Python Summary

This page lists Python types, functions, and operators that have been covered in class. The online Python References are linked for each sections and provide more complete coverage.

Mathematical Operators
Symbol	Meaning	Example
+	addition	4 + 5 is 9
-	subtraction	9 - 5 is 4
*	multiplication	3 * 5 is 15
/	division	6/3 is 2 6/4 is 1 6.0/4 is 1.5
%	mod/remainder	5 % 3 is 2
**	exponentiation	3**2 is 9 2**3 is 8
A shorthand version of each of these exists to update a variable that appears on both sides of the assignment. For example, the statement: x = x + 1 can be shortened to: x += 1		x = 1 sets x to 1 x += 1 sets x to 2 x *= 2 sets x to 4 x **= 2 sets x to 16
Sequence Operators
+	concatenation	"ab" + "cd" is "abcd" [1,2] + [3,4] is [1,2,3,4]
*	repeat	"xo" * 3 is "xoxoxo" [0] * 3 is [0,0,0]
[]	indexing note, indexing starts from 0 and negative values can be used to index from the sequence's end	"ab"[1] is "b" [1,2,3,4][-1] is [4]
[start:end]	slicing from start up to but not including end note, if start is left off, 0 is assumed and if end is left off, len(sequence) is assumed	"abcd"[1:3] is "bc" "abcd"[:-2] is "ab" [1,2,3,4][:] is [1,2,3,4]
[start:end:stride]	slicing with stride note, if stride is negative, the elements of the sequence are accessed in reverse order	"abcd"[::2] is "ac" "abcd"[::-2] is "db" [1,2,3,4,5][1:-1:2] is [2,4]
in	membership	"a" in "abcd" is True "ad" in "abcd" is False 5 not in [1,2,3,4] is True
Comparison Operators
==	is equal to	3 == 3 is True
!=	is not equal to	3 != 3 is False
>=	is greater than or equal to	4 >= 3 is True
<=	is less than or equal to	4 <= 3 is False
>	is strictly greater than	4 > 3 is True
<	is strictly less than	3 < 3 is False
Boolean Operators
x = 5
or	returns True if at least one part of it is True	(x < 3 or x > 7) is False (x < 3 or x < 7) is True
and	returns True only if both parts of it are True	(x > 3 and x < 7) is True (x > 3 and x > 7) is False
not	flips the value of a boolean	(not x == 5) is False
is	returns True only if the two things being compared are the same thing	x = 5 y = x (x is y) is True
Type Conversion Functions
int(x)	turn x into an integer value note, this can fail, e.g., int("abc") raises an error	int("123") is 123
float(x)	turn x into an float value note, this can fail, e.g., float("abc") raises an error	float("2.46") is 2.46
str(x)	turn x into a string value	str(432) is "432"
list(x)	turn x into a list value note, this can fail, e.g., list(123) raises an error	list("abc") is ["a", "b", "c"]
set(x)	turn x into a set value note, this can fail, e.g., set(123) raises an error	set("hello") is set(["h", "e", "l", "o"]
dict(x)	turn x into a dict value note, x must be a sequence of items with 2 elements	dict([("a",2), ("b",4)]) is {"a":2, "b":4}
type(x)	returns the type of x	type(1) is int type(1.2) is float
String Functions
s = "colorful"
Name	Returns	Example
.find(str)	returns index of first occurrence of str in s	s.find("o") is 1 s.find("e") is -1
.count(str)	returns number of occurrences of str in s	s.count("o") is 2 s.count("r") is 1 s.count("e") is 0
.strip()	returns copy of s with leading and trailing whitespace removed	"   big ".strip() is "big"
.split()	returns list of "words" in s	"big bad dog".split() is ["big","bad", "dog"]
.split(seperater)	returns list of "items" in s that are separated by a seperater	"this,old,man".split(",") is ["this", "old", "man"]
In general split and strip take any string value, not just whitespace or comma, e.g., s.split(":") will split on a colon and s.strip("gat") will remove leading and trailing letters "g", "a", or "t".
.join(sequence)	returns all strings in sequence joined together with each separated by s	",".join(["this", "old", "man"]) is "this,old,man" " ".join(["big","bad", "dog"]) is "big bad dog"
.lower()	returns string with all lowercase letters	s.upper() is "colorful"
.upper()	returns string with all uppercase letters	s.upper() is "COLORFUL"
.startswith(prefix)	returns True only if starts with given string	s.startswith("color") is True s.startswith("cool") is False
.endswith(suffix)	returns True only if ends with given string	s.endswith("ful") is True s.endswith("fool") is False
.isalpha()	returns True only if all characters are alphabetic	s.isalpha() is True "123".isalpha() is False
.isdigit()	returns True only if all characters are digits	s.isdigit() is False "123".isalpha() is True
List Functions
l = [2,4,6,2,8,2,10]
Name	Returns	Example
sum(lst)	returns sum of numeric items	sum([1,2,4]) is 7
zip(lstA,lstB)	returns list of tuples created from corresponding items in each list	zip([1,2,4], [1,3,5]) is [(1,1), (2,3), (4,5)]
enumerate(lst)	returns list of tuples created from the index and value of items in given list	enumerate([1,2,4]) is [(0,1), (1,2), (2,4)]
.index(item)	returns index of first occurrence	l.index(6) is 2 l.index(7) is -1
.count(item)	returns number of occurrences	l.count(2) is 3 l.count(10) is 1 l.count(7) is 0
.insert(index, item)	add to list at at position index	l.insert(0, 7) is [7,2,4,6,2,8,2,10] l.insert(-1, 7) is [2,4,6,2,8,2,7,10]
.append(item)	add to list's end	l.append(7) is [2,4,6,2,8,2,10,7]
.extend(lst)	add every element of lst to list's end	l.extend([8,9]) is [2,4,6,2,8,2,10,8,9]
.pop(index)	remove and return element at position index in list default index is last value	l.pop(2) is 6, l is now [2,4,2,8,2,10] l.pop() is 10, l is now [2,4,6,2,8,2]
.remove(item)	remove first occurrence	l.remove(2) is [4,6,2,8,2,10]
Set Functions
s = set([1,3,5,7,9]), t = set([2,4,5,6,8])
.add(item)	adds the item into the set	s.add(2) is set([1,2,3,5,7,9])
.union(t)	returns set containing all elements in either s OR t, t can be any sequence	s.union(t) is set([1,2,3,4,5,6,7,8,9])
.intersection(t)	returns set containing only elements in both s AND t, t can be any sequence	s.intersection(t) is set([5])
.difference(t)	returns set containing only elements in s NOT t, t can be any sequence	s.difference(t) is set([1,3,7,9])
s | t	returns union of s and t, both must be sets	(s | t) is set([1,2,3,4,5,6,7,8,9])
s & t	returns intersection of s and t, both must be sets	(s & t) is set([5])
s - t	returns difference between s and t, all elements in s that are not in t	(s - t) is set([1,3,7,9])
Dictionary Functions
d = {"one": 1, "two": 2, "three": 3}
d[key]	returns value associated with key, error if key not in dictionary d	d["one"] is 1 d["four"] is an error
d.get(key)	returns value associated with key, error if key not in dictionary d	d.get("one") is 1 d.get("four") is an error
d.get(key, default)	returns value associated with key, returns default if key not in d	d.get("one",0) is 1 d.get("four",0) is 0
d.keys()	returns list of the keys in dictionary	d.keys() is ["one","two","three"]
d.values()	returns list of the values in dictionary	d.values() is [1,2,3]
d.items()	returns list of tuples, (key, item) pairs from dictionary	d.items() is [("one",2),("two",2),("three",3)]
General Functions
abs(x)	returns absolute value of the int or float x	abs(-33) is 33 abs(-33.5) is 33.5
len(x)	returns length of sequence x	len("duke") is 4
range(x)	returns list of integers starting at 0 and going up to but not including x	range(5) is [0,1,2,3,4]
range(start, stop)	returns list of integers starting at start and going up to but not including stop	range(3, 7) is [3,4,5,6]
sorted(x)	returns list that is sorted version of sequence x, does not change x	sorted("cat") is ["a","c","t"]
Note, sorted takes an optional parameter, reverse=True, which reverses the order of the resulting list		sorted("cat", reverse=True) is ["t", "c", "a"]
sorted(x,key=func)	returns list that is sorted version of sequence x, according to the given function, does not change x	sorted(["abc","z"],key=len) is ["z","abc"] sorted("cat", key=len,reverse=True) is ["abc", "z"]
Note, a popular function to use as a key is itemgetter, which returns the items associated with the given indices of the sub-list (assumming the elements of the list being sorted are sequences themselves).		itemgetter(1)[2,4,6] is 4 itemgetter(2,1,0)[2,4,6] is (6,4,2) sorted(["cat","dog","bear"], key=itemgetter(1)) is ["cat","bear","dog"]
min(x, y, z)	returns minimum value of all arguments	min(3, 1, 2) is 1 min("z", "b", "a") is "a"
max(x, y, z)	returns maximum value of all arguments	max(3, 1, 2) is 3 max("z", "b", "a") is "z"
In general, min and max take any sequence, e.g., a list or a string.
help(x)	prints documentation for module x
Random Functions
random.choice(lst)	returns a random element from lst, error if len(lst) == 0
random.randint(start, end)	returns a random integer between start and end Note, unlike range() and list slicing, the largest value it can return is end, not end-1
random.random()	returns a random float between 0 and 1
File Functions
open(path)	create file object from given file on disk	open("data.txt") open("c:\data\numbers.txt")
.read()	returns string that represents the entire contents of file
.readlines()	returns list of strings that is the entire contents of file separated by newline note, each string includes newline
.close()	closes file, no longer possible to read from file
Special "Escaped" Characters
'\t'	tab
'\n'	new line
'\\'	back slash