Lesson 28
Classes and Methods
Python Mini-Course
University of Oklahoma
Department of Psychology
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Python Mini-Course: Lesson 28
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Lesson objectives
1. Create methods inside class
definitions
2. Call methods using function syntax
and method syntax
3. Create custom __init__ and
__str__ methods
4. Use operator overloading
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Encapsulation
 Data and behaviors are packaged
together
 The object only reveals the
interfaces needed to interact with it
 Internal data and behaviors can
remain hidden
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Encapsulating the Time class
 Instead of using functions, we
want to use methods
 Move the functions inside the class
definition
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The print_time() method
class Time(object):
…
def print_time(self):
"""
Print the time in hour:minute:second format.
"""
print '%02d:%02d:%02d' % \
(self.hour, self.minute, self.second)
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Calling a method
 Using function syntax
t1 = Time(2,35)
Time.print_time(t1)
 Using method syntax
t1.print_time()
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Converting the other functions
to methods
 The valid_time method
 The increment method
 The add_time method
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The valid_time method
class Time(object):
...
def valid_time(self):
validity = True
# All values must be at least zero
if self.hour < 0 or self.minute < 0 \
or self.second < 0:
validity = False
# Minute and second must be base 60
if self.minute >= 60 or self.second >= 60:
validity = False
return validity
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The increment method
class Time(object):
...
def increment(self, t2):
# Check the input arguments
if type(t2) != Time:
raise AttributeError, \
'invalid argument passed to Time.increment()'
if
not t2.valid_time():
raise ValueError, \
'invalid Time object passed to Time.increment()'
# Add the times
self.hour
+= t2.hour
self.minute += t2.minute
self.second += t2.second
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Python Mini-Course: Lesson 28
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The add_time method
class Time(object):
...
def add_time(self, t2):
# Add the times
new_time = Time()
new_time.hour = self.hour + t2.hour
new_time.minute = self.minute + t2.minute
new_time.second = self.second + t2.second
# Return the sum
return new_time
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Using the Time class
t1 = Time(0,0,30)
t2 = Time(1,0,45)
t1.increment(t2)
t1.print_time()
t3 = t1.add_time(t2)
t3.print_time()
t4 = Time.add_time(t1, t2)
t4.print_time()
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Improving the Time class
 Problems:
1. Formatting
1. Minutes and seconds should always be
less than 60
2. Printing is awkward
3. Adding times is awkward
 Solutions: see time_oop1.py
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Keeping the right format
class Time(object):
...
def adjust_base_60(self):
# Increment minutes as necessary and adjust seconds
self.minute += self.second // 60
self.second = self.second % 60
# Increment hours as necessary and adjust minutes
self.hour += self.minute // 60
self.minute = self.minute % 60
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Controlling access to attributes
class Time(object):
...
def set_time(self, hour=0, minute=0, second=0):
self.hour = hour
self.minute = minute
self.second = second
self.adjust_base_60()
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Python Mini-Course: Lesson 28
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The __str__ method
class Time(object):
…
def __str__(self):
"""
Return the time in hour:minute:second format.
"""
return '%02d:%02d:%02d' % \
(self.hour, self.minute, self.second)
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The __str__ method
 The __str__ method is a special
method that is called by the str()
and print commands
t1 = Time(2,45)
str(t1)
print t1
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Why does this work?
 In Python, the most basic class, the
object class, defines the __str__
method
 Time is a sub-class of the object
class, and it inherits this behavior
 By defining our own __str__ method
we override the base class method
 This is polymorphism
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Operator overloading
 We can also define how a class
responds to standard operators
such as +, -, etc.
 This is called operator
overloading
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The __add__ method
class Time(object):
...
def __add__(self, other):
return self.add_time(other)
def add_time(self, t2):
new_time = Time()
new_time.hour = self.hour + t2.hour
new_time.minute = self.minute + t2.minute
new_time.second = self.second + t2.second
new_time.adjust_base_60()
return new_time
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Python Mini-Course: Lesson 28
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Using the __add__ method
t1 = Time(0,0,30)
t2 = Time(1,0,45)
t3 = t1 + t2
print t3
print t1 + t2
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Type-based dispatch
 Allow us to use different types of
arguments for the same method
(or function)
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class Time(object):
...
def increment(self, t2):
# Check the input arguments
if type(t2) == Time:
# Add the times
self.hour
+= t2.hour
self.minute += t2.minute
self.second += t2.second
elif type(t2) == int:
# Increment the seconds
self.second += t2
else:
raise AttributeError, \
'invalid argument passed to Time.increment()'
self.adjust_base_60()
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For more practice, try
 Think Python.
 Exercise 17.3,
 Exercise 17.4,
 Exercise 17.5,
 Exercise 17.6,
exercise)
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Python Mini-Course: Lesson 28
Chap 17
page
page
page
page
165
166
167
169 (debugging
6/17/09
Assignment
 Think Python. Chap 18
 Read text
 Type in and run code as you go
(save as poker.py)
 Do Exercises 18.2 and 18.3 in text
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