Python Dictionaries

A dictionary in Python is an unordered, mutable collection of key-value pairs. Each key is unique, and values can be of any data type. It is defined using curly braces {}.

my_dict = {"key1": "value1", "key2": "value2"}

A dictionary is defined using curly braces {} with key-value pairs separated by a colon :.

my_dict = {"name": "Alice", "age": 25}

You can access a value using its corresponding key inside square brackets [].

my_dict = {"name": "Alice", "age": 25}
print(my_dict["name"])  # Output: Alice

If you try to access a key that doesn't exist, a KeyError is raised

my_dict = {"name": "Alice", "age": 25}
print(my_dict["gender"])  # KeyError: 'gender'

You can use the get() method, which returns None (or a default value) if the key does not exist.

my_dict = {"name": "Alice", "age": 25}
print(my_dict.get("gender", "Not Found"))  # Output: Not Found

You can add a new key-value pair by simply assigning a value to a new key.

my_dict = {"name": "Alice", "age": 25}
my_dict["gender"] = "Female"

You can update the value of an existing key by reassigning a new value to that key.

my_dict = {"name": "Alice", "age": 25}
my_dict["age"] = 26

You can use the del keyword or the pop() method to remove a key-value pair

my_dict = {"name": "Alice", "age": 25}
del my_dict["age"]

• del removes the key-value pair and does not return anything.
• pop() removes the key-value pair and returns the value of the removed key.

my_dict = {"name": "Alice", "age": 25}
age = my_dict.pop("age")  # Removes "age" and returns its value

The clear() method removes all items from the dictionary.

my_dict = {"name": "Alice", "age": 25}
my_dict.clear()  # Dictionary is now empty

You can use the in keyword to check if a key exists in the dictionary

my_dict = {"name": "Alice", "age": 25}
print("name" in my_dict)  # Output: True

You can use the keys() method to get all the keys.

my_dict = {"name": "Alice", "age": 25}
print(my_dict.keys())  # Output: dict_keys(['name', 'age'])

You can use the values() method to get all the values

my_dict = {"name": "Alice", "age": 25}
print(my_dict.values())  # Output: dict_values(['Alice', 25])

You can use the items() method to get both keys and values.

my_dict = {"name": "Alice", "age": 25}
print(my_dict.items())  # Output: dict_items([('name', 'Alice'), ('age', 25)])

You can iterate over the dictionary using a for loop. You can iterate over keys, values, or both.

my_dict = {"name": "Alice", "age": 25}
for key, value in my_dict.items():
    print(key, value)

No, dictionary keys must be unique. If you assign a new value to an existing key, the previous value will be overwritten.

my_dict = {"name": "Alice", "age": 25}
my_dict["age"] = 26  # 'age' value is now 26

Dictionary keys must be immutable (e.g., strings, numbers, tuples). Lists and other mutable types cannot be used as keys.

You can merge dictionaries using the update() method or the | operator (Python 3.9+).

dict1 = {"name": "Alice"}
dict2 = {"age": 25}
dict1.update(dict2)  # Merges dict2 into dict1

The setdefault() method returns the value of a key if it exists; otherwise, it inserts the key with a specified default value.

my_dict = {"name": "Alice"}
print(my_dict.setdefault("age", 25))  # Output: 25

The fromkeys() method creates a new dictionary from a sequence of keys, with each key having the same default value.

keys = ["name", "age"]
my_dict = dict.fromkeys(keys, "N/A")

You can copy a dictionary using the copy() method.

original = {"name": "Alice", "age": 25}
new_dict = original.copy()

You can use the sorted() function to sort the dictionary by keys.

my_dict = {"b": 2, "a": 1, "c": 3}
sorted_dict = sorted(my_dict.items())  # Sorts by keys

You can use the sorted() function and pass a key argument to sort by values

my_dict = {"a": 3, "b": 1, "c": 2}
sorted_dict = sorted(my_dict.items(), key=lambda x: x[1])  # Sorts by values

• items() returns a view of key-value pairs.
• keys() returns a view of the keys.
• values() returns a view of the values.

my_dict = {"name": "Alice", "age": 25}
print(my_dict.items())  # [('name', 'Alice'), ('age', 25)]

The popitem() method removes and returns the last key-value pair from the dictionary as a tuple.

my_dict = {"name": "Alice", "age": 25}
print(my_dict.popitem())  # ('age', 25)

Nested dictionaries can be accessed using multiple keys

my_dict = {"person": {"name": "Alice", "age": 25}}
print(my_dict["person"]["name"])  # Output: Alice

No, dictionaries themselves are not sets, but their keys can be used to create a set.

You can check if a dictionary is empty by checking its length or using not in a condition.

my_dict = {}
if not my_dict:
    print("Dictionary is empty")

Dictionary comprehension allows you to create dictionaries using an expression inside curly braces.

my_dict = {x: x ** 2 for x in range(5)}  # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

A dictionary can store keys and values of any data type, including lists, tuples, strings, and numbers.

Yes, you can reverse iterate over a dictionary using reversed() or items() with reversed().

my_dict = {"a": 1, "b": 2, "c": 3}
for key, value in reversed(my_dict.items()):
    print(key, value)

You can merge multiple dictionaries using the update() method or the ** unpacking operator (Python 3.5+).

dict1 = {"a": 1}
dict2 = {"b": 2}
dict3 = {"c": 3}
merged_dict = {**dict1, **dict2, **dict3}

You can use the len() function to get the number of key-value pairs

my_dict = {"name": "Alice", "age": 25}
print(len(my_dict))  # Output: 2

You can use the dict() constructor to convert a list of tuples into a dictionary

my_list = [("a", 1), ("b", 2)]
my_dict = dict(my_list)

Dictionaries provide fast lookups, allow efficient searching, and enable storing data in key-value pairs, making them ideal for situations where data retrieval speed is important.