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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 onto another 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: Sunday, April 30, 2017
Types of Objects
Everything you create in Python is an
object – strings, tuples, lists, dictionaries,
sets, integers, floating-point numbers,
etc.
Examples:
Variables in Python do not store values.
Variables always store a reference to an
object! A variable name is an alias.
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)
Python Range
range( [start=0, ] stop [ ,step=1 ] )
the start and the step are optional
r = range(2,11,2) # r is <class ‘range’>
t = list(r) t--> [2, 4, 6, 8, 10 ]
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
()
[]
{}
{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
Note: strings may be enclosed in single,
double or triple quotes.
Python Array
Python does not have an array type.
However, you can use a list.
values = [ 0 for x in range(5) ]
values --> [0, 0, 0, 0, 0]
Valid subscripts are 0 – 4 inclusive
String slicing
[start] : [stop] [: step]
s=’Hello World’ s[:] --> ‘Hello World’
s[:5] --> ‘Hello’ s[6:] --> ‘World’
Python Mutability
Most objects are immutable
String Type
string
<class ‘str’>
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
Mutable objects include:
 list, set, dict
Immutable objects include:

numeric types, string, tuple
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
type(‘s’) --> <class ‘str’>
type(5) --> <class ‘int’>
id( )
--> address of an object
Python 3 Quick Reference Guide
Last Update: Sunday, April 30, 2017
If Statement (selection structure)
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
Simple if
if test:
statement(s)
Example
if x < y:
x += 1
-------------------------------------------------------------------------------if / else
if test:
statement(s)
else:
statement
Example
if x < y:
x += 1
else:
x -= 1
--------------------------------------------------------------------------------
Loop Control:
 Expressions that are used to control a while loop:
 initialize
 test
 update (done automatically in a for loop)
if / elif / else (nested if)
if test:
statement(s)
elif test:
statement(s)
else:
statement(s)
Example
if x < y:
x += 1
elif x < z:
x -= 1
else:
y += 1
 Counter-controlled loops,
aka definite loops, work with a loop control variable
(lcv). Can be a for loop or a while loop
Range Expressions
 Sentinel-controlled loops,
aka indefinite loops, work with a sentinel value. A
while loop is used for indefinite loop
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
 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.
for x in range(1,6):
print( x**2, end=’ ‘ )
generates values in the range start to stop-1 [start, stop)
For Loop statement (repetition structure)
Form:
for iterable-expr:
suite
Empty Statement: pass
Use when a statement is
required but none is
needed or as a
placeholder.
isinstance function:
Null reference:
None
x=3
isinstance (x, int) --> True
isinstance (x, float) --> False
isinstance (x, str) --> False
x = None
# x is <class 'NoneType'>
x == None --> True
x is None --> True
[ pre-test loop ]
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] # list
 for x in (3,5,7,9,12) # tuple
While Loop statement (repetition structure)
Examples
if x < y:
print( ‘x < y’ )
range( [start=0,] stop [,step=1] )
[ 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