Looking Up Postage by Country

Looking Up Postage by Country

In this lesson, you looked up the postage for order 1042 by a key held in a variable, and ported Marta’s rates table.

• The country comes from Iris’s address, cut down to the two fields this step needs, so the key is in a variable. Python’s RATES[country] is RATES[country] in JavaScript too: the brackets use the variable’s value.
• Ported the way the order was, with the quotes off the keys, the table keeps HOME_COUNTRY bare, as it was in Python. The port stops with TypeError: Cannot read properties of undefined (reading 'toFixed'): the lookup gave undefined, because in JavaScript a key that isn’t there gives undefined instead of raising a KeyError.
• Logged on its own, the table has a key called HOME_COUNTRY and no GB. In an object literal, a bare key is always the key’s own name, as the order’s id: 1042 made a key called id. It’s the name you’d write after a dot, because an object is also a record: order.id reads the key that id: 1042 wrote.
• Python reads a dict literal differently: a bare word there is a variable, so Python looks up HOME_COUNTRY and uses its value, and the dict prints with 'GB' first. The same line means the value in Python and the name in JavaScript.
• To use a variable’s value as a key, wrap it in brackets inside the literal: [HOME_COUNTRY]: 3.2, a computed property name. The brackets work out what’s inside and use the result, as Python’s bare word did.
• Brackets mean “the value of this”, whether you read a key or write one in a literal. Dots and bare keys mean “this name”, as a dot does on any Python object, so RATES.country looks for a key called country and gives undefined. Write a dot when you know the key as you write the code, as in address.country, and brackets when the key comes from data.

Here’s Marta’s Python step:

HOME_COUNTRY = "GB"
RATES = {
    HOME_COUNTRY: 3.20,
    "IE": 7.50,
    "DE": 8.90,
    "US": 14.50,
    "ROW": 16.50,
}

address = {"city": "Leeds", "country": "GB"}
country = address["country"]
rate = RATES[country]
print(f"Postage: £{rate:.2f}")