Master Python Dictionaries: Syntax, Methods, Updates, Sorting & Merging

What Is a Dictionary in Python?

A Python dictionary is a mutable, unordered collection of key‑value pairs. Keys must be immutable (e.g., strings, numbers, tuples), while values can be any data type. Dictionaries are declared with curly braces:

students = {"Tim": 18, "Charlie": 12, "Tiffany": 22, "Robert": 25}

Python 3.7+ preserves insertion order, but the core concept remains the same: each key maps uniquely to a value.

Key Properties

• Keys are unique; duplicate keys overwrite previous entries.
• Keys must be immutable (strings, numbers, tuples).
• Key comparison is case‑sensitive.

Dictionary Syntax

Dict = { 'Tim': 18, 'xyz': 25 }

Keys and values are separated by colons; pairs are comma‑delimited.

Common Dictionary Methods

copy()

Creates a shallow copy of the dictionary.

original = {'Tim': 18, 'Charlie': 12}
copy_dict = original.copy()

update()

Updates the dictionary with key‑value pairs from another mapping or iterable.

students.update({'Sarah': 9})

items()

Returns a view of the dictionary’s key‑value pairs.

list(students.items())

len()

Returns the number of key‑value pairs.

len(students)

Membership Test (in)

Checks if a key exists in the dictionary.

"email" in my_dict

Updating and Deleting Entries

Add or Modify a Key

students.update({"Sarah": 9})

Delete a Key

del students["Charlie"]

Iterating and Sorting

While dictionaries are unordered, you can sort keys for display.

for name in sorted(students):
    print(f"{name}:{students[name]}")

Conversion to String

Use str() to get a printable representation.

print(str(students))

Merge Two Dictionaries

Two popular methods:

• Using update():

dict1.update(dict2)

• Using the ** unpacking operator (Python 3.5+):

merged = {**dict1, **dict2}

Sample Code Snippets

Python 3 Example – Create and Print a Dictionary

students = {"Tim": 18, "Charlie": 12, "Tiffany": 22, "Robert": 25}
print(students)

Check Key Existence

if "Tim" in students:
    print("Tim is present")

Method Reference Table

Method	Description	Syntax
copy()	Shallow copy of the dictionary	dict.copy()
update()	Merge or add key‑value pairs	dict.update(other)
items()	View of key‑value tuples	dict.items()
len()	Number of entries	len(dict)
in	Check key existence	key in dict

Key Takeaways

• Dictionary keys are unique, immutable, and case‑sensitive.
• Values can be any data type, including nested dictionaries or lists.
• Use copy(), update(), and ** for flexible data manipulation.
• Membership tests with in are fast and Pythonic.
• Python 3.7+ guarantees insertion order, simplifying iteration.