_registered(["Ada", "Grace"], "Ada"), True)

    def test_unregistered_name_is_not_found(self):
        self.assertIs(is_registered(["Ada", "Grace"], "Alan"), False)

    def test_empty_list_has_no_registrations(self):
        self.assertIs(is_registered([], "Ada"), False)

    def test_two_attendees_leave_eight_seats(self):
        self.assertEqual(seats_left(["Ada", "Grace"], 10), 8)

    def test_full_room_leaves_zero_seats(self):
        self.assertEqual(seats_left(["Ada", "Grace"], 2), 0)

Solution

def is_registered(attendees, name):
    return name in attendees


def seats_left(attendees, capacity):
    return capacity - len(attendees)


print(is_registered(["Ada", "Grace"], "Ada"))
print(seats_left(["Ada", "Grace"], 10))

Solution hidden. Give it a real try first.

Discussion

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main.py

Console

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