Lists

A list is an ordered, changeable collection that may hold duplicates and mixed types.

marks = [78, 92, 65, 92]
mixed = [1, "two", 3.0, True]
empty = []

Indexing and slicing

marks[0]        # 78    first
marks[-1]       # 92    last
marks[1:3]      # [92, 65]     end excluded, as with range
marks[:2]       # [78, 92]     from the start
marks[2:]       # [65, 92]     to the end
marks[::-1]     # [92, 65, 92, 78]   reversed

An index out of range raises IndexError. A slice out of range does not — it just gives you what exists. That asymmetry surprises people:

marks[10]       # IndexError
marks[10:20]    # []   no error

Lists are mutable

marks[0] = 80           # change in place
marks.append(55)        # add to the end
marks.insert(1, 99)     # insert at position
marks.remove(92)        # remove the FIRST 92 only
popped = marks.pop()    # remove and return the last
del marks[0]            # delete by index

remove() removing only the first match is worth remembering — with duplicates it does not do what many students expect.

Useful operations

len(marks)          sum(marks)      max(marks)      min(marks)
sorted(marks)       # NEW sorted list, original untouched
marks.sort()        # sorts IN PLACE, returns None
marks.reverse()     # in place
marks.count(92)     # how many times 92 appears
marks.index(65)     # position of first 65
92 in marks         # True/False membership

sorted() versus .sort() is a common exam point:

new = sorted(marks)     # new is the sorted list
new = marks.sort()      # new is None — a classic bug

Anything that mutates in place tends to return None in Python. It is a deliberate signal: "I changed the thing, I did not make you a new one."

The reference trap

This is the most important idea in the topic.

a = [1, 2, 3]
b = a               # b refers to the SAME list
b.append(4)
print(a)            # [1, 2, 3, 4]  — a changed too

b = a does not copy. Both names point at one list, so changing it through either name changes what both see. To copy:

b = a.copy()        # or a[:]  or list(a)
b.append(4)
print(a)            # [1, 2, 3]  unchanged

This connects to Module 1's point that a Python variable is a name for an object, not a box holding a value.

Building lists in a loop

squares = []
for i in range(1, 6):
    squares.append(i * i)       # [1, 4, 9, 16, 25]

Or as a comprehension, which you will meet in reading even if the syllabus does not require you to write them:

squares = [i * i for i in range(1, 6)]

Don't modify a list while iterating it

for x in numbers:
    if x < 0:
        numbers.remove(x)       # skips elements — silently wrong

Removing shifts everything left while the loop's internal index keeps advancing, so items get skipped. Build a new list instead:

numbers = [x for x in numbers if x >= 0]