Python Mini-Course University of Oklahoma Department of Psychology Lesson 28 Classes and Methods 6/17/09 Python Mini-Course: Lesson 28 1

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 6/17/09 Python Mini-Course: Lesson 28 2

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 6/17/09 Python Mini-Course: Lesson 28 3

Encapsulating the Time class Instead of using functions, we want to use methods Move the functions inside the class definition 6/17/09 Python Mini-Course: Lesson 28 4

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) 6/17/09 Python Mini-Course: Lesson 28 5

Calling a method Using function syntax t1 = Time(2,35) Time.print_time(t1) Using method syntax t1.print_time() 6/17/09 Python Mini-Course: Lesson 28 6

Converting the other functions to methods The valid_time method The increment method The add_time method 6/17/09 Python Mini-Course: Lesson 28 7

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 6/17/09 Python Mini-Course: Lesson 28 8

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 6/17/09 Python Mini-Course: Lesson 28 9

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 6/17/09 Python Mini-Course: Lesson 28 10

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() 6/17/09 Python Mini-Course: Lesson 28 11

Improving the Time class Problems: 1. Formatting 1. Minutes and seconds should always be less than Printing is awkward 3. Adding times is awkward Solutions: see time_oop1.py 6/17/09 Python Mini-Course: Lesson 28 12

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 6/17/09 Python Mini-Course: Lesson 28 13

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() 6/17/09 Python Mini-Course: Lesson 28 14

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) 6/17/09 Python Mini-Course: Lesson 28 15

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 6/17/09 Python Mini-Course: Lesson 28 16

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 6/17/09 Python Mini-Course: Lesson 28 17

Operator overloading We can also define how a class responds to standard operators such as +, -, etc. This is called operator overloading 6/17/09 Python Mini-Course: Lesson 28 18

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 6/17/09 Python Mini-Course: Lesson 28 19

Using the __add__ method t1 = Time(0,0,30) t2 = Time(1,0,45) t3 = t1 + t2 print t3 print t1 + t2 6/17/09 Python Mini-Course: Lesson 28 20

Type-based dispatch Allow us to use different types of arguments for the same method (or function) 6/17/09 Python Mini-Course: Lesson 28 21

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() 6/17/09 Python Mini-Course: Lesson 28 22

For more practice, try Think Python. Chap 17 Exercise 17.3, page 165 Exercise 17.4, page 166 Exercise 17.5, page 167 Exercise 17.6, page 169 (debugging exercise) 6/17/09 Python Mini-Course: Lesson 28 23

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 6/17/09 Python Mini-Course: Lesson 28 24