Lesson 26
Classes and Objects
Python Mini-Course
University of Oklahoma
Department of Psychology
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Python Mini-Course: Lesson 26
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Lesson objectives
1. Write a class definition
2. Instantiate objects
3. Assign values to attributes
4. Change attribute values
5. Copy objects
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Class definition
 Use the class keyword
 Example:
class Point(object):
"""
represents a point in 2-D space
"""
print Point
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Python Mini-Course: Lesson 26
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Instantiating an object
 Call the class constructor by
using the class name as if it were
a function
 Example:
p1 = Point()
print p1
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Python Mini-Course: Lesson 26
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Assigning attribute values
 Use the object name and the
attribute name with dot notation
 Example:
p1.x = 3.0
p1.y = 4.0
print p1.x, p1.y
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Assigning attribute values
 These are called instance
attributes
p2 = Point()
p2.x, p2.y = 12.0, 13.0
print p1.x, p1.y
print p2.x, p2.y
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Note on attributes
 Python handles attributes
somewhat differently from other
OOP languages
 All attributes are "public"
 Attributes can be created on the fly
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Using attributes
 Object attributes are just like any
other variable
print '(%g, %g)' % (p1.x, p1.y)
from math import sqrt
distance = sqrt(p1.x**2 + p2.y**2)
print distance
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Note:
 Objects are data types
 You can pass objects to functions
and return them from functions, just
like any other data type
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Example:
def print_point(p):
print '(%g, %g)' % (p.x, p.y)
print_point(p1)
NB: this example violates the principle
of encapsulation
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Encapsulation
 Examples
 rectangle1.py
 rectangle2.py
 rectangle3.py
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Aliasing
Try this:
box = Rectangle(10.0, 200.0)
box1 = box
box1.width = 100.0
print box.width, box1.width
box1 is box
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Copying
 Use the copy module
import copy
box2 = copy.copy(box)
box2.width = 50.0
print box.width, box2.width
box2 is box
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Limitations of copying
box2.corner.x = 2.0
print box.corner.x
box2.corner is box.corner
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Deep copying
 Use the copy module
box3 = copy.deepcopy(box)
box3.corner.x = 1.0
print box.corner.x, box3.corner.x
box3.corner is box.corner
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