AmI 2017 - Python basics

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

#include <stdio.h>
int main()
{
printf("Hello, world!");
return 0;
}
3/8/2017 Python basics 2

print("Hello, world!")

# this will print "Hello, world!"
print("Hello, world!")
inline comment

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

Variables
language_name = "Python"
naming convention: joined_lower
this is a string

Variables
version = '3.6.0'
introduced = 1991
is_awesome = True

Type Inference
language_name = "Python" # string
version = '3.6.0' # another string
introduced = 1991 # integer
is_awesome = True # boolean
actual type can be checked with type()
play_with_types.py

String
some_string = "I'm a string"
another_string = 'I'm a string, too'

String
some_string = "I'm a string"
another_string = 'I'm a string, too'
# SyntaxError: invalid syntax

String
another_string = 'I am a string, too'
another_strig = 'I'm a string, too'
escape sequence

String
long_string = """I am a long string.
I span over two lines."""
long_string = '''I am another long
string.
I span over three lines.
I am composed by three sentences.'''

If Statement
people = 20
cats = 30
if people < cats:
print("Too many cats! We are doomed!")
if people > cats:
print("Not many cats! We are safe!")
4 spaces
4 spaces

If Statement
people = 20
cats = 30
if people < cats:
print("Too many cats! We are doomed!")
elif people > cats:
print("Not many cats! We are safe!")
else:
print("We can't decide.")

Comparators and Booleans
Operators
print(2 == 1)
print('string' == "string")
print(not False)
print(2==1 and True)
print(2==1 or True)

Comparators and Booleans
Operators
print(2 == 1) # False
print('string' == "string") # True
print(not False) # True
print(2==1 and True) # False
print(2==1 or True) # True

Characters
for char in "hello":
print(char)
h
e
l
l
o

Characters
say_hello = "hello!"
print(say_hello[1])
e
index

Characters
say_hello = "hello!"
print(type(say_hello[1]))
<class 'str'>

Combining Strings
version = '3.6.0'
python_version = language_name + version
# python_version is Python3.6.0
print("my " + "name") # my name
concatenation

Combining Strings
a_lot_of_python = language_name*3
# a_lot_of_python is PythonPythonPython
repetition

Building Complex Strings
a = 3
b = 5
# 3 times 5 is 15
print(a, "times", b, "is", a*b)
works with print(), only

a = 3
b = 5
# 3 times 5 is 15
result = a + " times " + b + " is " + a*b

a = 3
b = 5
# 3 times 5 is 15
result = a + " times " + b + " is " + a*b
#TypeError: unsupported operand type(s)

a = 3
b = 5
# 3 times 5 is 15
result = str(a) + " times " + str(b) + "
is " + str(a*b)

String Interpolation
a = 3
b = 5
# 3 times 5 is 15
result = "%d times %d is %d" %(a, b, a*b)
Specifiers
• %s, format strings
• %d, format numbers
• %r, raw representation
tuplespecifiers.py

String Interpolation
a = 3
b = 5
# 3 times 5 is 15
result = "{} times {} is {}".format(a, b,
a*b)
new way!

String Immutability
# hello
say_hello = "helko"
# ops…
say_hello[3] = "l"

String Immutability
# hello
say_hello = "helko"
# ops…
say_hello[3] = "l"
# TypeError

String Immutability
# hello
say_hello = "helko"
# ops…
say_hello = "hello"
Other operations with strings? Python docs

Getting Input
print("How old are you?")
age = input() # age is a string
print("You are " + age + " years old")

Getting Input
print("How old are you?")
age = input() # age is a string
# I want "age" to be a number!
age = int(input())

Getting Input
age = input("How old are you? ")

List
fruits = ["apples", "oranges", "pears"]
count = [1, 2, 3, 4, 5]
change = [1, "pennies", 2, "dimes"]
a datatype to store multiple items, in sequence

Dictionary
legs = {"ant": 6, "snake": 0, "cow": 4}
states = {"Italy": "IT", "Germany": "DE"}
a datatype to store multiple items, not in sequence
key, immutable
value

Loops
doctor = 1
while doctor <= 13:
exterminate(doctor)
doctor += 1

For Loop: Strings
for char in "hello":
print(char)
h
e
l
l
o

For Loop: Ranges
for number in range(0,5):
print(number)
0
1
2
3
4

For Loop: Ranges
for number in range(0,25,5):
print(number)
0
5
10
15
20

For Loop: Lists
fruits = ["apples", "oranges", "pears"]
for fruit in fruits:
print("I love", fruit)
I love apples
I love oranges
I love pears

For Loop: Dictionaries
legs = {"ant": 6, "snake": 0, "cow": 4}
for (animal, number) in legs.items():
print("{} has {} legs".format(animal,
number))
ant has 6 legs
snake has 0 legs
cow has 4 legs

Printing a List
to_buy = ["eggs", "milk"]
print(to_buy)
['eggs', 'milk']

Printing a List
print(to_buy[0])
eggs

Modifying a List
print(to_buy[0])
to_buy[0] = "butter"
print(to_buy[0])
eggs
butter

Modifying a List
# I need to buy chocolate!
to_buy.append("chocolate")
['eggs', 'milk', 'chocolate']

Modifying a List
to_buy.extend(["flour", "cheese"])
['eggs', 'milk', 'chocolate', 'flour', 'cheese']

Modifying a List
to_buy = to_buy + ["flour", "cheese"]
['eggs', 'milk', 'chocolate', 'flour', 'cheese']
concatenation

Modifying a List
to_buy = ["eggs", "milk", "chocolate",
"flour", "cheese"]
print(to_buy[1:3])
['milk', 'chocolate']
slice operator

Modifying a List
"flour", "cheese"]
# make a full copy of the list
remember = to_buy[:]
works with strings, too

Modifying a List
"flour", "cheese"]
# I don't need cheese!
to_buy.pop()
# … neither milk, by the way!
to_buy.pop(1)

Modifying a List
"flour", "cheese"]
# I don't need cheese!
to_buy.remove("cheese")
# … neither milk, by the way!
to_buy.remove("milk")

Modifying a List
"flour", "cheese"]
# I want my original list back!
del to_buy[2:6]
['eggs', 'milk']

Strings vs. Lists
A string is a sequence of characters…
… but a list of characters is not a string
# string to list
name = list(language_name)

Strings vs. Lists
sentence = "this is AmI"
# break a string into separate words
words = sentence.split()
['this', 'is', 'AmI']

Copying Lists
fruits = ['apple', 'orange']
favorite_fruits = fruits
# add a fruit to the original list
fruits.append('banana')
print('The fruits now are:', fruits)
print('My favorite fruits are', favorite_fruits)
Fruits are: ['apple', 'orange', 'banana']
My favorite fruits are: ['apple', 'orange',
'banana']
???

Copying Lists
fruits = ['apple', 'orange']
favorite_fruits = fruits
# add a fruit to the original list
fruits.append('banana')
print('The fruits now are:', fruits)
print('My favorite fruits are', favorite_fruits)
We do not make a copy of
the entire list, but we only
make a reference to it!

Copying Lists (For Real!)
# option 1: slice
favorite_fruits = fruits[:]
#option 2: create a new list - best!
favorite_fruits = list(fruit)
#extend an empty list
favorite_fruits.extends(fruit)
Other operations with lists? Python docs

Printing a Dictionary
legs = {"ant": 6, "snake": 0 }
print(legs)
{'ant': 6, 'snake': 0}

Modifying a Dictionary
legs["spider"] = 273
{'ant': 6, 'snake': 0, 'spider': 273}

legs["spider"] = 273 # basically, run!
legs["spider"] = 8 # better!
{'ant': 6, 'snake': 0, 'spider': 8}

legs = {"ant": 6, "snake": 0, "spider": 8}
# I don't like spiders
del legs["spider"]
# Clear all the things!
legs.clear()

Retrieving a Value from a
Dictionary
legs = {"ant": 6, "snake": 0}
# get "ant"!
legs["ant"] # 6
# get "spider"
legs["spider"]

Dictionary
# get "ant"!
legs["ant"] # 6
# get "spider"
legs["spider"]
# KeyError: spider

Dictionary
# check if "spider" is in the dictionary
"spider" in legs # False
# get "spider" without throwing errors
legs.get("spider") # None
# get "spider" with a custom value
legs.get("spider", "Not present")

Functions
def say_hello():
print("Hello!")
say_hello()
definition
call

Functions with Parameters
def say_hello_to(name):
print("Hello", name)
say_hello_to("AmI students")

Default Parameter Values
def say_hello_to(name="AmI"):
print("Hello", name)
say_hello_to() # Hello AmI
say_hello_to("students") # Hello students

Returning Values
def build_greetings(name="AmI"):
return "Hello" + name
greeting = build_greetings()
print(greeting) # Hello AmI

Returning Multiple Values
return ("Hello", name)
(greeting, person) = build_greetings()
print(greeting + " to " + person)
# Hello to AmI

Documenting Functions
'''Build a greeting in the format
Hello plus a given name'''
return ("Hello", name)
docstring

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 math module resides in a file named math.py

Importing a Module
import math # import the math module
print math.pi # print 3.141592…
from math import pi # import pi, only!
print pi # print 3.141592…
from math import * # import all the names
print pi DO NOT USE

Command Line Parameters
from sys import argv
script, first = argv
print("The script is called:", script)
print("The parameter is:", first)
> python my_script.py one
The script is called: my_script.py
The parameter is: one
unpacking

Reading Files
filename = argv[1]
txt = open(filename)
print("Here's your file %r:", % filename)
print(txt.read())
open the file
show the file content

Writing Files
filename = argv[1]
# open in write mode and empty the file
target = open(filename, "w")
# write a string into the file
target.write("This is the new content")
target.close() # close the file

References and Links
• Python Documentation, http://docs.python.org/3
• The Python Tutorial,
http://docs.python.org/3/tutorial/
• Online Python Tutor, http://pythontutor.com
• «Think Python: How to think like a computer
scientist», 2nd edition, Allen Downey, Green Tea
Press, Needham, Massachusetts
• «Dive into Python 3», Mark Pilgrim
• «Learning Python» (5th edition), Mark Lutz, O'Reilly

Questions?
01QZP AMBIENT INTELLIGENCE
Luigi De Russis
luigi.derussis@polito.it

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AmI 2017 - Python basics

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