Dictionaries in Python

Creating a Dictionary

A dictionary is written using curly braces {}, with each key and its value separated by a colon, and pairs separated by commas.

person = {"name": "Alice", "age": 30, "city": "Delhi"}
print(person)
print(type(person))   # <class 'dict'>

Accessing Values by Key

Use square brackets with the key name to read a value. If the key does not exist, Python raises a KeyError.

print(person["name"])   # Alice
print(person["age"])    # 30

print(person["email"])  # raises KeyError: 'email'

The Safer get() Method

The .get() method looks up a key without raising an error if it is missing. It returns None by default, or a fallback value that you provide.

print(person.get("name"))                    # Alice
print(person.get("email"))                   # None - no error
print(person.get("email", "not provided"))  # not provided

Adding, Updating, and Removing Entries

Assigning to a new key adds an entry; assigning to an existing key updates it. The del keyword or the .pop() method removes an entry.

person["email"] = "alice@example.com"   # add a new key
person["age"] = 31                        # update an existing key
print(person)

del person["city"]                     # remove a key
print(person)

Common Dictionary Methods

Method	Description
.keys()	Returns a view of all the keys in the dictionary.
.values()	Returns a view of all the values in the dictionary.
.items()	Returns a view of (key, value) pairs.
.update(other)	Merges keys and values from another dictionary into this one.
.pop(key)	Removes a key and returns its value.

person = {"name": "Alice", "age": 30}

print(person.keys())     # dict_keys(['name', 'age'])
print(person.values())   # dict_values(['Alice', 30])
print(person.items())    # dict_items([('name', 'Alice'), ('age', 30)])

person.update({"city": "Delhi", "age": 31})
print(person)            # {'name': 'Alice', 'age': 31, 'city': 'Delhi'}

age = person.pop("age")
print(age)               # 31
print(person)            # {'name': 'Alice', 'city': 'Delhi'}

Looping Over a Dictionary

The most common way to loop over a dictionary is with .items(), which gives both the key and the value on each iteration.

person = {"name": "Alice", "age": 30, "city": "Delhi"}

for key, value in person.items():
    print(key, "-", value)

# name - Alice
# age - 30
# city - Delhi

Dictionary Comprehensions

Just like list comprehensions, a dictionary comprehension builds a new dictionary in a single line, usually by transforming an existing one.

prices = {"pen": 10, "book": 50, "bag": 200}

doubled = {k: v * 2 for k, v in prices.items()}
print(doubled)   # {'pen': 20, 'book': 100, 'bag': 400}

Checking If a Key Exists

Use the in keyword to check whether a key exists in a dictionary, before it is used, to avoid a KeyError.

person = {"name": "Alice", "age": 30}

if "name" in person:
    print("Name is present")

if "email" not in person:
    print("Email is not present")