count in Python: Count a Value or a Nonoverlapping Substring

Start with a small, explicit task

Count accepted responses in a fictional feedback sequence, then inspect the substring ana in banana. There are two starting positions where those letters can be seen, but they overlap. Predict the built-in text count before running it, and compare the result with a simple repeated-letter count.

Run the complete example

This original example was checked with Python 3.12.14. Save it as python_count.py and run it with Python 3.12. It uses only the standard library. The assertions check the stated results.

responses = ["yes", "no", "yes", "later", "yes"]
yes_count = responses.count("yes")
text = "banana"
assert yes_count == 3 and len(responses) == 5
assert responses.count("missing") == 0
assert text.count("ana") == 1
assert text.count("a") == 3
assert text.count("a", 2, 5) == 1
print("yes:", yes_count, "total:", len(responses))
print("ana:", text.count("ana"), "a:", text.count("a"))
print("a in [2:5]:", text.count("a", 2, 5))

Explain the result

The list has five responses but only three equal yes. A missing value contributes zero occurrences without raising an error. That differs from list.index, which is a position lookup and fails when no match exists. The two methods should not be used interchangeably when validating input.

banana has three individual a characters, but its two visible ana fragments overlap at one character. str.count counts nonoverlapping matches, yielding one. The bounded call examines positions 2 through 4, which form nan and contain one a; the stop index 5 is excluded. These results are case-sensitive literal comparisons.

Avoid these mistakes

Calling count repeatedly for every distinct item walks the data repeatedly; a frequency table such as Counter can be clearer when you need all values. Also decide what an occurrence means before counting: literal nonoverlapping substrings, overlapping patterns and tokenized words are different tasks.