Lists, tuples, dicts and sets

Why four of them

C gives you one container: the array. Fixed size, one type, indexed by number. Everything else you build yourself. Python ships four, because the four answer four different questions.

list - the default

Use a list when you have several things of the same kind and the order matters.

marks = [72, 65, 88, 91]

marks.append(55)          # add at the end
marks.insert(0, 100)      # add at position 0
marks.remove(65)          # remove the first 65 by value
last = marks.pop()        # remove and return the last item

print(marks)              # [100, 72, 88, 91]
print(len(marks))         # 4
print(marks[0], marks[-1])   # 100 91
print(marks[1:3])         # [72, 88]  - from 1, up to but not including 3

marks[-1] is the last item. Negative indices count from the right. This saves you writing marks[len(marks) - 1] everywhere.

A list can hold mixed types, but if yours does, you probably want a dict or a small class instead.

a = [1, 2, 3]
b = a
b.append(4)
print(a)          # [1, 2, 3, 4]  - a changed too

c = a.copy()
c.append(5)
print(a)          # [1, 2, 3, 4]  - a is safe

tuple - a list that cannot change

point = (3, 4)
x, y = point          # unpacking
print(x, y)           # 3 4

A tuple is fixed once created. point[0] = 9 raises TypeError. That sounds like a limitation, and it is the point: use a tuple when the number of items is part of the meaning. A 2D point always has exactly two numbers. A student record is always (roll number, name, marks).

Tuples are also the only one of these four that can be a dict key, because keys must be immutable.

A tuple of one item needs a trailing comma: (5,) is a tuple, (5) is just the number 5 in brackets.

dict - lookup by name

A dict maps a key to a value. This is the container you will use most once your programs get real.

student = {
    "roll": "21CS045",
    "name": "Ravi Kumar",
    "cgpa": 8.2,
}

print(student["name"])              # Ravi Kumar
print(student.get("branch", "CSE")) # CSE  - default if key missing

student["cgpa"] = 8.4               # update
student["branch"] = "CSE"           # add

for key, value in student.items():
    print(f"{key}: {value}")

set - membership and uniqueness

a = {1, 2, 3, 4}
b = {3, 4, 5}

print(a | b)      # {1, 2, 3, 4, 5}   union
print(a & b)      # {3, 4}            intersection
print(a - b)      # {1, 2}            difference

nums = [1, 2, 2, 3, 3, 3]
print(set(nums))         # {1, 2, 3}
print(len(set(nums)))    # 3 distinct values

An empty set is set(), not {}. {} is an empty dict.

The reason this lesson matters: speed

Checking whether an item is present is the difference between a program that finishes and one that does not.

big_list = list(range(1000000))
big_set = set(big_list)

# 999999 in big_list  ->  scans every element, ~1,000,000 comparisons
# 999999 in big_set   ->  one hash computation, ~1 comparison

in on a list is O(n). in on a set or a dict is O(1) on average, because they use a hash table. For 1,000,000 items that is the difference between milliseconds and minutes.

The rule to remember: if your program asks "have I seen this before?", use a set or a dict, never a list.