Handle a zero denominator explicitly

Your task

Write a rate function and test both ordinary and undefined cases. Explain the expected output before running the code.

Hint

Read with csv.DictReader so column names are explicit. CSV values are strings until converted. Keep parsing failures visible.

Worked solution

54/900 is 0.06. A zero denominator returns None, meaning unavailable, rather than claiming a 0% measured conversion rate. Validate nonnegative counts separately.

import csv
with open('retail-orders.csv',newline='',encoding='utf-8') as f:
    rows=list(csv.DictReader(f))
def rate(numerator,denominator):
    if denominator==0: return None
    return numerator/denominator
assert rate(54,900)==0.06
assert rate(0,0) is None

Extend the test

Change one input in a disposable copy and predict which check should fail. Restore the original before comparing results.

Make the reasoning yours.

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