🗂️ Data Structures

Dictionaries

डिक्शनरी

⏱ 3 hr3 topicsInteractive
🎯 By the end: You can create dictionaries, read and update values by key, use the main dict methods, and explain the rule for valid keys.

A dictionary stores data as key–value pairs, like a real dictionary maps a word (key) to its meaning (value). Instead of looking things up by position (index 0, 1, 2…), you look them up by a meaningful name. It's one of the most useful structures in all of programming — and we'll finish by building a Student Result Manager with it.

1Key-value pairs

A dictionary uses curly braces, with each item written as key: value, separated by commas. You access a value by its key (not a number):

student = {"name": "Asha", "age": 16, "grade": "A"}
print(student["name"])     # Asha
student["age"] = 17       # change a value
student["city"] = "Delhi" # add a new pair
print(student)
The rule for keys: keys must be unique and immutable (strings, numbers or tuples work as keys; a list cannot be a key because it's mutable). Values can be anything and can repeat. A dictionary is mutable — you can add, change and remove pairs.
A dictionary▶ runs real Python
main.py
Output
Click Run to execute.
Key points
  • A dictionary stores key–value pairs in curly braces; you look up values by key, not index.
  • Keys must be unique and immutable (string, number, tuple — not a list).
  • Dictionaries are mutable: add/change a pair with d[key] = value.

2Dictionary methods and looping

MethodDoes
keys()All the keys
values()All the values
items()All key-value pairs (great for looping)
get(key)Value for key, or None if absent (no error)
update(d2)Merge in another dictionary
clear()Empty the dictionary
Use get(key) instead of d[key] when a key might not exist — d["x"] crashes with an error if 'x' is missing, but d.get("x") safely returns None.

Looping over a dictionary with items() gives you each key and value together:

Methods & looping▶ runs real Python
main.py
Output
Click Run to execute.
Key points
  • keys(), values(), items() return the keys, values, and pairs; items() is ideal for looping.
  • get(key) safely returns None for a missing key; d[key] raises an error.
  • update() merges another dict; clear() empties it.

3Project: Student Result Manager

Dictionaries are perfect for records. This Student Result Manager stores a student's subject marks, then computes their total, average and grade. Run it, then add a subject or change the marks:

Student Result Manager▶ runs real Python
main.py
Output
Click Run to execute.
Key points
  • Dictionaries naturally model a record (a student, with named fields).
  • Read values by key, then compute totals/averages from them.
  • round(value, 2) rounds a number to 2 decimal places for a tidy average.

★ Project: extend the Result Manager

Using the playground above:

  1. Add two more subjects to the dictionary and update the total/average to include them.
  2. Use a for loop over items() to print each subject and its mark.
  3. Use get() to safely look up a subject that might not be there.
  4. Bonus: store several students (a list of dictionaries) and print each one's average.

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