Lab 02 — Python Refresher

Session: Week 1 · Session 2 · 90 min CLO: CLO-1 Difficulty: Beginner

Learning objectives

By the end of this lab you can:

1. Work fluently with lists, dicts, tuples, and sets.
2. Write for loops, conditionals, and list comprehensions.
3. Define and call functions with default arguments.
4. Read a small data file and do simple summary math in pure Python.

Problem statement

You receive sales records as a plain-text file (one row per line, product,units,price). Before anyone introduces pandas, the team wants a pure-Python script that reads the file, computes totals per product, and prints a clean report — so the logic is fully understood by hand.

Dataset requirements

Create sales.txt yourself with the starter code below (or type it by hand). It is a small, hand-made dataset — 8 rows.

Step-by-step tasks

1. Write a function parse_line(line) that turns "coffee,3,2.50" into ("coffee", 3, 2.5) — a tuple.
2. Write a function read_sales(path) that reads the file, skips the header line, and returns a list of parsed tuples. Handle a blank line gracefully.
3. Write totals_by_product(rows) returning a dict product -> total revenue (units × price).
4. Write print_report(totals) that prints one line per product sorted by revenue (highest first) plus a grand total line.
5. Use a list comprehension somewhere in steps 1–4 and comment it.
6. Run the full script; compare your output with the expected output.

Starter code

# sales.py
def parse_line(line):
    """'coffee,3,2.50' -> ('coffee', 3, 2.5). Return None for blank lines."""
    line = line.strip()
    if not line:
        return None
    # your code here: split on ',', convert types, return a tuple


def read_sales(path):
    rows = []
    with open(path) as f:
        next(f)                     # skip header
        for line in f:
            row = parse_line(line)
            if row is not None:
                rows.append(row)
    return rows


def totals_by_product(rows):
    totals = {}
    for product, units, price in rows:
        # your code here: accumulate units * price per product
        pass
    return totals


def print_report(totals):
    # your code here: sort by revenue descending, print, grand total
    pass


if __name__ == "__main__":
    rows = read_sales("sales.txt")
    totals = totals_by_product(rows)
    print_report(totals)

# sales.txt (create this file)
product,units,price
coffee,3,2.50
tea,5,1.20
coffee,2,2.50
cake,1,4.00
tea,2,1.20
juice,4,3.00
cake,2,4.00
coffee,1,2.50

Expected output

cake         12.00
juice        12.00
coffee       15.00
tea           8.40
grand total  47.40

(Sorting order: your choice between alphabetical and by revenue — say which you used in a comment.)

Questions

1. What type does parse_line return, and why is a tuple a good fit?
2. Why do we skip the first line of the file?
3. What does units * price produce if units is a string? Why must you convert types?
4. Rewrite totals_by_product with defaultdict — how does it change the code?
5. A customer buys tea,0,1.20 — what should the report show?

Challenge task

Extend the report with a max_product(totals) function that returns the best-selling product by units (not revenue). Print it as best by units: coffee. Then add a --sort alpha command-line flag (sys.argv) that sorts the report alphabetically instead of by revenue.

Marking rubric

Criteria	Max	Notes
parse_line correct types & blank handling	2	returns tuple or None
read_sales skips header, no errors	2	8 rows parsed
totals_by_product correct math	3	values match expected
print_report sorted + grand total	3	formatting close to expected
Comprehension used & commented	1	anywhere in script
Answers to questions	2	Q2, Q3 correct
Challenge: units max + --sort flag	3	both work
Total	16