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Lesson 8: Dictionaries (10 minutes)

Welcome to Lesson 8! In this lesson, we'll explore Python dictionaries and their usage. Dictionaries are powerful data structures that store key-value pairs and allow fast retrieval of values based on their keys.

Creating and Accessing Dictionaries:

In Python, dictionaries are defined using curly braces {} and consist of key-value pairs separated by colons :.

Example 1: Creating a Dictionary

# Empty dictionary
empty_dict = {}

# Dictionary with key-value pairs
person = {"name": "John", "age": 30, "occupation": "developer"}

To access the value associated with a specific key in a dictionary, you can use square brackets [] with the key inside.

Example 2: Accessing Values in a Dictionary

person = {"name": "John", "age": 30, "occupation": "developer"}

print(person["name"])       # Output: John
print(person["age"])        # Output: 30
print(person["occupation"]) # Output: developer

If the key is not present in the dictionary, accessing it will raise a KeyError. To avoid this, you can use the get() method, which returns a default value if the key is not found.

Example 3: Using get() method to Handle Missing Keys

person = {"name": "John", "age": 30}

print(person.get("name"))         # Output: John
print(person.get("occupation"))   # Output: None (key not found, returns None)
print(person.get("occupation", "-"))  # Output: - (key not found, returns default value "-")

Modifying and Adding Elements in a Dictionary:

You can modify the value associated with a key or add a new key-value pair to a dictionary using the assignment operator =.

Example 4: Modifying and Adding Elements in a Dictionary

person = {"name": "John", "age": 30}

person["age"] = 32       # Modify the value of "age"
person["occupation"] = "developer"  # Add a new key-value pair

print(person)           # Output: {"name": "John", "age": 32, "occupation": "developer"}

Removing Elements from a Dictionary:

To remove a key-value pair from a dictionary, you can use the del keyword or the pop() method.

Example 5: Removing Elements from a Dictionary

person = {"name": "John", "age": 30, "occupation": "developer"}

del person["age"]      # Remove "age" key and its value using del

# Remove "occupation" key and its value using pop()
occupation = person.pop("occupation")

print(person)         # Output: {"name": "John"}
print(occupation)     # Output: developer

Key Differences for C# Developers:

  • Python dictionaries are similar to C#'s dictionary or hash table data structure.
  • In Python, dictionaries use curly braces {} for defining key-value pairs, while C#'s dictionary initialization uses the Dictionary<TKey, TValue> class.

Now that you've learned about dictionaries in Python, you're ready to move on to Lesson 9, where we'll explore modules and packages, essential for organizing code in Python.

Additional Resources:

Practice Project:

  • Write a Python script that takes a list of words as input and creates a dictionary where the keys are the words, and the values are the lengths of the corresponding words.

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