Code: draft it, read it, debug it

Write a Python function that reads a CSV file and prints the average
of the "marks" column.
Then explain every line as if teaching a beginner.
Finally, tell me what happens if one cell in that column is blank,
and show the smallest change that handles it.

import csv

def average_marks(path):
    total, count = 0, 0
    with open(path, newline="") as f:
        for row in csv.DictReader(f):
            value = row.get("marks", "").strip()
            if value:                       # skip blank cells
                total += float(value)
                count += 1
    return total / count if count else 0

print(average_marks("students.csv"))