Class 12 Computer Science - Python Dictionaries

A Dictionary is an unordered, mutable collection of key-value pairs.

1. Creating a Dictionary

student = {
    "name": "Rahul",
    "roll_no": 101,
    "marks": 95
}

2. Important Methods

• dict.keys(): Returns all the keys.
• dict.values(): Returns all the values.
• dict.items(): Returns key-value pairs as tuples.
• dict.get(key): Returns value for a key without raising an error if key is missing.
• dict.pop(key): Removes the specified key and returns the value.

3. Merging Dictionaries

In Python 3.9+, you can use the union operator:

dict3 = dict1 | dict2

4. Dictionary Comprehension

squares = {x: x*x for x in range(1, 6)}
# Output: {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

5. Board Exam Typical Question

Question: Write a function to count the frequency of each character in a string using a dictionary.

def count_freq(s):
    freq = {}
    for char in s:
        freq[char] = freq.get(char, 0) + 1
    return freq

Tip: Remember that keys in a dictionary must be immutable (Strings, Numbers, Tuples). 🐍💻