5. Lighthouse

Level 4 · Pick the right shipping fee

From Megan Collins, Head of Sales

“Our shipping policy: business accounts always ship free. Everyone else pays $7.95 under $50, $4.95 from $50 up to (not including) $75, and nothing from $75. What does this cart pay?”

Your goal

Set result to the shipping fee (a float) for the given subtotal and segment: 0.0 for segment 'business'; otherwise 7.95 if subtotal is below 50, 4.95 if it is at least 50 and below 75, and 0.0 if it is 75 or more.

  • Store a number in result

Your code

Python

Loading the editor…

    ResultPress Run to see yours

    Mote checks your result

    Press Run. Each row that matches the goal becomes a firefly in your lantern.

    The lesson

    True and False

    A boolean (bool) has only two values: True and False. You get them from comparisons:

    62.40 >= 50          # True
"consumer" == "business"   # False
    operator meaning
    == equal to
    != not equal to
    < <= less than, less than or equal
    > >= greater than, greater than or equal

    String comparisons are exact and case-sensitive: "Business" == "business" is False. That’s why you’ll often see .lower() before a comparison on data typed by people.

    Combining conditions

    and is true only when both sides are true. or is true when at least one side is. not flips a value.

    subtotal = 62.40
subtotal >= 50 and subtotal < 75    # True
segment == "business" or subtotal >= 75
not (subtotal < 50)                 # True

    Python also lets you chain a range check, which reads like maths: 50 <= subtotal < 75.

    if, elif, else

    An if statement runs a block of code only when its condition is true. The block is the indented lines under it, and the line before it ends with a colon:

    if subtotal >= 75:
    shipping = 0.0
elif subtotal >= 50:
    shipping = 4.95
else:
    shipping = 7.95

    Python checks the conditions top to bottom and stops at the first true one. That’s why the second condition can simply say subtotal >= 50: if the cart were 75 or more, the first branch would already have run. elif (else if) can repeat as many times as you need, and else catches everything left over. Both are optional.

    Indentation is not decoration in Python; it defines which lines belong to the branch. Use four spaces consistently.

    Order your rules from most specific

    Business rules usually have exceptions (“business accounts always ship free”). Put the exceptions first, otherwise a general rule may grab the case before the exception is reached:

    if segment == "business" or subtotal >= 75:
    shipping = 0.0
elif subtotal >= 50:
    shipping = 4.95
else:
    shipping = 7.95

    Watch the boundaries

    “From $50 up to $75” is where bugs live. Is $75 itself in the middle band or free? The policy says free from $75, so the check is >= 75, not > 75. When you read a rule, always ask what happens exactly on the edge, and test a value right on it.

    Common mistakes

    • Writing = instead of == in a condition. if segment = "business": is a syntax error.
    • Missing the colon at the end of if, elif or else lines.
    • Getting the band order wrong, for example checking subtotal >= 50 before subtotal >= 75, so a $120 cart pays $4.95.
    • Writing if segment == "business" or "enterprise":. The right side is a non-empty string, which always counts as true. Repeat the comparison: segment == "business" or segment == "enterprise".