Lists in Python

A list holds an ordered run of values. They can be of different types, though in practice you usually keep one kind in one list.

marks = [88, 72, 95, 61]
mixed = [1, "two", 3.0, True]
empty = []

Getting things out

marks[0]     # 88  — counting starts at zero
marks[3]     # 61
marks[-1]    # 61  — last item
marks[-2]    # 95  — second from the end

Negative indexes count backwards. marks[-1] is the idiomatic way to get the last item; do not write marks[len(marks) - 1].

Slicing

marks[1:3]    # [72, 95]   — from 1, stop before 3
marks[:2]     # [88, 72]   — from the start
marks[2:]     # [95, 61]   — to the end
marks[:]      # a full copy

The end is always exclusive, same as range(). Once that clicks, slicing stops being confusing.

The methods you will actually use

marks.append(70)        # add one at the end
marks.insert(0, 100)    # add at a position
marks.remove(72)        # delete by value (first match only)
last = marks.pop()      # remove and return the last
marks.sort()            # sort in place
marks.reverse()         # flip in place
len(marks)              # how many
95 in marks             # True or False

Note which ones return something and which change the list in place. sorted(marks) gives you a new sorted list; marks.sort() returns None and changes the original. Writing marks = marks.sort() sets marks to None, and then nothing works. We see it most weeks.

The trap worth knowing early

Never use a list as a default argument:

def add_student(name, batch=[]):     # wrong
    batch.append(name)
    return batch

add_student("Ravi")     # ['Ravi']
add_student("Simran")   # ['Ravi', 'Simran'] — the same list!

The default is created once, when the function is defined, not each time it is called. Do this instead:

def add_student(name, batch=None):
    if batch is None:
        batch = []
    batch.append(name)
    return batch

This one appears in interviews, so it is worth being able to explain rather than just avoid.