Python Cheatsheet

A refresher of the basic Python data structures from DSCI 511.

You already met Python’s built-in data structures in DSCI 511. In this course we ask how fast each one is, and why. This page is a quick refresher of what each one can and cannot do.

1. The four basic python data structures

	List	Tuple	Set	Dictionary
Syntax	[1, 2, 3]	(1, 2, 3)	{1, 2, 3}	{"a": 1, "b": 2}
Ordered?	Yes	Yes	No	Yes (insertion order)
Indexed?	Yes, by position: x[0]	Yes, by position: x[0]	No	Yes, by key: x["a"]
Mutable? (can change after creation)	Yes	No	No change allowed at each item-level, but can add and remove items	Yes
Duplicates?	Allowed	Allowed	Not allowed	Keys not allowed, values allowed

Two words you will see a lot:

Word	Meaning	Example
Mutable	Can be changed in place after it is created	my_list.append(4) works
Indexed	Each item is reached through a position (or key)	my_list[0] gives the first item

1.1 List

An ordered, mutable collection. Allows duplicates. The everyday workhorse.

drivers = ["Ana", "Ben", "Chen"]

drivers[0]              # 'Ana'         -> indexed by position
drivers[-1]             # 'Chen'        -> negative index counts from the end
drivers[0:2]            # ['Ana', 'Ben'] -> slicing
drivers.append("Ben")   # add to the end (duplicates are fine)
drivers[1] = "Bo"       # change an item in place
len(drivers)            # 4 because we appended another Ben in the end above.
"Chen" in drivers       # True

1.2 Tuple

An ordered, immutable collection. Allows duplicates. Like a list you cannot change: good for a fixed group of values that belong together.

pickup = (49.26, -123.25)   # (latitude, longitude)

pickup[0]                   # 49.26         -> indexed by position
lat, lon = pickup           # unpacking
pickup[0] = 50.0            # TypeError: tuples cannot be changed

1.3 Set

An unordered, unindexed collection with no duplicates. You can add and remove items, but you cannot ask for “the item at position 0”.

riders = {"Ana", "Ben", "Ana"}   # the second "Ana" is dropped

len(riders)                      # 2 -> no duplicates (print order may vary)
riders.add("Chen")               # add an item
"Ben" in riders                  # True
riders[0]                        # TypeError: sets are not indexed

Sets also do maths on groups:

Operation	Meaning	Example
a & b	In both (intersection)	{1, 2, 3} & {2, 3, 4} gives {2, 3}
a | b	In either (union)	{1, 2, 3} | {2, 3, 4} gives {1, 2, 3, 4}
a - b	In a but not b (difference)	{1, 2, 3} - {2, 3, 4} gives {1}

1.4 Dictionary

A collection of key: value pairs. Ordered (by insertion), mutable, and indexed by key. Keys are unique; values can repeat.

ratings = {"Ana": 4.9, "Ben": 4.7}

ratings["Ana"]          # 4.9          -> look up a value by its key
ratings["Chen"] = 4.8   # add a new key
ratings["Ben"] = 4.5    # change an existing key's value
ratings.get("Zoe")      # None         -> no error if the key is missing
ratings["Zoe"]          # KeyError
list(ratings)           # ['Ana', 'Ben', 'Chen'] -> the keys, in insertion order