Python 3 Quick Reference Guide
Comment Symbol is a hash ( # )
# this is a comment (single line)
Multiline string character ( ‘ ’ ’ )
(3 single quotes)
‘ ‘ ‘ this is a multiline string. It can span
multiple
lines ‘ ‘ ‘
Line continuation character - backslash ( \ )
Python’s line continuation character is a
Backslash. This is required if a statement
is continued on the next line(s).
Arithmetic Operators
+ Addition
Subtraction
* Multiplication
% Modulus (integer remainder)
/ Division (floating-point)
// Integer quotient (int or float operands)
** Exponentiation
+
string concatenation operator
Remember to distinguish between
integers and floating-point numbers.
Integers are stored differently than floatingpoint numbers in memory and they are
different object types:
integer
floating-point
<class ‘int’>
<class ‘float’>
 integers:
2, 3, -5, 0, 8
 floating-point: 2.0, .5, -7., 3.1415
Relational Operators
< Less than
<= Less than or equal to
> Greater than
>= Greater than or equal to
!= Not equal to
== Equal to (compares objects)
Logical Operators
not / and / or
Assignment Operators
= simple assignments
+= compound assignment
(also -=, *=, /=, %=)
x += 10 --> x = x + 10
-= subtraction/assignment
*= multiplication/assignment
/= division/assignment
%= modulus/assignment
+= string concatenation
“my_opinion” += " take it or leave it… "
Special Operators
is compares references # a is b [ vs a == b ]
in tests membership
# x in list_c
Last Update: Friday, March 11, 2016
Types of Objects
Everything you create in Python is an
object – strings, tuples, lists, dictionaries,
sets, integers, floating-point numbers,
etc.
Python Range
range( [start=0, ] stop [ ,step=1 ] )
the start and the step are optional
Examples:
Variables in Python do not store values.
Variables always store a reference to an
object! A variable name is an alias.
Python Collections
a = (1,2,3)
# a is a tuple
b = [4,5,6]
# b is a list
c = {7,8,9}
# c is a set
d = { ‘a’:1, ‘b’:2} # d is a dict
x = 3 x stores a reference to a <class ‘int’>
object with the value 3
y = 5.4 y stores a reference to a <class
‘float’> object with the value 5.4
s = ’hi’ s stores a reference to a <class
‘str’> object with the value ‘hi’
Numeric Types
integer
<class ‘int’>
floating-point
<class ‘float’>
complex <class ‘complex’> e.g. (3+5j)
String Type
string
<class ‘str’>
Note: strings may be enclosed in single,
double or triple quotes.
String slicing
[start] : [stop] [: step]
s=’Hello World’ s[:] --> ‘Hello World’
s[:5] --> ‘Hello’ s[6:] --> ‘World’
Sequence Types
tuple
list
set
range
string
(3,4,5) <class ‘tuple’>
[3,4,5] <class ‘list’>
{5,3,4} <class ‘set’>
range(1,6)
<class ‘range’>
‘abcdefghijk’
<class ‘str’>
A dict is a map type key maps to value
dict
{ ‘A’:65, ‘B’:66 } <class ‘dict’>
‘A’ is a key, 65 is a value
Class Conversions
( string <--> int <--> float )
p = int( input(‘Enter an integer: ‘) )
q = float( input('Enter your hourly rate') )
r = int( ‘123’ )
s = float( "88.26" )
t = str( 2013 )
Popular Python Imports
import math (precede functions with math.)
 ceiling(x), floor(x), sqrt(x)
 math.e, math.pi
import random
 random.randint( first, last )
 random.random( ) --> [0, 1)
Import sys
 sys.exit(), sys.stdin, sys.stdout
r = range(5)
# r is <class ‘range’>
()
[]
{}
{k:v}
Collection Conversions:
set <--> tuple <--> list
e = tuple(c)
# set --> tuple
f = list(a)
# tuple --> list
g = set(b)
# list --> set
h = tuple(d)
# dict --> tuple
I = set(r)
# range --> set
Iteration over a Collection
 for j in a: # over a tuple
 for j in b: # over a list
 for j in c: # over a set
 for j in d: # over a dict
 for j in r
# over a range
Python Mutability
 A list is mutable
 All other ojects in Python are
immutable
Input statement
input( prompt ) --> returns a string
n_str = input(“Enter an integer”)
n = int(n_str) # convert str --> int
Print Statement
print( items [ ,sep= ] [ ,end= ] )
default sep = ’ ‘, default end = ‘\n’
m = 11, d = 6, y = 2015
print(m, d, y) --> 11 6 2015 (‘\n’)
print(m +’/’ + d + ‘/’ + y)
prints --> 11/6/2015 (‘\n’)
Formatted Output
print( string [ .format( )] )
print( “{} are a {}”.format(‘You’, 10) )
prints --> You are a 10 (‘\n’)
Python Attributes
type( ) --> class of an object
input()
type(‘s’) --> <class ‘str’>
type(5) --> <class ‘int’>
id( )
print()
--> address of an object
Python 3 Quick Reference Guide
Selection and Loop Structures
Selection:
 Unary or single selection
 Binary or dual selection
 Case structure possible when branching on a variable
 Simple selection
 One condition
 Compound selection
 Multiple conditions joined with and / or operators
Looping:
 Python uses Pre-test loops
 The test precedes the body of the loop
 while
 for
Loop Control:
 Expressions that are used to control a while loop:
 initialize
 test
 update (done automatically in a for loop)
 Counter-controlled loops,
aka definite loops, work with a loop control variable
(lcv). Can be a for loop or a while loop
 Sentinel-controlled loops,
aka indefinite loops, work with a sentinel value. A
while loop is used for indefinite loop
Last Update: Friday, March 11, 2016
If Statement (selection structure)
Simple if
if test:
statement(s)
-------------------------------------------------------------------------------if / else
if test:
statement(s)
else:
statement
 the continue statement skips the rest of the
statements in the body of the loop and proceeds to
the test expression. The loop is not exited unless the
test expression evaluates to False.
Python suite:
Unlike C++ and Java, curly braces are not used to create
a block. Python uses indentation to create a suite.
Examples
if x < y:
print( ‘x < y’ )
for x in range(1,6):
print( x**2, end=’ ‘ )
Popular Functions
round(num, digits)
sum(iter) --> sum( [1,2] )
len(obect)
ord(‘A’), chr(65)
help( )
Example
if x < y:
x += 1
else:
x -= 1
-------------------------------------------------------------------------------if / elif / else (nested if)
if test:
statement(s)
elif test:
statement(s)
else:
statement(s)
Range Expressions
Example
if x < y:
x += 1
elif x < z:
x -= 1
else:
y += 1
range( [start=0,] stop [,step=1] )
generates values in the range start to stop-1 [start, stop)
range(1, 11)
# generates numbers 1 - 10
# start=1, stop = 11, step=1
range(2, 21 ,2)
# generates even numbers 2 – 20
# start=2, stop=21, step=2
Note: The break and continue statements can be used
to modify the behavior of a loop
 the break statement exits a loop and starts executing
the next statement
Example
if x < y:
x += 1
For Loop statement (repetition structure)
Form:
for iterable-expr:
suite
Example:
sum = 0
for n in range(1,11):
sum += n
Note: The for loop iterates over a collection
 for x in range(5)
 for x in ‘abcde’:
 for x in [1,2,3,4,5]:
 evens = (2,4,6,8,10)
for x in evens:
While Loop statement (repetition structure)
Python Empty Statement
pass
Use when a statement is
required but none is needed
or as a placeholder.
Python Null reference
None
x = None
# x is <class 'NoneType'>
[ pre-test loop ]
[ pre-test loop ]
Form:
Example:
initialize
n,sum = 1,0
while boolean-expr:
suite
(update-expr)
while n <= 10:
sum += n
n += 1
Python loops:
 may also include an else suite ( for/while … else )
 may use keywords break and continue to alter the flow
of execution in the loop
 no direct support for a post-test loop