A class or module should have one responsibility, which in practice means one reason to change. A report class that parses a file, applies business rules and formats text has to change whenever any of the three does, and testing its rules means building a file first.

class Invoice:
    def __init__(self, lines):
        self.lines = lines

    def total(self):
        total = 0
        for qty, price in self.lines:
            total += qty * price
        return total

def render(invoice):
    return f"Total: {invoice.total():.2f}"

print(render(Invoice([(2, 1.5), (1, 4.0)])))

Invoice knows the rules and render knows the layout. A new layout leaves Invoice alone.

Pin the current behaviour with tests, move one piece out, run the tests, and repeat. Every step leaves working code, so you can stop at any point.

Split SalesReport into parse_sales(csv_text) returning (region, amount) pairs, a class SalesTotals(rows) whose by_region() returns a dict of sums, and render_totals(totals) producing the old text. sales_report(csv_text) connects them and returns what SalesReport(csv_text).text() did.

The explanation opens here once every check passes.