Python Tutorial › Data Structures › Dictionaries

Python Dictionaries

Q: How do I check if a key exists in a dictionary?

Use in: if "email" in user:. Use get() when you also want the value.

Q: How do I merge two dictionaries in Python?

Use a | b (Python 3.9+) or {a, b} to create a new dict, or a.update(b) to change a in place.

Q: What is the difference between dict.get() and dict[key]?

dict[key] raises KeyError if the key is missing. dict.get(key) returns None or a default instead.

BeginnerLesson 23 of 4713 min read10 runnable examples

A dictionary stores data as key-value pairs, so you can look a value up by a meaningful key instead of a position. Dictionaries model JSON, configuration, database rows and counters, which makes them one of the most used types in Python.

In this lesson

Create dictionaries and read values safely
Add, update and delete keys
Loop over keys, values and items
Work with nested dicts and merging

Create a dictionary

Write key: value pairs inside curly braces. Keys are usually strings, but any immutable value works.

Python

user = {"name": "Asha", "role": "tester", "years": 4}print(user)print(len(user)) ports = dict(http=80, https=443)       # dict() with keyword argumentsprint(ports)

Output

{'name': 'Asha', 'role': 'tester', 'years': 4}
3
{'http': 80, 'https': 443}

Since Python 3.7, dictionaries keep the order in which keys were inserted.

Read values

Use square brackets with the key. A missing key raises KeyError, so use get() when a key may be absent.

Python

user = {"name": "Asha", "role": "tester"}print(user["name"])print(user.get("email"))                    # None instead of an errorprint(user.get("email", "not provided"))    # with a defaultprint("role" in user)                       # checks keys

Output

Asha
None
not provided
True

Python

user = {"name": "Asha"}print(user["email"])

Output

Traceback (most recent call last):
  File "main.py", line 2, in <module>
    print(user["email"])
          ~~~~^^^^^^^^^
KeyError: 'email'

Add, change and delete

Python

config = {"browser": "chromium", "headless": True} config["timeout"] = 30          # add a new keyconfig["browser"] = "firefox"   # replace an existing valueconfig.update({"retries": 2, "headless": False})print(config) removed = config.pop("retries")  # remove and returndel config["timeout"]           # removeprint(removed, config)

Output

{'browser': 'firefox', 'headless': False, 'timeout': 30, 'retries': 2}
2 {'browser': 'firefox', 'headless': False}

setdefault() returns a key’s value, adding it with a default first if it is missing:

Python

groups = {}for name, team in [("asha", "qa"), ("ravi", "dev"), ("meena", "qa")]:    groups.setdefault(team, []).append(name)print(groups)

Output

{'qa': ['asha', 'meena'], 'dev': ['ravi']}

Loop over a dictionary

Python

stock = {"pen": 40, "notebook": 0, "bag": 7} for item in stock:                  # keys    print(item, end=" ")print() print(list(stock.values())) for item, qty in stock.items():     # key and value together    if qty == 0:        print(item, "is out of stock")

Output

pen notebook bag 
[40, 0, 7]
notebook is out of stock

Count things with a dict

Counting is the classic dictionary job:

Python

text = "the quick brown fox jumps over the lazy dog the end"counts = {}for word in text.split():    counts[word] = counts.get(word, 0) + 1 print(counts["the"])top = max(counts, key=counts.get)print(top, counts[top])

Output

3
the 3

Tip

collections.Counter does this in one line: Counter(text.split()).most_common(1). See the Collections lesson.

Nested dictionaries

Values can be lists or other dictionaries, which is exactly the shape of JSON data.

Python

response = {    "status": 200,    "data": {        "user": {"id": 7, "name": "Ravi"},        "roles": ["admin", "tester"],    },}print(response["data"]["user"]["name"])print(response["data"]["roles"][-1])print(response.get("error", {}).get("message", "no error"))

Output

Ravi
tester
no error

Merge dictionaries

Python

defaults = {"browser": "chromium", "headless": True, "timeout": 30}overrides = {"headless": False} settings = defaults | overrides      # Python 3.9+print(settings)print({**defaults, **overrides} == settings)

Output

{'browser': 'chromium', 'headless': False, 'timeout': 30}
True

When both have the same key, the value from the right-hand dict wins.

Dictionary methods

Method	What it does
`get(k, default)`	Value for `k`, or `default` if missing
`keys()`, `values()`, `items()`	Views of the keys, values or pairs
`update(other)`	Adds or replaces keys from another dict
`pop(k)`	Removes `k` and returns its value
`popitem()`	Removes and returns the last inserted pair
`setdefault(k, d)`	Returns `k`‘s value, inserting `d` first if missing
`copy()`	Shallow copy
`clear()`	Removes everything
`dict.fromkeys(keys, v)`	New dict with each key set to `v`

Sort a dictionary

Python

scores = {"Asha": 88, "Ravi": 74, "Meena": 93}print(sorted(scores))                                    # keysprint(sorted(scores.items(), key=lambda kv: kv[1], reverse=True))ranked = dict(sorted(scores.items(), key=lambda kv: kv[1], reverse=True))print(ranked)

Output

['Asha', 'Meena', 'Ravi']
[('Meena', 93), ('Asha', 88), ('Ravi', 74)]
{'Meena': 93, 'Asha': 88, 'Ravi': 74}

Check your understanding

Predict the output

d = {"a": 1, "b": 2}d["a"] += 10d.setdefault("c", 3)d.setdefault("a", 99)print(d, d.get("z", 0))

Quick quiz

1. What does d.get("x") return when “x” is not a key?

get() returns None, or the default you pass as the second argument.

2. Which loop gives both keys and values?

items() yields (key, value) pairs.

3. Which of these can NOT be a dictionary key?

4. In a | b, if both dicts have key “x”, whose value is kept?

Exercise: invert a dictionary

Swap keys and values of a country-code dict, then look up the code for “India”.

Input

{"IN": "India", "US": "United States", "JP": "Japan"}

Expected output

{'India': 'IN', 'United States': 'US', 'Japan': 'JP'}
IN

Hint 1

A dict comprehension: {v: k for k, v in codes.items()}.

Python

codes = {"IN": "India", "US": "United States", "JP": "Japan"}by_name = {name: code for code, name in codes.items()}print(by_name)print(by_name["India"])

Output

{'India': 'IN', 'United States': 'US', 'Japan': 'JP'}
IN

The comprehension walks over each (code, name) pair and builds a new dict with the name as the key. This only works safely when the values are unique.

Program Practice

94 Python Dictionary Programs

Every program has a step-by-step explanation, colour-coded code and its real output.

Start practising →

#1EasyAdd elements to a dictionary #2EasySquare of every value #3EasyMove items to another dictionary #4EasyConcatenate two dictionaries #5EasyOdd and even keys #6EasyDictionary from two lists

Related sets:All practice sets

Python dictionary FAQ

Are Python dictionaries ordered?

Yes. Since Python 3.7, dictionaries preserve insertion order.

How do I check if a key exists in a dictionary?

Use in: if "email" in user:. Use get() when you also want the value.

How do I merge two dictionaries in Python?

Use a | b (Python 3.9+) or {a, b} to create a new dict, or a.update(b) to change a in place.

What is the difference between dict.get() and dict[key]?

dict[key] raises KeyError if the key is missing. dict.get(key) returns None or a default instead.