Python Dictionary Operations

Table of Contents

In the last lesson, we learned about Python Introduction to Dictionaries. Now let’s read, add, update, remove, and check dictionary items.

Let’s Do More with Dictionaries

We know how to create a dictionary. We know how to access values. We know how to add and change items.

But there is more we can do. We can merge dictionaries. We can check if a key exists. We can get values safely. We can remove items in different ways.

Let’s go through the common dictionary operations.

Checking If a Key Exists in Python

We already saw in. It checks if a key is in the dictionary.

person = {"name": "Alex", "age": 25}
print("name" in person)

Output

True

print("city" in person)

Output

False

We can also use not in.

print("city" not in person)

Output

True

This is very common. We will use it all the time to avoid KeyError.

Getting a Value Safely with get() in Python

If we try to access a key that does not exist, we get a KeyError. But get() is safer. It returns None if the key is not found. Or we can give it a default value.

person = {"name": "Alex", "age": 25}
print(person.get("name"))

Output

Alex

print(person.get("city"))

Output

None

print(person.get("city", "Unknown"))

Output

Unknown

That is very useful. Let’s use get() when we are not sure if the key exists.

Setting a Default with setdefault() in Python

setdefault() is like get(), but it also adds the key if it does not exist.

person = {"name": "Alex", "age": 25}
person.setdefault("city", "London")
print(person)

Output

{'name': 'Alex', 'age': 25, 'city': 'London'}

If the key already exists, setdefault() does nothing. It returns the existing value.

person.setdefault("name", "Sam")
print(person)

Output

{'name': 'Alex', 'age': 25, 'city': 'London'}

The name is still “Alex”. Because it already existed.

Length of a Python Dictionary

We use len() to get the number of key-value pairs.

person = {"name": "Alex", "age": 25, "city": "London"}
print(len(person))

Output

Merging Dictionaries in Python

We can merge two dictionaries using the update() method.

dict1 = {"a": 1, "b": 2}
dict2 = {"b": 3, "c": 4}
dict1.update(dict2)
print(dict1)

Output

{'a': 1, 'b': 3, 'c': 4}

Notice that b was overwritten. Because dict2 had a value for b. The update method takes the second dictionary’s values for any overlapping keys.

If we do not want to modify the original, we can create a new dictionary.

dict1 = {"a": 1, "b": 2}
dict2 = {"b": 3, "c": 4}
merged = {**dict1, **dict2}
print(merged)

Output

{'a': 1, 'b': 3, 'c': 4}

The ** unpacks the dictionaries. This is a clean way to merge without changing the originals.

In Python 3.9 and later, we can use the | operator.

merged = dict1 | dict2
print(merged)

Output

{'a': 1, 'b': 3, 'c': 4}

That is even cleaner. But it only works in Python 3.9+.

Copying a Python Dictionary

We can copy a dictionary using copy().

original = {"a": 1, "b": 2}
copied = original.copy()
print(copied)

Output

{'a': 1, 'b': 2}

We can also use dict().

copied = dict(original)

Both create a shallow copy. If the dictionary contains mutable objects, they will be shared. For deep copies, we use copy.deepcopy().

Removing Items from a Python Dictionary

We already know del and pop(). Let’s see popitem() and clear().

popitem() removes and returns the last inserted key-value pair. In Python 3.7+, it removes the last item.

person = {"name": "Alex", "age": 25, "city": "London"}
item = person.popitem()
print(item)
print(person)

Output

('city', 'London')
{'name': 'Alex', 'age': 25}

clear() removes all items.

person.clear()
print(person)

Output

{}

Checking Equality in Python Dictionaries

We can compare two dictionaries with ==.

dict1 = {"a": 1, "b": 2}
dict2 = {"b": 2, "a": 1}
print(dict1 == dict2)

Output

True

Because dictionaries compare by content, not order.

Iterating Through Python Dictionaries

We already saw looping. But let’s recap the methods.

keys() returns a view of all keys.
values() returns a view of all values.
items() returns a view of all key-value pairs as tuples.

person = {"name": "Alex", "age": 25, "city": "London"}

for key in person.keys():
    print(key)

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

for key, value in person.items():
    print(f"{key}: {value}")

Output

name
age
city
Alex
25
London
name: Alex
age: 25
city: London

These views are dynamic. If we change the dictionary, the view updates.