Computer Lab 10, objects and classes I

A circle is a 2D geometry shape characterized by the position of its centre and a radius. How can it be represented in Python?

1. Store the centre (x and y coordinates) and radius (r) as separate variables.
2. Store x, y and r as elements in a list or a tuple.
3. Create a new type to represent circles as objects.

The last option is the prefered one. It might be more difficult to implement a class/object but offers much more additional possibilities.

• A class starts with a class keyword:

class Circle:
    """represents a circle in 2D space"""

• A construction usually initializes object's data. Objects/class data are called attributes.

def __init__(self, x, y, r):
        self.x = x
        self.y = y
        self.r = r

• Now, let's create 2 circles objects:

if __name__ == "__main__":
    c1 = Circle(1, 2, 2)
    c2 = Circle(4, 6, 4)
    print(c1.x, c1.y, c1.r)
    print(c2.x, c2.y, c2.r)

• Object methods operate on the object. Object/class functions are called methods.

class Circle:
 
(...)
 
    def get_area(self):
        area = 3.14 * (self.r ** 2)
        return area
 
if __name__ == "__main__":
    c1 = Circle(1, 2, 2)
 
    print(c1.get_area())

• Create a function (in global scope) which takes 2 circles as arguments and returns True/False if these two circles intersect.

def is_intersect(c1, c2):
    """Tells if the two circles intersect or not.
 
    :param c1: Circle object
    :param c2: Circle object
    :return: boolean: True if the c1, c2 intersect, otherwise False
    """
 
if __name__ == "__main__":
    c1 = Circle(1, 2, 2)
    c2 = Circle(4, 6, 4)
 
    print(is_intersect(c1,c2))

• Usually, it is better to include all the functions operating on the objects directly in the corresponding class (thus making them methods):

class Circle:
 
(...)
 
    def insersects_with(self, other):
        """Tells if this and other circles intersect or not.
 
        :param other: Circle object
        :return: boolean: True if self and other intersect, otherwise False
        """
 
if __name__ == "__main__":
    c1 = Circle(1, 2, 2)
    c2 = Circle(4, 6, 4)
 
    print(c1.insersects_with(c2))      

• Next, create a method accepting 2 parameters: dx and dy that define a shift of the circle's centre. This method should return a copy of itself with a shifted centre.

class Circle:
 
(...)
 
    def get_shifted_copy(self, dx, dy):
 
 
if __name__ == "__main__":
    c1 = Circle(1, 2, 2)
    print(c1.x, c1.y, c1.r)
 
    c1_shifted = c1.get_shifted_copy(5, -4)
    print(c1.x, c1.y, c1.r)
    print(c1_shifted.x, c1_shifted.y, c1_shifted.r)

1 2 2
1 2 2
6 -2 2

• Let's try to use our object in a print() function:

if __name__ == "__main__":
    c1 = Circle(1, 2, 2)
    print(c1)

<__main__.Circle object at 0x0000029E0C649E50>

• Well, we don't really care in which address in the memory the c1 object is located. Let's implement the __str__(self) function to overwrite this behaviour:

class Circle:
 
(...)
 
    def __str__(self):
        return "Circle: x={}, y={}, r={}".format(self.x, self.y, self.r)
 
if __name__ == "__main__":
    c1 = Circle(1, 2, 2)
    c2 = Circle(-5, 0, 6)
    print(c1)
    print(c2)

Circle: x=1, y=2, r=2
Circle: x=-5, y=0, r=6

class Book:
    """represents a book"""
 
    def __init__(self, title, author, year):
        """
        :param title: string, book title
        :param author: string, book author
        :param year: int, year of publication
        """
 
    def __str__(self):
        """
        >>> book1 = Book("I, Robot", "Isaac Asimov", 1950)
        >>> print(book1)
        Book: I, Robot by Isaac Asimov (1950)
        >>> book2 = Book("Pride and Prejudice", "Jane Austen", 1813)
        >>> print(book2)
        Book: Pride and Prejudice by Jane Austen (1813)
        """
 
    def has_same_author(self, other):
        """Compares authors of this and the other book.
 
        :param other: Book object
        :return: boolean, True if self and other have the same author, otherwise False
        >>> book1 = Book("I, Robot", "Isaac Asimov", 1950)
        >>> book2 = Book("Pride and Prejudice", "Jane Austen", 1813)
        >>> book3 = Book("Sense and Sensibility", "Jane Austen", 1811)
        >>> book1.has_same_author(book3)
        False
        >>> book2.has_same_author(book3)
        True
        """
 
if __name__ == "__main__":
    import doctest
    doctest.testmod(verbose=True)