Python Practice 5/9

Dictionaries

A dict maps keys to values: {"apple": 2, "bread": 3} looks up a price by name instead of by position, the way a list would. prices["apple"] reads the value stored under the key "apple" - and it throws a KeyError if that key doesn't exist, so you're always looking up something that's actually there in this lesson.

sum(x for x in items) totals a generator expression - like a list comprehension without the brackets, built to be summed (or looped) instead of collected into a list first.

Sometimes a key might not be there, and a KeyError would be the wrong behavior. prices.get(key, default) reads a value the same way [] does, but returns default instead of raising when the key is missing - the safe way to look something up when you're not sure it exists.

Your task: write total_cost(items, prices), which takes a list of item names and returns the sum of their prices, looked up in prices. Then write total_cost_safe(items, prices), doing the same thing but treating any item not in prices as costing 0 instead of raising.

You'll practice:

Looking up values in a dict with []
Summing a generator expression
Falling back to a default with .get(key, default) when a key might be missing

prices = {"apple": 2, "bread": 3, "milk": 4}

# Write total_cost(items, prices): sum of prices[item] for each item in items.
def total_cost(items, prices):
    pass

# Write total_cost_safe(items, prices): same, but items missing from prices count as 0.
def total_cost_safe(items, prices):
    pass