Python Dictionaries

A dictionary stores pairs of a key and a value: a word and its meaning, a name and a phone number, a product and its price. You look a value up by its key, not by a position. Dictionaries are written with curly brackets, with a colon between each key and its value. Together with lists, they are the most important collection in Python.

Creating and reading

student = {"name": "Asha", "age": 21, "city": "Pune"}
print(student["name"])
print(student.get("age"))
print(student.get("email"))
print(student.get("email", "not given"))
print(len(student), "city" in student)

Asha
21
None
not given
3 True

Square brackets give the value for a key and raise a KeyError when the key does not exist. get returns None, or the default you give it, instead. in checks whether a key is in the dictionary.

Adding, changing and deleting

prices = {"tea": 20, "coffee": 35}
prices["juice"] = 50        # a new key: added
prices["tea"] = 25          # an existing key: changed
del prices["coffee"]
removed = prices.pop("juice")
print(prices, removed)
prices.update({"cake": 60, "tea": 22})
print(prices)

{'tea': 25} 50
{'tea': 22, 'cake': 60}

Keys are unique: assigning to an existing key replaces its value. Dictionaries remember the order in which keys were first added.

Looping over a dictionary

stock = {"apples": 12, "bananas": 0, "mangoes": 7}
for item in stock:
    print(item)
for item, count in stock.items():
    status = "in stock" if count else "sold out"
    print(f"{item:8} {count:3}  {status}")
print(list(stock.keys()), sum(stock.values()))

apples
bananas
mangoes
apples    12  in stock
bananas    0  sold out
mangoes    7  in stock
['apples', 'bananas', 'mangoes'] 19

A plain loop gives the keys. items() gives key and value pairs, keys() the keys and values() the values.

Counting with a dictionary

A very common job is counting how often each thing appears. get with a default of 0 makes it a one-line loop body:

text = "to be or not to be"
counts = {}
for word in text.split():
    counts[word] = counts.get(word, 0) + 1
print(counts)

{'to': 2, 'be': 2, 'or': 1, 'not': 1}

Values can be anything, even lists or other dictionaries, so dictionaries can describe complex data such as the contents of a JSON file. Keys must be immutable: strings, numbers or tuples.

Practice exercises

1. Make a dictionary of three countries and their capitals, and print the capital of one of them.
2. Add a fourth country, change one capital, and print all pairs with a loop.
3. Count the letters of the word "banana" in a dictionary.
4. Ask for a fruit name and print its price from a dictionary, or "not sold here" when it is missing.