Python Dictionaries: Accessing Values and Understanding Dictionary Slicing

Python Programming Tutorial for Students

In this lesson, we will learn how to create a Python dictionary, access values using dictionary keys, and understand why dictionary slicing such as d['bar':'baz'] produces an error.

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1. What is a Dictionary in Python?

A dictionary is a Python data structure that stores data as key-value pairs.

For example:

d = {'foo': 100, 'bar': 200, 'baz': 300}

In this dictionary:

• 'foo' is a key and 100 is its value.
• 'bar' is a key and 200 is its value.
• 'baz' is a key and 300 is its value.

The dictionary can be visualized as:

Key	Value
foo	100
bar	200
baz	300

2. Accessing a Dictionary Value

To access a value in a dictionary, place the key inside square brackets [].

d = {'foo': 100, 'bar': 200, 'baz': 300}

print(d['bar'])
print(d['baz'])

The output is:

200
300

When Python sees:

d['bar']

it searches the dictionary for the key 'bar' and returns its corresponding value, which is 200.

3. What Happens If We Try to Slice a Dictionary?

Students who are familiar with lists and strings may try something like:

d['bar':'baz']

However, this is not valid dictionary syntax.

Python interprets 'bar':'baz' as a slice. Dictionaries do not support slicing in this way.

You may therefore see an error similar to:

KeyError: slice('bar', 'baz', None)

4. Why Does This Error Occur?

Slicing is commonly used with sequences such as lists, strings and tuples.

For example:

numbers = [10, 20, 30, 40, 50]

print(numbers[1:4])

Output:

[20, 30, 40]

Here, [1:4] means "give me the elements starting from index 1 up to, but not including, index 4."

A dictionary, however, is normally accessed using its keys, not by slicing between keys.

5. Correct Way to Retrieve Multiple Dictionary Entries

If you want the values associated with 'bar' and 'baz', you can access them individually:

d = {'foo': 100, 'bar': 200, 'baz': 300}

print(d['bar'])
print(d['baz'])

Output:

200
300

6. Selecting Several Specific Keys

We can also create a new dictionary containing only the keys we want:

d = {'foo': 100, 'bar': 200, 'baz': 300}

result = {key: d[key] for key in ['bar', 'baz']}

print(result)

Output:

{'bar': 200, 'baz': 300}

7. If You Really Want to Slice Dictionary Entries

You can first convert the dictionary's items into a list and then use normal list slicing.

d = {'foo': 100, 'bar': 200, 'baz': 300}

print(list(d.items())[1:3])

Output:

[('bar', 200), ('baz', 300)]

Notice that we used list(d.items()) first. This converts the dictionary's key-value pairs into a list, and lists support slicing.

8. Important Difference to Remember

Expression	Meaning	Result
d['bar']	Access the value using the key bar	200
d['baz']	Access the value using the key baz	300
d['bar':'baz']	Attempts dictionary slicing	❌ Error
list(d.items())[1:3]	Converts items to a list and slices the list	✅ Works

9. Complete Example

# Creating a dictionary
d = {'foo': 100, 'bar': 200, 'baz': 300}

# Accessing individual values
print(d['bar'])
print(d['baz'])

# Selecting dictionary items by position
print(list(d.items())[1:3])

Output:

200
300
[('bar', 200), ('baz', 300)]

Key Takeaway for Students

Dictionary → access values using keys.
List/String/Tuple → can be sliced using positions.

10. Practice Exercise

Create the following dictionary:

student = {
    'name': 'John',
    'age': 20,
    'department': 'Computer Engineering',
    'level': 300
}

Then write Python code to:

1. Print the student's name.
2. Print the student's age.
3. Print the student's department.
4. Print the student's level.
5. Try using a slice such as student['age':'level'] and observe the error.

Conclusion

Python dictionaries are designed around key-value pairs. To retrieve a value, use its key inside square brackets, such as d['bar'].

Unlike lists, strings and tuples, dictionaries do not support traditional slicing such as d['bar':'baz']. If you need to select entries by position, you can convert the dictionary items to a list first.

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