Python introduction 2

Introduction to Python -2
Ahmad Hussein
ahmadhussein.ah7@gmail.com

Class Declaration
• The simplest class possible is shown in the following example:
class Person:
pass # An empty block

Class Methods
• The self:
Class methods have only one specific difference from ordinary functions.
they must have an extra first name that has to be added to the beginning
of the parameter list, but you do not give a value for this parameter when
you call the method, Python will provide it. This particular variable refers to
the object itself, and by convention, it is given the name self.
The self in Python is equivalent to the this pointer in C++ and the this reference in
Java and C#.

Class Methods
class Person:
def say_hi(self):
print('Hello, how are you?')
p = Person()
p.say_hi()
# Hello, how are you?

The __init__ method
• The __init__ method is run as soon as an object of a class is created.
The method is useful to do any initialization you want to do with your
object.
class Person:
def __init__(self, name):
self.name = name
def say_hi(self):
print('Hello, my name is', self.name)
p = Person(‘Yasser')
p.say_hi()
# Hello, my name is Yasser

Class And Object Variables
• Class variables are shared - they can be accessed by all instances of
that class.
• Object variables are owned by each individual object/instance of the
class. In this case, each object has its own copy of the field i.e. they
are not shared and are not related in any way to the field by the same
name in a different instance.

class Robot:
"""Represents a robot, with a name."""
# A class variable, counting the number of robots
population = 0
def __init__(self, name):
"""Initializes the data."""
self.name = name
print("(Initializing {})".format(self.name))
# When this person is created, the robot
# adds to the population
Robot.population += 1

def die(self):
"""I am dying."""
print("{} is being destroyed!".format(self.name))
Robot.population -= 1
def say_hi(self):
print("Greetings, my masters call me {}.".format(self.name))
@classmethod
def how_many(cls):
"""Prints the current population."""
print("We have {:d} robots.".format(cls.population))

droid1 = Robot("R2-D2")
droid1.say_hi()
Robot.how_many()
droid2 = Robot("C-3PO")
droid2.say_hi()
Robot.how_many()
droid1.die()
droid2.die()
Robot.how_many()

Output
(Initializing R2-D2)
Greetings, my masters call me R2-D2.
We have 1 robots.
(Initializing C-3PO)
Greetings, my masters call me C-3PO.
We have 2 robots.
R2-D2 is being destroyed!
C-3PO is being destroyed!
We have 0 robots.

Inheritance
class SchoolMember:
def __init__(self, name, age):
self.name = name
self.age = age
print('(Initialized SchoolMember: {})'.format(self.name))
def tell(self):
print('Name:"{}" Age:"{}"'.format(self.name, self.age), end=" ")

Inheritance
class Teacher(SchoolMember):
def __init__(self, name, age, salary):
SchoolMember.__init__(self, name, age)
self.salary = salary
print('(Initialized Teacher: {})'.format(self.name))
def tell(self):
SchoolMember.tell(self)
print('Salary: "{:d}"'.format(self.salary))

Inheritance
class Student(SchoolMember):
def __init__(self, name, age, marks):
SchoolMember.__init__(self, name, age)
self.marks = marks
print('(Initialized Student: {})'.format(self.name))
def tell(self):
SchoolMember.tell(self)
print('Marks: "{:d}"'.format(self.marks))

Inheritance
t = Teacher('Ahmad', 40, 1234)
s = Student('Khaled', 25, 75)
members = [t, s]
for member in members:
# Works for both Teachers and Students
member.tell()

Inheritance
Output:
(Initialized SchoolMember: Ahmad)
(Initialized Teacher: Ahmad)
(Initialized SchoolMember: Khaled)
(Initialized Student: Khaled)
Name:"Ahmad" Age:"40" Salary: "1234"
Name:"Khaled" Age:"25" Marks: "75"

Input from user
text = input("Enter text: ")
# Enter text: Ahmad

Files
poem = '''
Programming is fun
When the work is done
if you wanna make your work also fun:
use Python!
'''
# Open for 'w'riting
f = open('poem.txt', 'w')
# Write text to file
f.write(poem)
# Close the file
f.close()

Files
# If no mode is specified,
# 'r'ead mode is assumed by default
f = open('poem.txt')
while True:
line = f.readline()
# Zero length indicates EOF
if len(line) == 0:
break
print(line, end='')
# close the file
f.close()

Files
Output
Programming is fun
When the work is done
if you wanna make your work also fun:
use Python!

Errors
• Consider a simple print function call. What if we misspelt print as
Print? Note the capitalization. In this case, Python raises a syntax
error.
Print("Hello World")
# NameError: name 'Print' is not defined
print("Hello World")
# Hello World

Exceptions
• Exceptions occur when exceptional situations occur in your program.
For example, what if you are going to read a file and the file does not
exist? Or what if you accidentally deleted it when the program was
running? Such situations are handled using exceptions.
10 * (1/0)
# ZeroDivisionError: division by zero

Handling Exceptions
try:
10 * (1/0)
except ZeroDivisionError:
print("Can't division by zero")
# Can't division by zero

Handling Exceptions
try:
x = int(input("Please enter a number: "))
except ValueError:
print("Oops! That was no valid number.")
# Please enter a number: df
# Oops! That was no valid number.

Python introduction 2

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Python introduction 2