Dictionaries

Introduction

A dictionary stores pairs: a key and its value. Instead of asking for "the item at position 2", you ask for "the value for this key", like looking up a name in a phone book.

Lists find things by position, but much of the world is organised by name: a word and its meaning, a student and their mark. A dictionary finds a value directly from its key, however large the collection grows.

How it works

Keys and values

ages = {"Rahim": 21, "Karim": 22}
ages["Nadia"] = 20
print(ages["Rahim"])
print(len(ages))

The objects panel draws the dictionary as a two-column table, keys on the left and values on the right.

• ages["Rahim"] looks up a value by its key.
• ages["Nadia"] = 20 adds a new pair, or replaces the value if the key already exists.
• len(ages) counts the pairs. Each key appears only once.

Looping over pairs

stock = {"pens": 12, "pencils": 30}
for item, amount in stock.items():
    print(item, amount)

items() gives each key together with its value, so the loop gets two variables per round.

Syntax

ages = {"Rahim": 21}
ages["Nadia"] = 20
ages.get("Karim", 0)
"Rahim" in ages
for name, age in ages.items():
    print(name, age)

Example

text = "a b a c b a"
counts = {}
for word in text.split():
    counts[word] = counts.get(word, 0) + 1
print(counts)

This counts how often each word appears. counts.get(word, 0) gives the current count, or 0 if the word has not been seen yet; adding 1 and storing it back updates the count. Step through and watch rows appear and change in the table: a reaches 3, b 2 and c 1.

Line by line

1. text is the sentence to count.
2. counts = {} starts with an empty dictionary.
3. text.split() gives the words, and the loop visits each one.
4. counts.get(word, 0) gives the count so far, or 0 the first time.
5. Adding 1 and storing it back updates that word's count.
6. print(counts) shows the finished tally.

Expected result

{'a': 3, 'b': 2, 'c': 1}

In the real world

A dictionary can count votes, store settings or hold a cache of answers. JSON, the format used by almost every web service, maps directly onto Python dictionaries and lists.

Common mistakes

• ages["Tanvir"] for a key that does not exist is a KeyError. Check with "Tanvir" in ages, or use ages.get("Tanvir").
• Using a list as a key is a TypeError; keys must not change, so use strings, numbers or tuples.
• Expecting two values for one key: assigning to an existing key replaces the old value.

Best practices

• Use get or in when a key might be missing.
• Use strings, numbers or tuples as keys.
• Use items() when you need keys and values together.

Summary

A dictionary stores key and value pairs and finds values directly by key. Assigning to a key adds or replaces it, get supplies a default, and items() loops over both at once.

Practice

Read a line of words and print the word that appears most often. If several words tie, print the one that appeared first.

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