Class 11 • Chapter 10

Dictionaries

Chapter 10 covers Dictionaries, Python's powerful Key-Value data structure. It explains how to map labels to data, preventing KeyErrors, and utilizing dictionary-specific methods.

15 note blocks4 exam topics

🎯 Exam Focus Areas

Understand the rule that Dictionary Keys must be Immutable.Explain how the `get()` method prevents KeyErrors.Know how to extract both Keys and Values using a `for` loop and the `.items()` method.Perform basic operations like adding a new pair or updating an existing value.

Lists and Tuples are accessed using integer indices (0, 1, 2). This is great for sequences, but terrible if you want to look up data based on a label (e.g., finding the grade of 'Basant' without knowing his exact position in the list). Dictionaries solve this problem. A Dictionary is an unordered, mutable collection of Key-Value pairs. It acts like a real-world dictionary: you look up a word (the Key) to find its definition (the Value).

10.1 Creating Dictionaries

Dictionaries are enclosed in curly braces `{}`. Each item consists of a Key and a Value separated by a colon `:`. Items are separated by commas. Keys MUST be unique and Immutable (like Strings, Integers, or Tuples). Values can be anything (including Lists or other Dictionaries).

# Creating a dictionary
student_grades = {
    "Basant": 95,
    "Alice": 88,
    "Bob": 72
}

# Creating an empty dictionary
empty_dict = {}

10.2 Accessing and Modifying Data

You access a value not by its index number, but by providing its Key inside square brackets. Because dictionaries are Mutable, you can easily add new Key-Value pairs or update existing ones.

inventory = {"Apples": 10, "Bananas": 5}

# Accessing a value
print(inventory["Apples"])  # Output: 10

# Adding a new Key-Value pair
inventory["Oranges"] = 20

# Updating an existing value
inventory["Bananas"] = 8

print(inventory) # Output: {'Apples': 10, 'Bananas': 8, 'Oranges': 20}

10.3 The get() Method

If you try to access a key that does not exist using `dict['key']`, Python will instantly crash with a KeyError. To safely retrieve data, use the `.get(key)` method. If the key is missing, it returns `None` (or a default value) instead of crashing.

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

# print(user["email"])  <-- CRASH! KeyError

print(user.get("email"))                 # Output: None
print(user.get("email", "Not Provided")) # Output: Not Provided

10.4 Dictionary Methods

1dict.keys(): Returns a view of all the keys in the dictionary.
2dict.values(): Returns a view of all the values.
3dict.items(): Returns a view of all Key-Value pairs as Tuples. (Highly useful for loops).
4dict.pop(key): Removes the specified key and returns its value.
5dict.update(other_dict): Merges another dictionary into the current one.

10.5 Traversing a Dictionary

You can loop through a dictionary in several ways depending on whether you need the keys, the values, or both.

scores = {"Math": 90, "Science": 85}

# Looping through keys (Default behavior)
for subject in scores:
    print(subject)

# Looping through both Keys and Values using .items()
for subject, mark in scores.items():
    print(f"Got {mark} in {subject}")

📝 Quick Revision Points

1Dictionaries use curly braces `{}`.
2Format: `{"Key": "Value"}`.
3Keys MUST be unique and Immutable. Values can be anything.
4`dict["Key"] = newValue` updates or adds data.
5`dict.get("Key")` prevents crashing if Key is missing.
6`for k, v in dict.items():` loops through everything.

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inventory = {"Apples": 10, "Bananas": 5} # Accessing a value print(inventory["Apples"]) # Output: 10 # Adding a new Key-Value pair inventory["Oranges"] = 20 # Updating an existing value inventory["Bananas"] = 8 print(inventory) # Output: {'Apples': 10, 'Bananas': 8, 'Oranges': 20}

user = {"name": "John", "age": 30} # print(user["email"]) <-- CRASH! KeyError print(user.get("email")) # Output: None print(user.get("email", "Not Provided")) # Output: Not Provided

scores = {"Math": 90, "Science": 85} # Looping through keys (Default behavior) for subject in scores: print(subject) # Looping through both Keys and Values using .items() for subject, mark in scores.items(): print(f"Got {mark} in {subject}")