AmI 2015 - Python basics

Python
BASICS
Introduction to Python programming, basic
concepts: formatting, naming conventions,
variables, etc.

Editing / Formatting
• Python programs are
text files
• The end of a line
marks the end of a
statement
• Comments:
– Inline comments start
with a #
3/12/2015 Python basics 2
print 1+1 #statement
# inline comment

Editing / Formatting
• Code blocks are
defined through
identation
– mandatory
– 4 spaces strategy
• Use 4 spaces for code
identation
• Configure the text
editor to replace tabs
with 4 spaces (default
in PyDev)
• Exploit automatic
identation
def print_name():
# this is a block
name = 'sheldon'
surname = 'cooper'
print name, surname
4 spaces
4 spaces
4 spaces
4 spaces

Keywords
• and
• del
• from
• not
• while
• as
• elif
• global
• or
• with
• assert
• else
• if
• pass
• yield
• break
• except
• import
• print
• class
• exec
• in
• raise
• continue
• finally
• is
• return
• def
• for
• lambda
• try

Numbers and math
Operator Description
+ plus Sum
- minus Subtraction
/ slash Floor division
* asterisk Multiplication
** double asterisk Exponentiation
% percent Remainder
< less-than Comparison
> greater-than Comparison
<= less-than-equal Comparison
>= greater-than-equal Comparison

Numbers and math
print "I will now count my chickens:"
print "Hens", 25 + 30 / 6
print "Roosters", 100 - 25 * 3 % 4
print "Now I will count the eggs:"
print 3 + 2 + 1 - 5 + 4 % 2 - 1 / 4 + 6
print "Is it true that 3 + 2 < 5 - 7?"
print 3 + 2 < 5 - 7
print "What is 3 + 2?", 3 + 2
print "What is 5 - 7?", 5 - 7
print "Oh, that's why it's False."
print "How about some more."
print "Is it greater?", 5 > -2

Order of operations
• PEMDAS
– Parenthesis
– Exponentiation
– Multiplication
– Division
– Addition
– Subtraction
• Same precedence
– Left to right execution

Naming conventions
• joined_lower
– for functions,
variables, attributes
• joined_lower or
ALL_CAPS
– for constants
• StudlyCaps
– for classes
#variables
my_variable = 12
my_second_variable = 'Hello!'
#functions
my_function(my_variable)
my_print(my_second_variable)

Variables
• Variable types are not
explicitly declared
• Runtime type-checking
• The same variable can
be reused for holding
different data types
#integer variable
a = 1
print a
#float variable
a = 2.345
print a
#re-assignment to string
a = 'my name'
print a
# double quotes could be
# used as well
a = "my name"
print a

More variables
• Actual type can be
checked through the
interpreter
• Check the first result,
what happened?
• Display 01,010,01010
• Display 08
• Octal numbering system?

Examples
cars = 100
space_in_a_car = 4.0
drivers = 30
passengers = 90
cars_not_driven = cars - drivers
cars_driven = drivers
carpool_capacity = cars_driven * space_in_a_car
average_passengers_per_car = passengers / cars_driven
print 'There are', cars, 'cars available.'
print 'There are only', drivers, 'drivers available.'
print 'There will be', cars_not_driven, 'empty cars today.'
print 'We can transport', carpool_capacity, 'people today.'
print 'We have', passengers, 'to carpool today.'
print 'We need to put about', average_passengers_per_car,'in each car.'

Strings
• Defined by using quotes
– "first string"
– 'second string'
• Immutable
• Each character in a string is assigned a number
– the number is called index
• Mathematical operators cannot be applied
• Exceptions
– + : means concatenation
– * : means repetition

Strings
name = 'Anthony "Tony" Stark'
age = 45 # not a lie
height = 174 # cm
weight = 78 # kg
eyes = 'brown'
teeth = 'white'
hair = 'brown'
print "Let's talk about %s." % name
print "He's %d cm tall." % height
print "He's %d pounds heavy." % weight
print "Actually that's not too heavy."
print "He's got %s eyes and %s hair." % (eyes, hair)
print "His teeth are usually %s depending on the coffee." % teeth
# this line is tricky, try to get it exactly right
print "If I add %d, %d, and %d I get %d." % (age, height, weight, age + height + weight)

Strings
name = 'Anthony "Tony" Stark'
age = 45 # not a lie
height = 174 # cm
weight = 78 # kg
eyes = 'brown'
teeth = 'white'
hair = 'brown'
print "Let's talk about %s." % name
print "He's %d cm tall." % height
print "He's %d pounds heavy." % weight
print "Actually that's not too heavy."
print "He's got %s eyes and %s hair." % (eyes, hair)
print "His teeth are usually %s depending on the coffee." % teeth
# this line is tricky, try to get it exactly right
print "If I add %d, %d, and %d I get %d." % (age, height, weight, age + height + weight)
Specifiers
• %s, format strings
• %d, format numbers
• %r, raw representation
Tuple

More strings
x = "There are %d types of people." % 10
binary = "binary"
do_not = "don't"
y = "Those who know %s and those who %s." % (binary, do_not)
print x
print y
print "I said: %r." % x
print "I also said: '%s'." % y
hilarious = False
joke_evaluation = "Isn't that joke so funny?! %r"
print joke_evaluation % hilarious
w = "This is the left side of..."
e = "a string with a right side."
print w + e

Escape sequences
• n
– Line feed + Carriage return
•
– Prints a «»
• We want to print «Hello»
– print "I said: "Hello" "
– Syntax error: no difference between quotes
• Solution: using escape sequences
– print "I said: “Hello" "

Getting input from people
• Asking questions
– We want to ask the user’s age
– We want to ask the user’s height
• The raw_input()function allows to read from
the console
print "How old are you?",
age = raw_input()
print "How tall are you?",
height = raw_input()
print "You are %s years old, and you are about %s cm tall." % (age, height)

More input
height = int(raw_input("How tall are you? "))
name = raw_input("What's your name? ")
print type(height)
print type(name)
print("Hello %s, you are about %d tall" %(name, height) )

Command-line parameters
• Python scripts can receive launch parameters
– Placed just after the script name
– Any number
– Accessible through sys.argv
• sys
– Python module to handle system-level operations
• argv
– Argument variable
– for handling command-line parameters

Command-line parameters
from sys import argv
script, first, second, third = argv
print 'The script is called:', script
print 'Your first variable is:', first
print 'Your second variable is:', second
print 'Your third variable is:', third

Functions
• A function is a named sequence of statements
that performs a computation
– Definition first:
• specify the name and the sequence of statements
– Then usage:
• “call” the function by name
• Examples
– Type conversion functions
• int(‘32’)  32
• str(3.2479)  ‘3.2479’

Math functions
• Located in the math module
import math
signal_power = 10.0
noise_power = 0.01
ratio = signal_power / noise_power
print "ratio:", ratio
decibels = 10 * math.log10(ratio)
print "decibels:", decibels
radians = 0.7
height = math.sin(radians)
print height
Function call

String functions
• len()
– Gets the length (the number of characters) of a
string
• lower()
– Gets rid of all the capitalization in a string
• upper()
– Transform a string in upper case
• str()
– Transform «everything» in a string

String functions: an example
course_name = 'Ambient Intelligence'
string_len = len(course_name)
print string_len # 20
print course_name.lower() # ambient intelligence
print course_name.upper() # AMBIENT INTELLIGENCE
pi = 3.14
print "the value of pi is around " + str(pi) without str()
it gives an error

New functions
• Can be defined by developers
• Typically used to group homogeneous code
portions
– i.e., code for accomplishing a well-defined operation
• Enable re-use
– Same operation can be re-used several times
• Defined using the keyword def

New functions
• Compute the area of a disk, given the radius
import math
def circle_area(radius):
return radius**2*math.pi
radius = raw_input('Please, insert the radiusn')
print 'Radius: ', radius
print 'Area: ', circle_area(radius) Function call
Function definition

Docstring
• Optional, multiline comment
• Explains what the function does
• Starts and ends with """ or '''
import math
'''Compute the circle area given its radius'''
radius = raw_input('Please, insert the radiusn')
print 'Radius: ', radius
print 'Area: ', circle_area(radius)

Modules
• A way to logically organize the code
• They are files consisting of Python code
– they can define (and implement) functions, variables,
etc.
– typically, the file containing a module is called in the
same way
• e.g., the module math resides in a file named math.py
• We already met them
import math

Importing modules
• import module_name
– allows to use all the items present in a module
import math
…
Import the math module
Call the pi variable from
the math module

Importing modules
• from module_name import name
– it only imports name from the specified module
• from module_name import *
– it imports all names from a module
– do not use!
from math import pi
return radius**2*pi
…
Import pi from the math module
Use the pi variable

Playing with files
• Python script can read and write files
• First, open a file
– You can use the open() function
• Then, you can read or write it
– With read(), readline(), or write()
• Finally, remember to close the file
– You can use the close() function

Reading files
• Read a file taking its name from command line
filename = argv[1]
txt = open(filename)
print “Here’s your file %r:", % filename
print txt.read()
print “nType the filename again:”
file_again = raw_input(“> ”)
txt_again = open(file_again)
print txt_again.read()
Open the file
Show the file content

Writing files
script, filename = argv
print "We're going to erase %r." % filename
print "Opening the file..."
target = open(filename, 'w')
print "… truncating the file. Goodbye!"
target.truncate()
print "nNow I'm going to ask you for two lines."
line1 = raw_input("line 1: ")
line2 = raw_input("line 2: ")
print "I'm going to write these to the file."
target.write(line1)
target.write("n")
target.write(line2)
target.write("n")
print "And finally, we close it."
target.close()
Open the file in write mode
Empties the file
Write a string to the file

Conditionals and control flow
• Control flow gives the ability to choose among
outcomes
– based off what else is happening in the program
• Comparators
– Equal to  ==
– Not equal to  !=
– Less than  <
– Less than or equal to  <=
– Greater than  >
– Greater than or equal to  >=

Comparators: an example
print 2 == 1 # False
print 2 == 2 # True
print 10 >= 2 # True
print 2 < 10 # True
print 5 != 5 # False
print 'string' == "string" # True
number = 123
print number > 100 # True

Boolean operators
• They are three:
– not
– and
– or
• Not evaluated from left to right
– not is evaluated first
– and is evaluated next
– or is evaluated last

Boolean operators: an example
print 2 == 1 and True # False
print 2 == 2 or True # True
print 10 >= 2 and 2 != 1 # True
print not True # False
print 10 > 5 and 10 == 10 or 5 < 2 # True
print not False and True # True

Conditional statement
• if is a statements that executes some code
after checking if a given expression is True
• Structure
if expression:
do something
people = 20
cats = 30
if people < cats:
print 'Too many cats! The world
is doomed!'
if people > cats:
print 'Not many cats! The world
is saved!'

More “if”
• Let’s try to “improve” the previous example
• Chained conditionals
– To express more than two possibilities
– Each condition is checked in order
people = 20
cats = 30
if people < cats:
print 'Too many cats! The world is doomed!'
elif people > cats:
print 'Not many cats! The world is saved!'
else:
print "We can’t decide.”
else if

Loops and lists
• Loop
– An easy way to do repetitive things
– A condition to start and stop the loop is required
– e.g., for and while loops
• List
– A datatype for storing multiple items
• a sequence of values
– You can assign items to a list in this way:
list_name = [item1, item2, …]

Loops and lists: an example
the_count = [1, 2, 3, 4, 5]
fruits = ['apples', 'oranges', 'pears', 'apricots']
change = [1, 'pennies', 2, 'dimes', 3, 'quarters']
# this first kind of for-loop goes through a list
for number in the_count:
print 'This is count %d' % number
# same as above
for fruit in fruits:
print 'A fruit of type: %s' % fruit
# we can go through mixed lists too
# notice that we have to use %r since we don't know what's in it
for i in change:
print 'I got %r' % i
Three lists

Loops and lists: an example
the_count = [1, 2, 3, 4, 5]
fruits = ['apples', 'oranges', 'pears', 'apricots']
change = [1, 'pennies', 2, 'dimes', 3, 'quarters']
# this first kind of for-loop goes through a list
for number in the_count:
print 'This is count %d' % number
# same as above
for fruit in fruits:
print 'A fruit of type: %s' % fruit
# we can go through mixed lists too
# notice that we have to use %r since we don't know what's in it
for i in change:
print 'I got %r' % i
Structure of a for loop
• for variable in collection:
• indent for the loop body

More “for”
# we can also build lists: start with an empty one…
elements = []
# then use the range function to do 0 to 5 counts
for i in range(0, 6):
print 'Adding %d to the list.' % i
# append() is a function that lists understand
elements.append(i)
# now we can print them out
for i in elements:
print 'Element was: %d' % i
Empty list
Repeat 6 times

Lists
• Mutable
• Do not have a fixed length
– You can add items to a list at any time
• Accessible by index
letters = [‘a’, ‘b’, ‘c’]
letters.append(‘d’)
print letters # a, b, c, d
print letters[0] # a
print len(letters) # 4
letters[3] = ‘e’
print letters # a, b, c, e

More lists
• List concatenation
– with the + operator
• List slices
– to access a portion of a list
– with the [:] operator
a = [1, 2, 3]
b = [4, 5, 6]
c = a + b
print c # 1, 2, 3, 4, 5, 6
c = [1, 2, 3, 4, 5, 6]
d = c[1:3] # d is [2, 3]
e = c[:3] # e is [1, 2, 3]
f = c[:] # f is a full copy of c

More lists
• List concatenation
– with the + operator
• List slices
– to access a portion of a list
– with the [:] operator
a = [1, 2, 3]
b = [4, 5, 6]
c = a + b
print c # 1, 2, 3, 4, 5, 6
c = [1, 2, 3, 4, 5, 6]
d = c[1:3] # d is [2, 3]
e = c[:3] # e is [1, 2, 3]
f = c[:] # f is a full copy of c
works with string, too

List functions
• append()
– add a new element to the end of a list
– e.g., my_list.append(‘d’)
• sort()
– arrange the elements of the list from low to high
– e.g., from a to z, from 1 to infinite, etc.
• extend()
– takes a list as an argument and appends all its
elements
– e.g., first_list.extend(second_list)

Deleting elements from a list
• Several ways to delete elements from a list
• If you know the index of the element to
remove: pop()
– without providing an index, pop() delete the last
element
• If you know the element to remove (but not the
index): remove()
• To remove more than one element: del()
– with a slice index
• e.g., del my_list[5:8]

Strings vs. lists
• A string is a sequence of character, but a list of
character is not a string
• To convert a string into a list of characters: list()
– e.g., my_list = list(my_string)
• To break a string into separate words: split()
– split a list according to some delimiters (default:
space)
– e.g., my_list = my_string.split()
– The inverse function is join()

Copying lists
• What happens here?
fruits = ['apple', 'orange', 'pear', 'apricot']
print 'The fruits are:', fruits
favourite_fruits = fruits
print 'My favourite fruits are', favourite_fruits
# add a fruit to the original list
fruits.append(‘banana’)
print 'The fruits now are:', fruits

Copying lists
• What happens here?
fruits = ['apple', 'orange', 'pear', 'apricot']
print 'The fruits are:', fruits
favourite_fruits = fruits
# add a fruit to the original list
fruits.append(‘banana’)
print 'The fruits now are:', fruits
We do not make a copy of
the entire list, but we only
make a reference to it!

Copying lists
• How to make a full copy of a list?
• Various methods exist
– you can entirely slice a list
• favourite_fruits = fruits[:]
– you can create a new list from the existing one
• favourite_fruits = list(fruit)
– you can extend an empty list with the existing one
• favourite_fruits.extend(fruit)
• Prefer the list() method, when possible!

Dictionaries
• Similar to lists, but you can access values by
looking up a key instead of an index
– A key can be a string or a number
• Example
– A dictionary with 3 key-value pairs
dict = { ‘key1’ : 1, ‘key2’ : 2, ‘key3’ : 3 }
• Mutable, like lists
– They can be changed after their creation

Dictionaries: an example
# create a mapping of U.S. state to abbreviation
states = {
'Oregon' : 'OR',
'Florida' : 'FL',
'California' : 'CA'
}
print 'States:', states
print 'Is Oregon available?', 'Oregon' in states
# add some more states
states['New York'] = 'NY'
states['Michigan'] = 'MI'
# print two states
print "New York’s abbreviation is: ", states[‘New York’]
print "Florida’s abbreviation is: ", states[‘Florida’]
Create a dictionary with 3 key-value pairs
Add two more key-value pairs

More dictionaries
# states is a dictionary defined as before
# print every state abbreviation
for state, abbrev in states.items():
print "%s is abbreviated %s", % (state, abbrev)
# safely get an abbreviation of a state that might not be there
state = states.get('Texas', None) # None is the default
if not state:
print "Sorry, no Texas."
# get a state abbreviation with a default value
next_state = states.get('Massachusetts', 'Does Not Exist')
print "Massachusetts is abbreviated %s", % next_state

Dictionary functions
• len()
– dictionary length: the number of key-value pairs
• del()
– remove a key-value pair
• e.g., del my_dict[my_key]
• clear()
– remove all items from a dictionary
• keys() and values()
– return a copy of the dictionary’s list of key and value,
respectively

References and Links
• The Python Tutorial,
http://docs.python.org/2/tutorial/
• «Think Python: How to think like a computer
scientist», Allen Downey, Green Tea Press,
Needham, Massachusetts
• «Dive into Python 2», Mark Pilgrim
• «Learn Python the Hard Way», Zed Shaw
• «Learning Python» (5th edition), Mark Lutz, O'Reilly
• The Google Python course,
https://developer.google.com/edu/python
• Online Python Tutor, http://pythontutor.com

Questions?
01PRD AMBIENT INTELLIGENCE: TECHNOLOGY AND DESIGN
Luigi De Russis and Dario Bonino
luigi.derussis@polito.it
bonino@isbm.it

License
• This work is licensed under the Creative Commons “Attribution-
NonCommercial-ShareAlike Unported (CC BY-NC-SA 3,0)” License.
• You are free:
– to Share - to copy, distribute and transmit the work
– to Remix - to adapt the work
• Under the following conditions:
– Attribution - You must attribute the work in the manner specified by the
author or licensor (but not in any way that suggests that they endorse you
or your use of the work).
– Noncommercial - You may not use this work for commercial purposes.
– Share Alike - If you alter, transform, or build upon this work, you may
distribute the resulting work only under the same or similar license to this
one.
• To view a copy of this license, visit
http://creativecommons.org/license/by-nc-sa/3.0/
3/12/2015 Version Control with Git 59

AmI 2015 - Python basics

Recommended

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to AmI 2015 - Python basics (20)

More from Luigi De Russis (20)

Recently uploaded (20)

AmI 2015 - Python basics