Dictionaries in Python

🟢 Beginner

📖 Definition

A Dictionary in Python is a mutable, key-value mapping collection enclosed in curly braces {} with the syntax {"key": "value"}. Dictionary keys must be unique and immutable/hashable (strings, numbers, tuples), while values can be any data type.

🇮🇳 Hindi

Dictionary key-value pairs ka structure hota hai ({"name": "Aarav", "age": 25}). Index position ke bajaye data Key ke dwara search kiya jata hai (dict["key"]). Key missing rehne par error se bachne ke liye safe method .get("key", default) ka use karein.

🚩 Marathi

Dictionary madhye data Key-Value स्वरूपात (pair) saathavla jato. Key missing aslyas error yenaypasun vachnyasathi .get() method vapara.

🧠 Simple Explanation

Think of a Python dictionary as a real physical language dictionary. When you want to find the meaning of "algorithm", you don’t read page by page from the beginning—you directly look up the key word "algorithm" to instantly fetch its associated definition value.

📝 1. Accessing & Modifying Dictionary Entries

user = {
    "id": 101,
    "name": "Rahul Verma",
    "role": "Developer",
    "is_active": True
}

# 1. Direct Bracket Access (Raises KeyError if key doesn't exist!)
print(user["name"]) # "Rahul Verma"

# 2. Safe .get() Access (Returns default fallback if key is missing)
location = user.get("location", "Not Specified") # "Not Specified"

# 3. Adding or Updating Entries
user["location"] = "Mumbai" # Adds new key-value pair
user["role"] = "Lead Developer" # Updates existing key

# 4. Deleting Entries
removed_role = user.pop("role") # Removes and returns value

📝 2. Iterating Dictionaries (.keys(), .values(), .items())

scores = {"Aarav": 92, "Priya": 88, "Rohan": 95}

# Iterating Keys
for name in scores.keys():
    print(name)

# Iterating Values
for score in scores.values():
    print(score)

# Iterating Key-Value Pairs simultaneously (Best Practice)
for name, score in scores.items():
    print(f"{name} -> {score}")

💡 Complete Example: Student Grade Management Engine

# Student Record System
student_database = {
    "STU-101": {"name": "Aarav Mehta", "marks": 85, "city": "Mumbai"},
    "STU-102": {"name": "Sneha Kapoor", "marks": 92, "city": "Delhi"},
    "STU-103": {"name": "Rohan Sharma", "marks": 48, "city": "Bengaluru"}
}

print("=== STUDENT DATABASE MANAGER ===")

# Adding a new student record
new_student_id = "STU-104"
student_database[new_student_id] = {
    "name": "Ananya Roy",
    "marks": 78,
    "city": "Kolkata"
}

# Searching student record safely using .get()
target_id = "STU-102"
student = student_database.get(target_id)

if student:
    print(f"\nRecord Found for ID '{target_id}':")
    print(f" - Name  : {student['name']}")
    print(f" - Marks : {student['marks']}")
    print(f" - City  : {student['city']}")

# Iterating all students and updating pass status
print("\n--- CLASS ACADEMIC SUMMARY ---")
for student_id, details in student_database.items():
    status = "PASSED" if details["marks"] >= 50 else "FAILED"
    print(f"ID: {student_id} | Name: {details['name']:<15} | Marks: {details['marks']} | Status: {status}")

👀 Output

=== STUDENT DATABASE MANAGER ===

Record Found for ID 'STU-102':
 - Name  : Sneha Kapoor
 - Marks : 92
 - City  : Delhi

--- CLASS ACADEMIC SUMMARY ---
ID: STU-101 | Name: Aarav Mehta     | Marks: 85 | Status: PASSED
ID: STU-102 | Name: Sneha Kapoor    | Marks: 92 | Status: PASSED
ID: STU-103 | Name: Rohan Sharma    | Marks: 48 | Status: FAILED
ID: STU-104 | Name: Ananya Roy      | Marks: 78 | Status: PASSED

⚠️ Common Mistakes

  • KeyError: Accessing a non-existent key directly via dict["missing_key"] (use dict.get("missing_key", default) instead!).
  • Trying to use a mutable object (like a list) as a dictionary key ({"tags": ["a", "b"]} as key causes TypeError: unhashable type: 'list').

🛡️ Safety / Important Notes

In Python 3.7+, dictionaries are guaranteed to preserve insertion order. Iterating through dictionary keys or items will yield elements in the exact order they were inserted.

🌍 Real-World Usage

JSON REST API data payloads, database user record objects, configuration file settings, caches, and fast lookup tables ($O(1)$ key lookups).

🧪 Try It Yourself

  1. Create a dictionary inventory = {"apples": 10, "bananas": 5}. Add "oranges": 8 and update "apples": 12.
  2. Retrieve "mangoes" using .get("mangoes", 0) and print the result.

🎯 Mini Challenge

Write a program that takes a list of words ["apple", "banana", "apple", "cherry", "banana", "apple"] and builds a dictionary word-frequency count table.

🔗 Related Topics

🧭 Navigation

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