Lesson 28 Classes and Methods

1. Lesson 28Classes and Methods Python Mini-Course University of Oklahoma Department of Psychology Python Mini-Course: Lesson 28

2. Lesson objectives • Create methods inside class definitions • Call methods using function syntax and method syntax • Create custom __init__ and __str__ methods • Use operator overloading Python Mini-Course: Lesson 28

3. 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 Python Mini-Course: Lesson 28

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

5. 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) Python Mini-Course: Lesson 28

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

7. Converting the other functions to methods • The valid_time method • The increment method • The add_time method Python Mini-Course: Lesson 28

8. 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 Python Mini-Course: Lesson 28

9. 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 Python Mini-Course: Lesson 28

10. 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 Python Mini-Course: Lesson 28

11. 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() Python Mini-Course: Lesson 28

12. Improving the Time class • Problems: • Formatting • Minutes and seconds should always be less than 60 • Printing is awkward • Adding times is awkward • Solutions: see time_oop1.py Python Mini-Course: Lesson 28

13. 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 Python Mini-Course: Lesson 28

14. 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() Python Mini-Course: Lesson 28

15. 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) Python Mini-Course: Lesson 28

16. 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 Python Mini-Course: Lesson 28

17. 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 Python Mini-Course: Lesson 28

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

19. 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 Python Mini-Course: Lesson 28

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

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

22. 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() Python Mini-Course: Lesson 28

23. 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) Python Mini-Course: Lesson 28

24. 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 Python Mini-Course: Lesson 28