Lists & Tuples

Lists — Ordered & Mutable

stack = ["python", "django", "react"]

stack[0]          # 'python'   — zero-indexed
stack[-1]         # 'react'    — negative = from the end
len(stack)        # 3
"django" in stack # True — membership test

Slicing — Read Any Sub-Sequence

The pattern is list[start:stop:step] — start is included, stop is not:

nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

nums[2:5]      # [2, 3, 4]
nums[:3]       # [0, 1, 2]   — from the beginning
nums[7:]       # [7, 8, 9]   — to the end
nums[::2]      # [0, 2, 4, 6, 8] — every 2nd
nums[::-1]     # reversed copy
nums[:]        # full shallow copy

Mutating Lists

stack.append("fastapi")          # add to the end — O(1)
stack.insert(0, "html")          # add at index — O(n)
stack.extend(["sql", "git"])     # append many
stack.remove("html")             # remove first matching value
last = stack.pop()               # remove & return the last item
stack[1] = "django 5"            # replace by index
del stack[0]                     # delete by index

Sorting

scores = [82, 41, 99, 67]

sorted(scores)                  # NEW sorted list — original untouched
scores.sort()                   # sorts IN PLACE, returns None
scores.sort(reverse=True)

# sort by a key function
users = [("ravi", 82), ("asha", 99), ("dev", 41)]
users.sort(key=lambda u: u[1], reverse=True)
# [('asha', 99), ('ravi', 82), ('dev', 41)]

Lists Are References — the #1 Beginner Bug

a = [1, 2, 3]
b = a              # b is the SAME list, not a copy
b.append(4)
print(a)           # [1, 2, 3, 4]  — surprise!

c = a.copy()       # actual copy (or a[:], or list(a))
c.append(5)
print(a)           # unchanged

Tuples — Ordered & Immutable

A tuple is a frozen sequence. Use it for fixed-shape data — coordinates, RGB values, database rows:

point = (28.6139, 77.2090)
point[0]            # 28.6139
point[0] = 1.0      # TypeError — tuples can't change

Immutability is a feature: tuples can be dict keys and set members, and they signal “this data has a fixed structure”. Unpacking makes them ergonomic:

lat, lng = point

for name, score in [("ravi", 82), ("asha", 99)]:
    print(f"{name}: {score}")

List or Tuple?

• List — homogeneous items, length varies: a list of users, a queue of jobs.
• Tuple — heterogeneous fields, fixed shape: (name, score), (x, y).