Python: List

Syntax of List (Literal Expression to Create a List)

[x0, x1, x2 etc]

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

list can have items of different types.

Count Items

len(list)

Return the count of number of items.

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

Check Existence

x in list

check if exist.

x = [0, 1, 2]
print(2 in x)

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

x not in list

return true if not in list.

x = [0, 1, 2]
print(3 not in x)

x = ["a", "b", "c"]
print("m" not in x)

Get one Item

list[i]

Return item in list at index i.

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

Index starts at 0. Best to think of index as between items. The left of first item is 0.

Negative index counts from right.

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

Set/Change One Item

list[i] = new_value

Change a item's value.

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

Delete One Item

del list[i]

Delete one item.

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

Get Sublist

list[i:j]

Return a sublist from index i to j.

Note: index start at 0. Item at index 0 means first item. Ending index does not include itself.

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

list[:j]

From beginning to j.

list[i:]

From i to the end, including the end item.

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

Delete Sublist

del list[i:j]

delete the items between i to j (not including j). Same as list[i:j] = []

del list[i:j:k]

delete the items between i to j (not including j), with step k.

Replace Sublist

list[i:j] = list2

Replace i to j items by the contents of the 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

e = list(range(20))
e[0:10:2] = ["a"] * 5
print(e)
# ['a', 1, 'a', 3, 'a', 5, 'a', 7, 'a', 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

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

Note: there is also list methods append. see Python: List Methods

list1 + list2

Join 2 lists into 1.

x = ["a", "b"] + [7, 6]
print(x)
# ['a', 'b', 7, 6]