Lists, Sets, Tuples and Disctionaries

*Dicts preserve insertion order (Python 3.7+)

1. List

Definition: A list is an ordered, mutable collection that can hold duplicate values of any data type.

1orders = [101, 102, 103]
2orders.append(104)      # add at end
3orders.extend([105])    # add multiple
4orders.insert(1, 150)   # add at index
5orders.remove(102)      # remove first match
6orders.pop()            # remove & return last
7orders.sort()           # sort in place
8sorted(orders)          # returns new sorted list

Use for: ordered data you need to change (e.g., API results — often a list[dict]).

2. Tuple

Definition: A tuple is an ordered, immutable collection — once created, its values cannot be changed, added, or removed.

1row = (101, "Akash", 500)
2row[0]        # 100 — OK, read
3row.append(1) # ❌ Error — can't modify

Use for: fixed data that shouldn't change (e.g., a DB row, coordinates).

3. Set

Definition: A set is an unordered collection of unique, hashable elements — duplicates are automatically removed.

1ids = {101, 102, 101, 103}  # → {101, 102, 103}
2ids.add(104)
3ids.discard(101)   # no error if missing
4 
5a, b = {1,2,3}, {3,4,5}
6a.union(b)         # {1,2,3,4,5}
7a.intersection(b)  # {3}
8a.difference(b)    # {1,2}

Use for: deduplication, membership checks, comparing groups.

4. Dict

Definition: A dictionary is an unordered* collection of key-value pairs, where each key is unique and maps to a value. (Insertion order is preserved since Python 3.7)

1user = {"id": 101, "name": "Akash"}
2user.get("email")           # None (no KeyError)
3user.get("email", "n/a")    # default value
4user.update({"age": 26})
5user.pop("age")
6 
7for k, v in user.items():
8    print(k, v)

Use for: structured records — most JSON/API data maps here.

Quick Decision Guide

• Need to change order/values → list
• Need data that can't change → tuple
• Need only unique values → set
• Need key → value lookup → dict
• Processing API/JSON → list[dict]

Common Interview Qs

1. List vs tuple — when to use each?
2. remove() vs pop()?
3. Why can't a set hold a list?
4. sort() vs sorted()?
5. dict["key"] vs dict.get("key")?
6. How to dedupe records by ID?

Core takeaway:

1List  → ordered + mutable + duplicates
2Tuple → ordered + immutable
3Set   → unique values only
4Dict  → key → value