lesson 37 Dictionaries & Sets · beginner

Add, update and remove pairs

d[key] = value adds a new pair or replaces the old value. del d[key] removes a pair. Updating a key that isn't there raises KeyError.

You'll be able to

Add or replace a value with d[key] = value, and remove a pair with del
Update a value from its old value, with d[key] = d[key] - n or d[key] -= n
Apply a list of events to a dictionary, skipping keys that don't exist

The idea

Stock levels, latest prices and current addresses all change, and a dictionary changes with them, in place, like a list.

stock["P0164"] = 25 adds the pair if the key is new and replaces the value if it isn't. There is no separate add command.
del stock["P0659"] removes a pair.
stock["P0607"] = stock["P0607"] - 2 updates a value from its old value. The short form stock["P0607"] -= 2 does the same, and += adds.

Run each example to see what it prints. Running examples doesn't count toward progress.

example-1.py · Keeping stock levels current

stock = {"P0607": 12, "P0208": 3, "P0659": 0}
stock["P0607"] = stock["P0607"] - 2
stock["P0164"] = 25
del stock["P0659"]
print(stock)
stock["P0208"] += 5
print(stock["P0208"], len(stock))

example-2.py · Updating a key that isn't there

stock = {"P0607": 12}
stock["P0607"] -= 1
print(stock)
stock["P0123"] -= 1

In a real system, changes arrive as a list of events, such as sales. Apply them one at a time, check that each key exists, and react when a value crosses a limit.

example-3.py · Applying a day's sales

stock = {"P0607": 12, "P0208": 3, "P0659": 7}
sales = [("P0607", 2), ("P0208", 2), ("P0123", 1), ("P0659", 5)]
for product_id, quantity in sales:
    if product_id not in stock:
        print("no such product:", product_id)
        continue
    stock[product_id] = stock[product_id] - quantity
    if stock[product_id] < 3:
        print("reorder", product_id, "only", stock[product_id], "left")
print(stock)

When to use it: whenever values per key change over time: stock, latest status, the newest email address. Mistakes to avoid:

Using += or -= on a key that may not exist yet.
Expecting d[key] = value to fail for an existing key. It silently replaces the value.
Deleting a key that isn't there, which also raises KeyError.

Review

d[key] = value adds a new pair or replaces the value of an existing key. del d[key] removes a pair.
d[key] -= n and d[key] += n read the old value first, so the key must exist.
Apply events one at a time, check keys with in, and use continue to skip the unknown ones.
Next: walk every key and value of a dictionary to answer questions about all of it.

Practise this