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:
print 1+1 #statement
# inline comment
– Inline comments start with a #
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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 3/12/2015
def print_name(): # this is a block 4 spaces name = 'sheldon' 4 spaces surname = 'cooper' 4 spaces print name, surname 4 spaces
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Keywords • • • • • • • • • •
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and del from not while as elif global or with
• • • • • • • • • • •
assert else if pass yield break except import print class exec Python basics
• • • • • • • • • •
in raise continue finally is return def for lambda try 4
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
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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
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Order of operations • PEMDAS – – – – – –
Parenthesis Exponentiation Multiplication Division Addition Subtraction
• Same precedence – Left to right execution 3/12/2015
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Naming conventions • joined_lower – for functions, variables, attributes
• joined_lower or ALL_CAPS – for constants
• StudlyCaps
#variables my_variable = 12 my_second_variable = 'Hello!' #functions my_function(my_variable) my_print(my_second_variable)
– for classes
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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
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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?
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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.'
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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 3/12/2015
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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) 3/12/2015
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Strings Specifiers • %s, format strings %d, format name = 'Anthony•"Tony" Stark' numbers age = 45 # not a lie • %r, raw representation height = 174 # cm weight = 78 # kg eyes = 'brown' teeth = 'white' hair = 'brown'
Tuple
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) 3/12/2015
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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
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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\" " 3/12/2015
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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)
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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) )
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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 3/12/2015
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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
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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’ 3/12/2015
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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
Function call
radians = 0.7 height = math.sin(radians) print height 3/12/2015
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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 3/12/2015
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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)
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without str() it gives an error
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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
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New functions • Compute the area of a disk, given the radius import math def circle_area(radius): return radius**2*math.pi
Function definition
radius = raw_input('Please, insert the radius\n') print 'Radius: ', radius print 'Area: ', circle_area(radius) Function call
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Docstring • Optional, multiline comment • Explains what the function does • Starts and ends with """ or ''' import math def circle_area(radius): '''Compute the circle area given its radius''' return radius**2*math.pi radius = raw_input('Please, insert the radius\n') print 'Radius: ', radius print 'Area: ', circle_area(radius) 3/12/2015
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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 from sys import argv 3/12/2015
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Importing modules • import module_name – allows to use all the items present in a module import math
Import the math module
def circle_area(radius): return radius**2*math.pi
Call the pi variable from the math module
…
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Importing modules • from module_name import name – it only imports name from the specified module from math import pi def circle_area(radius): return radius**2*pi …
Import pi from the math module
Use the pi variable
• from module_name import * – it imports all names from a module – do not use! 3/12/2015
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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
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Reading files • Read a file taking its name from command line from sys import argv filename = argv[1] txt = open(filename)
Open the file
print “Here’s your file %r:", % filename print txt.read()
Show the file content
print “\nType the filename again:” file_again = raw_input(“> ”) txt_again = open(file_again) print txt_again.read()
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Writing files from sys import argv script, filename = argv print "We're going to erase %r." % filename print "Opening the file..." target = open(filename, 'w') Open the file in write mode print "… truncating the file. Goodbye!" target.truncate() Empties the file 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") Write a string to the file target.write(line2) target.write("\n") print "And finally, we close it." target.close() 3/12/2015
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Conditionals and control flow • Control flow gives the ability to choose among outcomes – based off what else is happening in the program
• Comparators – – – – – – 3/12/2015
Equal to == Not equal to != Less than < Less than or equal to <= Greater than > Greater than or equal to >= Python basics
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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
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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
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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
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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!'
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More “if” • Let’s try to “improve” the previous example people = 20 cats = 30 if people < cats: print 'Too many cats! The world is doomed!' else if elif people > cats: print 'Not many cats! The world is saved!' else: print "We can’t decide.”
• Chained conditionals – To express more than two possibilities – Each condition is checked in order 3/12/2015
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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, …]
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Loops and lists: an example the_count = [1, 2, 3, 4, 5] fruits = ['apples', 'oranges', 'pears', 'apricots'] change = [1, 'pennies', 2, 'dimes', 3, 'quarters']
Three lists
# 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 3/12/2015
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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 Structure of a for loop for fruit in fruits: • for variable in collection: print 'A fruit of type: %s' % fruit • indent for the loop body # 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 3/12/2015
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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)
Empty list
Repeat 6 times
# now we can print them out for i in elements: print 'Element was: %d' % i
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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 3/12/2015
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More lists • List concatenation – with the + operator a = [1, 2, 3] b = [4, 5, 6] c=a+b print c # 1, 2, 3, 4, 5, 6
• List slices – to access a portion of a list – with the [:] operator 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 3/12/2015
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More lists • List concatenation – with the + operator a = [1, 2, 3] b = [4, 5, 6] c=a+b print c # 1, 2, 3, 4, 5, 6
• List slices – to access a portion of a list – with the [:] operator 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 3/12/2015
works with string, too
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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) 3/12/2015
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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]
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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() 3/12/2015
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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 print 'My favourite fruits are', favourite_fruits
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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’)
We do not make a copy of the entire list, but we only make a reference to it!
print 'The fruits now are:', fruits print 'My favourite fruits are', favourite_fruits
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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! 3/12/2015
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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
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Dictionaries: an example # create a mapping of U.S. state to abbreviation states = { 'Oregon' : 'OR', Create a dictionary with 3 key-value pairs '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'
Add two more key-value pairs
# print two states print "New York’s abbreviation is: ", states[‘New York’] print "Florida’s abbreviation is: ", states[‘Florida’] 3/12/2015
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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
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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 3/12/2015
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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 3/12/2015
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Questions? 01PRD AMBIENT INTELLIGENCE: TECHNOLOGY AND DESIGN
Luigi De Russis and Dario Bonino
[email protected] [email protected]
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• To view a copy of this license, visit http://creativecommons.org/license/by-nc-sa/3.0/
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