Python Dictionaries: Adding, Updating, and Managing Key/Value Pairs

Python dictionaries are foundational data structures that store data as key/value pairs. Keys are immutable and unique, while values can be any data type, including nested structures.

Key Rules for Dictionary Keys

• Duplicate keys are overwritten: the last occurrence takes precedence.
• Allowed key types include strings, numbers, tuples, and other hashable objects. Mutable types like lists cannot be used as keys.
• Python’s built‑in functions, classes, and even functions themselves can serve as keys.

Adding Elements to a Dictionary

To add a new key/value pair, simply assign a value to a new key:

my_dict = {"username": "XYZ", "email": "xyz@example.com"}
my_dict["location"] = "Mumbai"
print(my_dict)

Output: {"username": "XYZ", "email": "xyz@example.com", "location": "Mumbai"}

When the value is a list, you can append items to that list:

my_dict = {"tags": []}
my_dict["tags"].append("python")
print(my_dict)

Output: {"tags": ["python"]}

Accessing Dictionary Elements

Retrieve a value by its key using square brackets. If the key does not exist, a KeyError is raised:

print(my_dict["username"])  # XYZ
print(my_dict["nonexistent"])  # KeyError

To avoid errors, use dict.get() or dict.setdefault():

print(my_dict.get("nonexistent", "default"))

Deleting Elements

• del keyword: del my_dict["username"]
• pop() method: removed = my_dict.pop("email") returns the removed value.
• clear() method: my_dict.clear() empties the dictionary.

Updating Existing Values

Assign a new value to an existing key to update it:

my_dict["username"] = "ABC"
print(my_dict)

Combining Dictionaries

Merge two dictionaries using update() or the union operator (|) in Python 3.9+:

dict_a = {"a": 1, "b": 2}
dict_b = {"b": 3, "c": 4}
dict_a.update(dict_b)
print(dict_a)  # {"a": 1, "b": 3, "c": 4}

# or
combined = dict_a | dict_b
print(combined)

Embedding a Dictionary Inside Another

Assign a whole dictionary to a new key to nest it:

parent = {"username": "XYZ"}
child = {"firstName": "Nick", "lastName": "Price"}
parent["name"] = child
print(parent)

Output: {"username": "XYZ", "name": {"firstName": "Nick", "lastName": "Price"}}

Summary

• Dictionary keys are unique, hashable, and immutable.
• Use assignment for adding or updating, del or pop() for removal, and clear() to empty.
• Methods like update() and the union operator simplify merging dictionaries.

For more details, refer to the official Python documentation: Python Data Structures – Dictionaries.