1. Recitation 6 Programming for Engineers in Python

2. Agenda Object Oriented Programming: Old McDonald Had a Farm Time Class 2

3. Example – Old McDonald 3 __str__ - Called by the str() built-in function and by the print statement to compute the “informal” string representation of an object: https://gist.github.com/1397677

4. Animal class 4 class Animal: def __init__(self, name, noise): self.name = name self.noise = noise

5. Farm class 5 class Farm: def __init__(self): self.animals = [] def add(self, animal): self.animals.append(animal)

6. Farm class 6 def __str__(self): s = “” for animal in self.animals: s += 'Old MacDonald had a farm, EE-I-EE-I-O,\n' s += 'And on that farm he had a '+animal.name+', EE-I-EE-I-O,\n' s += 'With a '+ animal.noise + animal.noise+' here and a ‘ s += animal.noise + animal.noise +' there\n' s += 'Here a '+ animal.noise +', there a ‘ s += animal.noise + ', everywhere a ‘ + animal.noise s += animal.noise + '\n' s += 'Old MacDonald had a farm, EE-I-EE-I-O.\n‘ return s

7. Sing the song! 7 >>> farm = Farm() >>> farm.add(Animal('cow','moo')) >>> print farm Old MacDonald had a farm, EE-I-EE-I-O, And on that farm he had a cow, EE-I-EE-I-O, With a moomoo here and a moomoo there Here a moo, there a moo, everywhere a moomoo Old MacDonald had a farm, EE-I-EE-I-O. >>> farm.add(Animal('duck','kwak')) >>> farm.add(Animal('dog','woof')) >>> farm.add(Animal('cat','miaoo'))

8. Sing the song! 8 >>> print farm Old MacDonald had a farm, EE-I-EE-I-O, And on that farm he had a cow, EE-I-EE-I-O, With a moomoo here and a moomoo there Here a moo, there a moo, everywhere a moomoo Old MacDonald had a farm, EE-I-EE-I-O, And on that farm he had a duck, EE-I-EE-I-O, With a kwakkwak here and a kwakkwak there Here a kwak, there a kwak, everywhere a kwakkwak Old MacDonald had a farm, EE-I-EE-I-O, And on that farm he had a dog, EE-I-EE-I-O, With a woofwoof here and a woofwoof there Here a woof, there a woof, everywhere a woofwoof Old MacDonald had a farm, EE-I-EE-I-O, And on that farm he had a cat, EE-I-EE-I-O, With a miaoomiaoo here and a miaoomiaoo there Here a miaoo, there a miaoo, everywhere a miaoomiaoo Old MacDonald had a farm, EE-I-EE-I-O.

9. A Time class 9 We’ll define a class called Time that records the time of day (The “Think Python” Book, ch. 16 & 17): class Time(object): """represents the time of day. attributes: hour, minute, second""“ def __init__(self, hour, minute, second): self.hour = hour self.minute = minute self.second = second >>> time = Time(11, 9, 30)

10. Time class – cont. 10 >>> time = Time(11, 59, 30) How would we print a Time instance? def __str__(self): return "%.2d:%.2d:%.2d" % (self.hour, self.minute, self.second) >>> print time 11:59:30 >>> time.hour = 0 >>> print time 00:59:30 String formatting: http://docs.python.org/release/2.5.2/lib/types seq-strings.html http://docs.python.org/release/2.5.2/lib/types seq-strings.html

11. Time class – cont. 11 We want to add times – for example, it’s 16:00:00 and I have a meeting in 2 hours: 16:00:00 + 02:00:00 = 18:00:00 def __add__(self, other): hour = self.hour+other.hour minute = self.minute+other.minute second = self.second+other.second return Time(hour, minute, second) Is this right? What will happen when adding 16:00:00 to 16:00:00?

12. Adding times – cont. 12 0 ≤hour<24, 0≤minute<60, 0≤second<60 100 minutes = 1 hour 40 minutes def __add__(self, other): hour = self.hour + other.hour minute = self.minute + other.minute second = self.second + other.second minute += second / 60 second = second % 60 hour += minute / 60 minute = minute % 60 hour = hour % 24 return Time(hour, minute, second)

13. Adding times – cont. 13 >>> t1 = Time(16,0,0) >>> t2 = Time(2,0,0) >>> print t1+t2 18:00:00 >>> print t1+t1 08:00:00

14. Subtracting times 14 Just like addition: def __sub__(self, other): hour = self.hour - other.hour minute = self.minute - other.minute second = self.second - other.second minute += second / 60 second = second % 60 hour += minute/60 minute = minute % 60 hour = hour % 24 return Time(hour, minute, second)

15. Even better 15 def __init__(self, hour, minute, second): minute = minute + second / 60 hour = hour + minute / 60 self.hour = hour % 24 self.minute = minute % 60 self.second = second % 60 def __add__(self, other): hour = this.hour + other.hour minute = this.minute + other.minute second = this.second + other.second return Time(hour, minute, second)

16. Debugging 16 We want to write a method that verifies that a Time instance is valid: def valid(self): if self.hour < 0 or self.minute < 0 or self.second < 0: return False if self.hour>=24 or self.minute >= 60 or self.second >= 60: return False return True

17. Exceptions 17 When the program reaches a state it can’t handle or an error it cannot recover from, it raises an exception. This means it sends an error to the user There are different types of errors. Examples: IndexError ZeroDivisionError ValueError TypeError … Full list: http://docs.python.org/library/exceptions.htmlhttp://docs.python.org/library/exceptions.html

18. Debugging – cont. 18 We want to verify that a time is valid before using it and raise an exception if a time is invalid: def __add__(self,other): if not self.valid() or not other.valid(): raise ValueError(‘invalid Time object in addition’) …. This will help us find errors and alert users of the code on wrong usage Optional: This can also be written using assert: def __add__(self,other): assert self.valid() and other.valid() ….

19. Comparing Time instances 19 We’d like to be able to compare times. Right now: >>> t1 = Time(16,0,0) >>> t2 = Time(5,0,0) >>> t1>t2 False >>> t1 = Time(16,0,0) >>> t1>t2 True This happens because python compares the addresses!

20. Comparing Time instances – cont. 20 We’d like to be able to compare times: def __cmp__(self, other): tup1 = (self.hour, self.minute, self.second) tup2 = (other.hour, other.minute, other.second) return cmp(tup1, tup2) >>> Time(16,0,0)>Time(11,30,0) True >>> Time(16,0,0)>Time(16,0,1) False

21. Object representation 21 >>> t1 = Time(16,0,0) >>> print t1 # calls __str__ 16:00:00 >>> t1 If we want a different output we must override the __repr__ method

22. Object representation – cont. 22 def __repr__(self): return self.__class__.__name__+" instance: "+self.__str__() >>> t = Time(16,0,0) >>> t Time instance: 16:00:00 Note the use of these attributes: >>> cls = t.__class__ >>> cls >>> cls.__name__ Time

23. Time as number of seconds 23 We note that the time of day can also be denoted by the number of seconds since the beginning of the day: def to_seconds(self): return self.hour*3600 + self.minute*60 + self.second >>> time = Time(16,0,0) >>> time.to_seconds() 5760 We already have the other direction from __init__ (see here)see here

24. Time as number of seconds – cont. 24 This also simplifies the _add__ and __sub__ methods: def __add__(self, other): secs = self.to_seconds() + other.to_seconds() return Time(0,0, secs) def __sub__(self,other): secs = self.to_seconds() - other.to_seconds() return Time(0,0, secs)

25. Comparing is also easier 25 We can also change __cmp__: def __cmp__(self, other): return cmp(self.to_seconds(), other.to_seconds()) >>> t1 = Time(16,0,0) >>> t2 = Time(21,0,0) >>> t1>t2 False

26. Type checking, adding integers 26 The next step is to allow __add__ to add either seconds or Time def __add__(self, other): secs = self.to_seconds() if isinstance(other, Time):# is other a Time? secs += other.to_seconds() elif isinstance(other, int):# is other an int? secs += other return Time(0,0,secs) # creating a new time

27. Type checking, adding integers 27 Now we can use the + operator with either int or Time: >>> time = Time(16,0,0) >>> print time+60 16:01:00 >>> print time+Time(0,1,0) 16:01:00

28. Type checking, adding integers 28 What if we add a float? Let’s change the if-else: def __add__(self, other): secs = self.to_seconds() if isinstance(other,Time): secs += other.to_seconds() elif isinstance(other,int): secs += other else: raise TypeError("Can't add Time and “ +other.__class__.__name__) return Time(0,0,secs)

29. Type checking, adding integers 29 >>> time + 5.0 Traceback (most recent call last): File " ", line 1, in t1+5. File "C:/python_course/Time.py", line 38, in __add__ raise TypeError("Can't add Time and "+other.__class__.__name__) TypeError: Can't add Time and float

30. Right-side addition 30 >>> print time + 60 16:01:00 >>> print 60 + time Traceback (most recent call last): File " ", line 1, in 60 + time TypeError: unsupported operand type(s) for +: 'int' and 'Time‘ We need to implement __radd__!

31. Right-side addition – cont. 31 def __radd__(self,other): return self.__add__(other) >>> print 60 + time 16:01:00 Alternatively we can write: def __radd__(self,other): return self+other

32. The benefits of overriding + 32 We can use the built-in sum: >>> sum([Time(1,0,1),Time(2,0,2), Time(3,0,3)]) Time instance: 06:00:06

33. Home work 6: Point class 33 We will write a class for a Point - 2-D vector Operator overloading: http://docs.python.org/library/operator.html All instructions in hw6.pdf Use what we learned: class Rational class Time