Python: List

Syntax of list

literal expression for list:

[x0, x1, x2, etc]

• Items can be any type.
• Items need not be all the same type.
• Items can be added, or removed. (the list can grow or shrink. Not fixed length)
• Access to any item is constant time.
• Python list is implemented as dynamic array.

x = [1, 2, "m"]
print(x)
# [1, 2, 'm']

0-based index

List index start at 0. Best to think of it as between items.

x = ["a", "b", "c", "d"]
#   0   1    2    3 ← index boundaries

Negative index counts from right.

Total number of items

len(list)

Return the count of number of items.

x = [0, 1, 2]
print(len(x))
# 3

Check exist

x in list

check if exist.

print(2 in [0, 1, 2])
# True

print(9 in [0, 1, 2])
# False

x not in list

return true if not in list.

print(3 not in [0, 1, 2])
# True

Get one item

list[i]

Return item at index i.

x = ["a", "b", "c"]
print(x[1])
# b

x = ["a", "b", "c"]
print(x[-1])
# c

Set or change one item

list[i] = newValue

Change a item's value.

x = [1, 2, 3]
x[2] = "b"
print(x)
# [1, 2, 'b']

Delete one item

del list[i]

Delete item at index i.

x = ["a", "b", "c"]
del x[1]
print(x)
# ['a', 'c']

Get a slice

list[i:j]

Return a slice from index i to j.

x = [0, 1, 2, 3, 4, 5]
print(x[1:4])
# [1, 2, 3]

list[i:j:k]

every kth.

x = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(x[1:10:2])
# [1, 3, 5, 7, 9]

list[:j]

From beginning to j.

list[i:]

From i to the end.

x = [0, 1, 2, 3, 4, 5]
print(x[3:])
# [3, 4, 5]

Delete a slice

del list[i:j]

delete the items between i to j.

🛑 WARNING: del is a statement, not a function. It cannot be used as expression in function argument.

xx = [1, 2, 3, 4, 5]
del xx[1:3]
print(xx)
# [1, 4, 5]

del list[i:j:k]

delete every kth item between i to j.

Replace a slice

list[i:j] = list2

Replace i to j items by the content of list2

x = [1, 2, 3, 4, 5]
x[2:4] = [99, 100]
print(x)
# [1, 2, 99, 100, 5]

list[i:j:k] = list2

Replace items list[i:j:k] by content of list2. (list2 must have same number of items)

# list slice alternate assignment example
xx = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
xx[0:10:2] = ["a", "a", "a", "a", "a"]
print(xx)
# ['a', 2, 'a', 4, 'a', 6, 'a', 8, 'a', 10, 11, 12]

Nested lists

Lists can be nested arbitrarily. Example:

x = [3, 4, [7, 8]]

Append extra bracket to get item of nested list.

x = [3, 4, [7, 8]]
print(x[2][1])
# 8

Join lists

list1 + list2

Join two lists into one.

see also append. Python: List Methods

x = [1, 2] + [7, 6]
print(x)
# [1, 2, 7, 6]