Project: Count the letters

Write a function called count_letters that takes a string and returns a dictionary showing how many times each letter appears in it. The string can contain characters that are not letters, and it can be empty. Count only letters, and count upper-case and lower-case as the same letter. The keys of the dictionary should be upper-case letters.

For example, given this string:

"AaBb"

the dictionary should hold these pairs:

{'A': 2, 'B': 2}

We have not covered the built-in isalpha(), so do not use it.

Save the file as count_letters.py.

Show a solution

Try the project on your own first. This is one working solution, followed by its tests. Press Run to run the tests. Read them to see what thorough testing looks like, then change the solution and run again to watch a test fail.

def count_letters(text: str) -> dict:
    """Count each letter in `text`, ignoring case. Keys are upper-case."""
    counts: dict = {}
    for character in text:
        letter: str = character.upper()
        if "A" <= letter <= "Z":
            if letter in counts:
                counts[letter] += 1
            else:
                counts[letter] = 1
    return counts

# ---- Tests ----
import sys
import unittest

class TestCountLetters(unittest.TestCase):
    def test_the_example_from_the_project(self):
        self.assertEqual(count_letters("AaBb"), {"A": 2, "B": 2})

def test_an_empty_string_gives_an_empty_dictionary(self):
        self.assertEqual(count_letters(""), {})

def test_a_single_letter_is_counted_once(self):
        self.assertEqual(count_letters("q"), {"Q": 1})

def test_repeated_letters_are_added_up(self):
        self.assertEqual(count_letters("banana"), {"B": 1, "A": 3, "N": 2})

def test_upper_and_lower_case_count_as_the_same_letter(self):
        self.assertEqual(count_letters("aAaA"), {"A": 4})

def test_the_keys_are_upper_case(self):
        self.assertEqual(list(count_letters("xyz")), ["X", "Y", "Z"])

def test_digits_spaces_and_punctuation_are_ignored(self):
        self.assertEqual(count_letters("123 !?-_"), {})

def test_letters_mixed_with_other_characters(self):
        expected = {"H": 1, "E": 1, "L": 3, "O": 2, "W": 1, "R": 1, "D": 1}
        self.assertEqual(count_letters("Hello, World!"), expected)

def test_the_first_and_last_letters_of_the_alphabet_are_counted(self):
        self.assertEqual(count_letters("azAZ"), {"A": 2, "Z": 2})

def test_characters_just_outside_the_alphabet_are_ignored(self):
        # "@" and "[" sit next to "A" and "Z" in the character table.
        # "`" and "{" sit next to "a" and "z".
        self.assertEqual(count_letters("@[`{"), {})

def test_accented_letters_are_not_counted(self):
        self.assertEqual(count_letters("éa"), {"A": 1})

# ---- Run the tests ----
loader = unittest.defaultTestLoader
suite = unittest.TestSuite(loader.loadTestsFromTestCase(case) for case in [TestCountLetters])
unittest.TextTestRunner(stream=sys.stdout, verbosity=2).run(suite)

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