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Add Dictionary Items

Dictionaries in Python are dynamic and mutable, which means you can add new key–value pairs at any time. Adding items is a very common operation when building real-world data structures.
This topic explains all correct ways to add dictionary items, with examples and best practices.

Add Item Using Assignment

The simplest and most common way.
python
student = {
    "name": "Jayesh",
    "age": 25
}

student["course"] = "Python"
print(student)
If the key already exists, the value is updated.
python
student["age"] = 26

Add Item Using update()

The update() method adds one or more items at once.
python
student.update({"city": "Pune"})
print(student)
python
student.update({"salary": 50000, "active": True})
print(student)

Add Items from Another Dictionary

python
extra = {"experience": 3, "company": "TechCorp"}

student.update(extra)
print(student)

Add Items Conditionally

python
if "email" not in student:
    student["email"] = "jayesh@example.com"

print(student)

Add Items Using Loop

python
keys = ["a", "b", "c"]
values = [1, 2, 3]

d = {}
for k, v in zip(keys, values):
    d[k] = v

print(d)

Add Items Using setdefault()

setdefault() adds a key only if it does not exist.
python
person = {"name": "Amit"}

person.setdefault("age", 30)
print(person)
If key exists, value is not changed.
python
person.setdefault("age", 40)
print(person)

Add Nested Dictionary Items

python
data = {
    "user": {
        "name": "Riya"
    }
}

data["user"]["age"] = 22
print(data)

Add Items Using Dictionary Comprehension

python
numbers = [1, 2, 3]
squares = {x: x*x for x in numbers}
print(squares)

Common Mistakes

Using Duplicate Keys

python
d = {"a": 1, "a": 2}
print(d)
The last value overwrites previous ones.

Using Mutable Keys

python
# d = {[1, 2]: "value"}   # TypeError
Keys must be immutable.

Best Practices

  • Use assignment for single item
  • Use update() for multiple items
  • Use setdefault() for safe defaults
  • Check key existence when needed
  • Keep keys meaningful and consistent

Summary

  • Dictionaries allow adding items dynamically
  • Assignment and update() are most common
  • setdefault() prevents overwriting
  • Keys must be unique and immutable
  • Supports nested and bulk additions