Challenge 2: Line Counter — Possible Solution
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def count_lines(filename):
    count = 0
    with open(filename, "r") as file:
        for line in file:
            count += 1
    return count

print(count_lines("output.txt"))

Output (using the 3-line file from Challenge 1):
3

WHY THIS WORKS AS AN ANSWER
------------------------------
with open(filename, "r") as file: reuses the chapter's own context
manager pattern, guaranteeing the file closes automatically once the
function is done reading it — even if something inside the loop
raised an exception, tying back to the with block's guarantee from
earlier in the chapter.

for line in file: reuses the exact line-by-line iteration the chapter
recommends specifically because it's memory-efficient — file.readlines()
would load every line into a list all at once before counting, while
iterating the file object directly processes one line at a time
without ever holding the whole file in memory, a real advantage for
very large files.

count starts at 0 (outside the with block, so it survives after the
block closes) and increments once per line processed — a running-total
accumulator pattern reused from the loop-based accumulation covered
back in Course 1, Chapter 4. Returning count only after the with block
has fully finished ensures the file has been read to completion before
the final number is reported.