Dictionaries

A dictionary maps keys to values, like a real dictionary maps words to meanings:

stock = {"apple": 5, "pear": 2}
stock["apple"]         # 5
stock["kiwi"] = 7      # add or update
stock.get("plum", 0)   # 0: a safe lookup with a default

for name, qty in stock.items():
    print(name, qty)

key in stock tests whether a key exists; key not in stock tests that it is missing. Before appending to a list stored in a dict, create [] for a missing key.

stock.items() gives key-value pairs in insertion order, so grouped lists can keep the original order.

See it run

ages = {"mo": 30, "li": 25}
ages["jo"] = 41
ages["mo"] = 31
for name in ages:
    print(name)

A dict maps keys to values. Same square-bracket assignment either adds or overwrites.

Practice dictionaries

Exercise 1

Use stock in the editor below.

Add "kiwi" with 7.
Sell 2 apples (subtract 2 from the apple count).
Loop with .items() and print each line as apple: 3.

Exercise 2

Count how often each word appears in words in the editor below. Build the dict counts (word to count) with a loop, then set top to the count for "the".

Exercise 3

Use pets in the editor below. The vet wants pets grouped by species. Build by_species so each species maps to a list of pet names, in the order they appear in pets: {"dog": ["Rex", "Fido"], "cat": ["Tom", "Kit"], "parrot": ["Polly"]}

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Practice

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Visualizer Watch your code run line by line

Run your code, then choose Play to follow its execution.

Memory · names → values

Variables appear here as Python creates them.

Output & plots