Python Dictionaries Explained

A Python dictionary stores data as key-value pairs. It is one of the most useful built-in data types in Python.

Dictionaries are great when your data has labels such as name, age, or price. Instead of finding a value by position, you look it up by its key.

For example:

student = {"name": "Ana", "age": 20}

In this dictionary:

• "name" is a key, and "Ana" is its value
• "age" is a key, and 20 is its value

If you are completely new to the term, see what a dictionary in Python is.

Quick example #

Use this example to see the most common dictionary tasks: create, read, update, add, and loop.

student = {"name": "Ana", "age": 20}

print(student["name"])
student["age"] = 21
student["city"] = "Madrid"

for key, value in student.items():
    print(key, value)

Expected output:

Ana
name Ana
age 21
city Madrid

What a dictionary is #

A dictionary:

• Stores data as key-value pairs
• Uses each key to look up a value
• Works well for labeled data
• Is written with curly braces {}

Example:

person = {"name": "Ana", "age": 20}

This is different from a list. A list uses positions like 0, 1, and 2. A dictionary uses keys like "name" and "age".

When to use a dictionary #

Use a dictionary when values need names instead of positions.

Common examples:

• A student record
• App settings
• Word counters
• Product details
• Grouped data

Example:

settings = {
    "theme": "dark",
    "language": "English",
    "notifications": True
}

A dictionary is often a better choice than a list when you want fast lookup by key.

How to create a dictionary #

You can create a dictionary with curly braces {}.

student = {"name": "Ana", "age": 20}
print(student)

Output:

{'name': 'Ana', 'age': 20}

Empty dictionary #

data = {}
print(data)

Output:

{}

Using dict() #

You can also create a dictionary with dict():

student = dict(name="Ana", age=20)
print(student)

Output:

{'name': 'Ana', 'age': 20}

If you want more ways to create one, see creating a dictionary in Python.

How to access dictionary values #

Use square brackets with a key:

student = {"name": "Ana", "age": 20}

print(student["name"])

Output:

Ana

Using get() #

If a key might not exist, use get().

student = {"name": "Ana", "age": 20}

print(student.get("name"))
print(student.get("city"))

Output:

Ana
None

This is safer than using square brackets when the key may be missing.

student = {"name": "Ana"}

# print(student["city"])   # This would raise an error
print(student.get("city"))

To learn more, see the Python dictionary get() method or how to access values in a dictionary in Python.

How to add and change items #

To add a new item, assign a value to a new key.

student = {"name": "Ana", "age": 20}
student["city"] = "Paris"

print(student)

Output:

{'name': 'Ana', 'age': 20, 'city': 'Paris'}

To change an existing item, assign a new value to an existing key.

student = {"name": "Ana", "age": 20}
student["age"] = 21

print(student)

Output:

{'name': 'Ana', 'age': 21}

You can also change multiple items with the update() method.

How to remove items #

Use pop() to remove a key and get its value back.

student = {"name": "Ana", "age": 20, "city": "Paris"}

removed_value = student.pop("city")

print(removed_value)
print(student)

Output:

Paris
{'name': 'Ana', 'age': 20}

Use del to remove a key without returning its value.

student = {"name": "Ana", "age": 20}

del student["age"]
print(student)

Output:

{'name': 'Ana'}

Use clear() to remove everything.

student = {"name": "Ana", "age": 20}

student.clear()
print(student)

Output:

{}

Be careful: removing a missing key can raise an error.

student = {"name": "Ana"}

# del student["age"]   # This would raise an error

For step-by-step help, see how to remove a key from a dictionary in Python.

Looping through a dictionary #

You can loop through keys, values, or both.

Loop through keys #

student = {"name": "Ana", "age": 20, "city": "Madrid"}

for key in student:
    print(key)

Loop through values #

student = {"name": "Ana", "age": 20, "city": "Madrid"}

for value in student.values():
    print(value)

Loop through key-value pairs #

This is often the easiest option when you need both the key and the value.

student = {"name": "Ana", "age": 20, "city": "Madrid"}

for key, value in student.items():
    print(key, value)

Possible output:

name Ana
age 20
city Madrid

You can learn more about items(), keys(), and values(), or see how to loop through a dictionary in Python.

Important dictionary rules #

Here are some important rules beginners should know:

• Keys must be unique
• Keys should be immutable
• Values can be any type
• Dictionaries are mutable

Keys must be unique #

If the same key appears more than once, the later value replaces the earlier one.

data = {"name": "Ana", "name": "Luis"}
print(data)

Output:

{'name': 'Luis'}

Keys should be immutable #

Good key types include:

• Strings
• Numbers
• Tuples

These work well because they do not change.

Values can be any type #

A dictionary value can be a string, number, list, or even another dictionary.

student = {
    "name": "Ana",
    "age": 20,
    "grades": [90, 85, 88],
    "address": {"city": "Madrid", "zip": "28001"}
}

print(student)

Dictionaries are mutable #

This means you can change them after creation.

student = {"name": "Ana"}
student["name"] = "Eva"

print(student)

Dictionary methods to learn next #

These methods are very useful:

• get() for safe lookup
• keys() to view all keys
• values() to view all values
• items() to work with key-value pairs
• update() to merge or change multiple items

Example:

student = {"name": "Ana", "age": 20}

print(student.keys())
print(student.values())
print(student.items())

student.update({"age": 21, "city": "Madrid"})
print(student)

Output:

dict_keys(['name', 'age'])
dict_values(['name', 20])
dict_items([('name', 'Ana'), ('age', 20)])
{'name': 'Ana', 'age': 21, 'city': 'Madrid'}

Common beginner problems #

Here are some common mistakes when working with dictionaries.

Using a key that does not exist #

This causes a KeyError if you use square brackets.

student = {"name": "Ana"}

# print(student["age"])   # KeyError
print(student.get("age"))  # Safer

If you are seeing this error, read KeyError in Python: causes and fixes.

Confusing dictionaries with lists #

A dictionary does not use list positions like 0 and 1.

student = {"name": "Ana", "age": 20}

# print(student[0])   # Wrong
print(student["name"])  # Correct

Trying to use duplicate keys #

Only the last value is kept.

data = {"color": "red", "color": "blue"}
print(data)

Output:

{'color': 'blue'}

Forgetting that keys are case-sensitive #

These are different keys:

data = {"Name": "Ana", "name": "Luis"}
print(data)

Output:

{'Name': 'Ana', 'name': 'Luis'}

Using brackets instead of get() when a key may be missing #

Use get() when you are not sure a key exists.

Useful debugging checks #

If your dictionary code is not working, these quick checks can help:

print(my_dict)
print(my_dict.keys())
print(my_dict.values())
print(my_dict.items())
print(type(my_dict))
print('name' in my_dict)

These are useful for spotting:

• Misspelled keys
• Wrong data type
• Missing keys
• Unexpected dictionary contents

book = {"title": "Python 101", "year": 2020}
book["author"] = "Ana"
book["year"] = 2021

print(book)
print(book.get("price"))

{'title': 'Python 101', 'year': 2021, 'author': 'Ana'}
None

FAQ #

What is a Python dictionary? #

A dictionary is a mutable data type that stores values as key-value pairs.

How is a dictionary different from a list? #

A list uses positions called indexes. A dictionary uses keys like names or labels.

Can dictionary keys be duplicated? #

No. If the same key appears more than once, the later value replaces the earlier one.

Can dictionary values have different data types? #

Yes. One dictionary can store strings, numbers, lists, or other dictionaries as values.

How do I safely get a value from a dictionary? #

Use get(), because it does not raise an error when the key is missing.

See also #

• What is a dictionary in Python?
• Python dictionary get() method
• How to add a key to a dictionary in Python
• How to check if a key exists in a dictionary in Python
• KeyError when accessing dictionary values: fix