## Python Reference Sheet for Compsci 101, Exam 3, Spring 2021

On this page we'll keep track of the Python types, functions, and operators that we've covered in class. You can also review the online Python References for more complete coverage, BUT NOTE there is way more python in the there then we will cover! The reference page below is all you should need to complete the exam.

Mathematical Operators
Symbol Meaning Example
+ addition 4 + 5 = 9
- subtraction 9 - 5 = 4
* multiplication 3*5 = 15
/ and // division 6/3 = 2.0
6/4 = 1.5
6//4 = 1
% mod/remainder 5 % 3 = 2
** exponentiation 3**2 = 9, 2**3 = 8
String Operators
+ concatenation "ab"+"cd"="abcd"
* repeat "xo"*3 = "xoxoxo"
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`
not flips/negates the value of a bool (not x == 5) is False
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
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
Type Conversion Functions
int(x) turn x into an integer value int("123") == 123
int can fail, e.g., int("abc") raises an error
float(x) turn x into an float value float("2.46") == 2.46
float can fail, e.g., float("abc") raises an error
str(x) turn x into a string value str(432) == "432"
type(x) the type of x type(1) == int
type(1.2) == float
String Functions
`s="colorful"`
Name Returns Example
.find(str) index of first occurrence s.find("o") == 1
s.find("e") == -1
.rfind(str) index of last occurrence s.rfind("o") == 3
s.rfind("e") == -1
.index(str) same as .find(str), error if str not in string s.index("o") == 3
.count(str) number of occurrences s.count("o") == 2
s.count("r") == 1
s.count("e") == 0
.strip() copy with leading/trailing whitespace removed "   big ".strip() == "big"
.split() list of "words" in s "big bad dog".split() == ["big","bad", "dog"]
.split(",") list of "items " in s that are separated by a comma
In general can split on any string, not just a comma, e.g., s.split(":") will split on a colon and s.split("gat") will split on the string "gat".
"this,old,man".split(",") == ["this", "old", "man"]
' '.join(lst) concatenate elements of lst, a list of strings, separated by ' ' or any string ':'.join(['a','b','c']) == "a:b:c"
.startswith(str) boolean if starts with string s.startswith("color") == True
s.startswith("cool") == False
.endswith(str) boolean if ends with string s.endswith("ful") == True
s.endswith("color") == False
.upper() uppercase of s s.upper() == "COLORFUL"
.lower() lowercase of s "HELLO".lower() == "hello"
.isupper() boolean is uppercase 'A'.isupper() == True
'a'.isupper() == False
.islower() boolean is lowercase 'A'.islower() == False
'a'.islower() == True
.isalpha() boolean is alphabetic character '3'.isalpha() == False
'?'.isalpha() == False
'z'.isalpha() == True
.capitalize() capitalized s s.capitalize() == "Colorful"
.replace(str1, str2) replace all occurrences of str1 with str2 s.replace('o','y') == "cylyrful"
.replace(str1, str2,n) replace the first n occurrences of str1 with str2 s.replace('o','y',1) == "cylorful"
Miscellaneous Functions
help(x) documentation for module x
len(x) length of sequence x, e.g., String/List len("duke") == 4
list(str) a list of the characters from string str list("cards") == ['c','a','r','d','s']
sorted(x) return list that is sorted version of sequence/iterable x, doesn't change x sorted("cat") == ['a','c','t']
range(x) a list of integers starting at 0 and going up to but not including x range(5) == [0, 1, 2, 3, 4]
range(start, stop) a list of integers starting at start and going up to but not including stop range(3, 7) == [3, 4, 5, 6]
range(start, stop, inc) a list of integers starting at start and going up to but not including stop with increment inc range(3, 9, 2) == [3, 5, 7]
min(x, y, z) minimum value of all arguments min(3, 1, 2) == 1
min("z", "b", "a") == "a"
max(x, y, z) maximum value of all arguments max(3, 1, 2) == 3
max("z", "b", "a") == "z"
abs(x) absolute value of the int or float x abs(-33) == 33
abs(-33.5) == 33.5
List Functions
sum(lst) returns sum of elements in list lst sum([1,2,4]) == 7
max(lst) returns maximal element in lst max([5,3,1,7,2]) == 7
lst.append(...) append an element to lst, changing lst [1,2,3].append(8) == [1,2,3,8]
lst.insert(pos,elt) append `elt` to `lst` at position `pos`, changing `lst` [1,2,3].insert(1,8) == [1,8,2,3]
lst.extend(lst2) append every element of lst2 to lst [1,2,3].extend([8,9]) == [1,2,3,8,9]
lst.remove(elt) remove first occurence of `elt` from `lst` [1,2,3,2,3,2].remove(2) == [1,3,2,3,2]
lst.sort() sorts the elements of lst
lst.index(elt) return index of elt in lst, error if elt not in lst [1,5,3,8].index(5) == 1
lst.count(elt) return number of occurrences of elt in `lst` [1,2,1,2,3].count(1) == 2
lst.pop() remove and return last element in `lst`, so has side-effect of altering list and returns value.
lst.pop(index) remove and return element at position index in `lst`, so has side-effect of altering list and returns value. Default index is last value.
Math Functions (import math)
math.pi 3.1415926535897931
math.sqrt(num) returns square root of num as float math.sqrt(9) == 3.0
File Functions
open("filename") opens a file, returns file object f = open("foo.txt")
open("filename",mode) specify mode of 'r', 'a', 'w', return file object f = open("foo.txt", "a")
f.read() returns the entire file as one string s = f.read()
Random Functions (import random)
random.choice(list_of_choices) returns a random element from list_of_choices. Gives an error if list_of_choices has length 0.
random.randint(start, end) Returns a random integer between start and end. 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.
Set Functions
set(lst) returns a set of the elements from list lst
s.add(item) adds the item into the set, and returns nothing.
s.update(lst) adds the elements in the list `lst` into the set, and returns nothing.
s.remove(item) removes the item from the set, error if item not there.
s.union(t) returns new set representing s UNION t, i.e., all elements in either s OR t, t can be any iterable (e.g., a list)
s.intersection(t) returns new set representing s INTERSECT t, i.e., only elements in both s AND t, t can be any iterable (e.g., a list)
s.difference(t) returns new set representing s difference t, i.e., elements in s that are not in t
s.symmetric_difference(t) returns new set representing elements in s or t, but not in both
s | t returns/evaluates to union of s and t, both must be sets.
s & t returns/evaluates to intersection of s and t, both must be sets.
s - t returns/evaluates to set with all elements in s that are not in t
s ^ t returns/evaluates to set with all elements from s and t that are not in both s and t
Dictionary Functions
d[key] returns the value associated with `key`, error if `key` not in dictionary `d`
d.get(key) returns value associated with `key`, error if `key` not in dictionary `d`
d.get(key,default) returns value associated with `key`, returns `default` if `key` not in `d`
d.keys() returns a list/view of the keys in dictionary
d.values() returns a list/view of values in dictionary
d.items() returns a list/view of tuples, (key,item) pairs from dictionary
d.iteritems() Use only with for loop, gives (key,item) tuple pairs from the dictionary
d.update(dict) updates the dictionary with another dictionary `dict`
Image Library Functions
Image.open(fname)opens and returns image
im.show() displays image im
im.getdata()returns generator of all pixels in im
im.putdata(pixlist)modifies image by setting all pixels to pixlist
im.sizereturns tuple that is (width,height) of image
Image.new("RGB",size) creates and returns a new image with dimensions of tuple size