Illinois Python Cheat Sheet
by Elizabeth de Sa e Silva, Tamara Nelson-Fromm, Wade Fagen-Ulmschneider
Basic Data Types
Integers are whole numbers
int1 = 8 int2 = -5
int3 = 0 int4 = int(4.0)
Floats have a decimal point
float1 = 5.5 float2 = 0.0
float3 = 1e6 float4 = float(2)
Strings
A string literal has quotes: ‘CS101’, ‘CS107’, ‘5.67’
(it’s literally the exact characters of the string)
A variable name does not: course_name, stat107, my_string
A string can be indexed the same way as a list
Example
my_string = ‘literal’ #’literal’ is the literal
print(‘my_string’) #prints “my_string”
print(my_string) #prints “literal”
print(literal) #ERROR
Slicing
Strings, lists, and other iterable data types (data with many elements) can be indexed over a
range of values, or sliced
Replace any [i] with a range to select many elements at once:
[start:stop:step]
Selects position start through position stop, not including stop, but only
elements step positions apart;
start defaults to zero, so [ :10:7 ] starts at 0
stop defaults to one past the last index, so [ 10: :2 ] selects through the end of the data
step defaults to one, so [ 1:5 ] steps by 1 (a negative step will count backwards)
Examples
my_string = ‘abcdefghijk’
my_string[2:4] == ‘cd’
my_string[:5] == ‘abcde’
my_string[5:] == ‘fghijk’
my_string[:] == ‘abcdefghijk’
my_string[2:8:2] == ‘ceg’
my_string[8:2:-2] == ‘ige’
Lists
Creating a new list
empty_list = []
my_list =[1,2,3]
Adding to a list (appending)
list_name.append(v) #adds just the
#element v to
#list_name
list_name += [v1,v2] #adds v1 and v2
#to the end of
#list_name
Indexing
list[i] is equal to the element in
list at zero-based index i
Negative index values count from
the end of the data
list[-i] is equal to
list[ len(list) - i ]
Changing a list
#changes the element
list[i] = v #in list at position
#i to the value v
Example
my_list = [10,20,30] #my_list is declared as [10,20,30]
my_list.append(40) #my_list becomes [10,20,30,40]
my_list += [50,60] #my_list becomes [10,20,30,40,50,60]
my_list[2] == 30 # True
my_list[4] = “fifty” #my_list becomes [1,2,3,4,”fifty”,60]
my_list[-1] == “fifty” # True
my_list[60] #ERROR
Booleans
Booleans are True or False values
x == y Is True if x is equal to y x in y is True if x is an element of y
not x == y Is True is x is not equal to y
And
True and True = True
True and False = False
False and False = False
Or
True or True = True
True or False = True
False or False = False
Dictionaries
Creating a new dictionary
my_dict = {key1:value1, key2:value2, …, keyn:valuen}
empty_dict = {} #keys and values can be any data type
Adding to a dictionary (appending)
dict_name[key] = value
#adds key:value to dict_name
Indexing
dict[key] is equal to the value in
dict with key key
Changing a dictionary
dict_name[key] = value #changes key’s value to v so dict_name
# now has the pair key:v
Getting Keys and Values
dict_name.keys() #returns a list of keys in dict_name
dict_name.values() #returns a list of values in dict_name
Example
my_dict = {‘a’:5, ‘b’:6} #my_dict is declared as {‘a’:5,’b’:6}
my_dict[‘c’] = ‘4’ #my_dict becomes {‘a’:5, 6:’b’, ‘c’:’4’}
my_dict[‘a’] == 5 # True
my_dict[‘b’] = ‘a’ #my_dict becomes {‘a’:5,‘b’:’a’,‘c’:’4’}
my_dict[5] #ERROR
my_dict.keys() #equal to [‘a’, ‘b’, ‘c’]
