1. Inheritance Copyright © Software Carpentry 2010 This work is licensed under the Creative Commons Attribution License See http://software-carpentry.org/license.html for more information. Classes and Objects

2. Inheritance Interpolating time series signals time strength

3. Classes and ObjectsInheritance Interpolating time series signals time strength step function linear interpolation

4. Classes and ObjectsInheritance Interpolating time series signals time strength step function linear interpolation A lot of the implementation is the same

5. Classes and ObjectsInheritance Interpolating time series signals time strength step function linear interpolation A lot of the implementation is the same How can we eliminate the reduncancy?

6. Classes and ObjectsInheritance First implementation class StepSignal(object):... def get(self, where): if where < self.values[0][0]: raise IndexError, '%f too low' % where for i in range(len(self.values)-1): x0, y0 = self.values[i] x1, y1 = self.values[i+1] if x0 <= where <= x1: return y0 raise IndexError, '%f too high' % where

7. Classes and ObjectsInheritance Second implementation class LinearSignal(object):... def get(self, where): if where < self.values[0][0]: raise IndexError, '%f too low' % where for i in range(len(self.values)-1): x0, y0 = self.values[i] x1, y1 = self.values[i+1] if x0 <= where <= x1: return y0 + (y1-y0) * (where-x0) / (x1-x0) raise IndexError, '%f too high' % where

8. Classes and ObjectsInheritance Second implementation class LinearSignal(object):... def get(self, where): if where < self.values[0][0]: raise IndexError, '%f too low' % where for i in range(len(self.values)-1): x0, y0 = self.values[i] x1, y1 = self.values[i+1] if x0 <= where <= x1: return y0 + (y1-y0) * (where-x0) / (x1-x0) raise IndexError, '%f too high' % where

9. Classes and ObjectsInheritance Refactor class StepSignal(object): def get(self, where): i = self.find(self, where) return self.values[i][1] class LinearSignal(object): def get(self, where): i = self.find(self, where) x0, y0 = self.values[i] x1, y1 = self.values[i+1] return y0 + (y1-y0) * (where-x0)/(x1-x0)

10. Classes and ObjectsInheritance Refactor class StepSignal(object): def get(self, where): i = self.find(self, where) return self.values[i][1] class LinearSignal(object): def get(self, where): i = self.find(self, where) x0, y0 = self.values[i] x1, y1 = self.values[i+1] return y0 + (y1-y0) * (where-x0)/(x1-x0) Where to put find?

11. Classes and ObjectsInheritance Refactor class StepSignal(object): def get(self, where): i = self.find(self, where) return self.values[i][1] class LinearSignal(object): def get(self, where): i = self.find(self, where) x0, y0 = self.values[i] x1, y1 = self.values[i+1] return y0 + (y1-y0) * (where-x0)/(x1-x0) Where to put find? Don't want to duplicate

12. Classes and ObjectsInheritance Use inheritance

13. Classes and ObjectsInheritance Use inheritance class Parent(object): def hello(self): print 'hello'

14. Classes and ObjectsInheritance Use inheritance class Parent(object): def hello(self): print 'hello' class Child(Parent): def goodbye(self): print 'goodbye'

15. Classes and ObjectsInheritance Use inheritance class Parent(object): def hello(self): print 'hello' class Child(Parent): def goodbye(self): print 'goodbye' Child inherits from Parent

16. Classes and ObjectsInheritance Use inheritance class Parent(object): def hello(self): print 'hello' class Child(Parent): def goodbye(self): print 'goodbye' c = Child() c.goodbye() goodbye Child inherits from Parent

17. Classes and ObjectsInheritance Use inheritance class Parent(object): def hello(self): print 'hello' class Child(Parent): def goodbye(self): print 'goodbye' c = Child() c.goodbye() goodbye c.hello() hello Child inherits from Parent

18. Classes and ObjectsInheritance Use inheritance class Parent(object): def hello(self): print 'hello' class Child(Parent): def goodbye(self): print 'goodbye' c = Child() c.goodbye() goodbye c.hello() hello p = Parent() p.hello() hello Child inherits from Parent

19. Classes and ObjectsInheritance Use inheritance class Parent(object): def hello(self): print 'hello' class Child(Parent): def goodbye(self): print 'goodbye' c = Child() c.goodbye() goodbye c.hello() hello p = Parent() p.hello() hello p.goodbye() AttributeError: 'Parent' object has no attribute 'goodbye' Child inherits from Parent

20. Classes and ObjectsInheritance Contents of memory p c Parent object Child hello goodbye

21. Classes and ObjectsInheritance Contents of memory p c Parent object Child hello goodbye

22. Classes and ObjectsInheritance Contents of memory p c Parent object Child hello goodbye

23. Classes and ObjectsInheritance Contents of memory p c Parent object Child hello goodbye

24. Classes and ObjectsInheritance Contents of memory p c Parent object Child hello goodbye

25. Classes and ObjectsInheritance Contents of memory p c Parent object Child hello goodbye

26. Classes and ObjectsInheritance class InterpolatedSignal(object): def find(self, where): if where < self.values[0][0]: raise IndexError, '%f too low' % where for i in range(len(self.values)-1): x0, y0 = self.values[i] x1, y1 = self.values[i+1] if x0 <= where <= x1: return i raise IndexError, '%f too high' % where

27. Classes and ObjectsInheritance class InterpolatedSignal(object): def find(self, where): if where < self.values[0][0]: raise IndexError, '%f too low' % where for i in range(len(self.values)-1): x0, y0 = self.values[i] x1, y1 = self.values[i+1] if x0 <= where <= x1: return i raise IndexError, '%f too high' % where Not much use on its own

28. Classes and ObjectsInheritance class InterpolatedSignal(object): def find(self, where): if where < self.values[0][0]: raise IndexError, '%f too low' % where for i in range(len(self.values)-1): x0, y0 = self.values[i] x1, y1 = self.values[i+1] if x0 <= where <= x1: return i raise IndexError, '%f too high' % where Not much use on its own Where does this come from?

29. Classes and ObjectsInheritance Derive specific interpolators

30. Classes and ObjectsInheritance Derive specific interpolators class StepSignal(InterpolatedSignal): def __init__(self, values): self.values = values[:] def get(self, where): i = self.find(where) return self.values[i][1]

31. Classes and ObjectsInheritance Derive specific interpolators class StepSignal(InterpolatedSignal): def __init__(self, values): self.values = values[:] def get(self, where): i = self.find(where) return self.values[i][1]

32. Classes and ObjectsInheritance Derive specific interpolators class StepSignal(InterpolatedSignal): def __init__(self, values): self.values = values[:] def get(self, where): i = self.find(where) return self.values[i][1]

33. Classes and ObjectsInheritance Derive specific interpolators class StepSignal(InterpolatedSignal): def __init__(self, values): self.values = values[:] def get(self, where): i = self.find(where) return self.values[i][1]

34. Classes and ObjectsInheritance Derive specific interpolators class StepSignal(InterpolatedSignal): def __init__(self, values): self.values = values[:] def get(self, where): i = self.find(where) return self.values[i][1] Fragile

35. Classes and ObjectsInheritance Derive specific interpolators class StepSignal(InterpolatedSignal): def __init__(self, values): self.values = values[:] def get(self, where): i = self.find(where) return self.values[i][1] Dependencies between classes should be explicit

36. Classes and ObjectsInheritance Have the parent class store the values

37. Classes and ObjectsInheritance Have the parent class store the values class InterpolatedSignal(object): def __init__(self, values): self.values = values[:] def get(self, where): raise NotImplementedError('Must provide get!') def find(self, where):...as before...

38. Classes and ObjectsInheritance Have the parent class store the values class InterpolatedSignal(object): def __init__(self, values): self.values = values[:] def get(self, where): raise NotImplementedError('Must provide get!') def find(self, where):...as before...

39. Classes and ObjectsInheritance The child's constructor relies on the parent's class StepSignal(InterpolatedSignal): def __init__(self, values): InterpolatedSignal.__init__(self, values) def get(self, where): i = self.find(where) return self.values[i][1]

40. Classes and ObjectsInheritance The child's constructor relies on the parent's class StepSignal(InterpolatedSignal): def __init__(self, values): InterpolatedSignal.__init__(self, values) def get(self, where): i = self.find(where) return self.values[i][1]

41. Classes and ObjectsInheritance Other classes are just as easy Class LinearSignal(InterpolatedSignal): def __init__(self, values): InterpolatedSignal.__init__(self, values) def get(self, where): i = self.find(where) return y0 + (y1-y0) * (where-x0)/(x1-x0)

42. Classes and ObjectsInheritance class InterpolatedSignal(object): def __init__(self, values): assert len(values) > 0, 'Must have some pairs' for i in range(len(values)): assert len(values[i]) == 2, 'Entries must be pairs' for i in range(len(values)-1)): x0 = values[i][0] x1 = values[i][1] assert x0 < x1, 'Samples must increase on x' >>> signal = StepSignal([ [1., 0.], [0., 2] ]) AssertionError: Samples must increase on x

43. Classes and ObjectsInheritance Child overrides parent method signal InterpolatedSignal object StepSignal __init__ find get

44. Classes and ObjectsInheritance Overriding in action

45. Classes and ObjectsInheritance Overriding in action class Parent(object): def hello(self): print 'hello' def goodbye(self): print 'goodbye'

46. Classes and ObjectsInheritance Overriding in action class Parent(object): def hello(self): print 'hello' def goodbye(self): print 'goodbye' class Child(Parent): def goodbye(self): print 'au revoir'

47. Classes and ObjectsInheritance Overriding in action class Parent(object): def hello(self): print 'hello' def goodbye(self): print 'goodbye' class Child(Parent): def goodbye(self): print 'au revoir' Child overrides Parent

48. Classes and ObjectsInheritance Overriding in action class Parent(object): def hello(self): print 'hello' def goodbye(self): print 'goodbye' class Child(Parent): def goodbye(self): print 'au revoir' p = Parent() p.hello() hello p.goodbye() goodbye

49. Classes and ObjectsInheritance Overriding in action class Parent(object): def hello(self): print 'hello' def goodbye(self): print 'goodbye' class Child(Parent): def goodbye(self): print 'au revoir' p = Parent() p.hello() hello p.goodbye() goodbye C = child() c.hello() hello

50. Classes and ObjectsInheritance Overriding in action class Parent(object): def hello(self): print 'hello' def goodbye(self): print 'goodbye' class Child(Parent): def goodbye(self): print 'au revoir' p = Parent() p.hello() hello p.goodbye() goodbye C = child() c.hello() hello c.goodbye() au revoir

51. January 2011 Copyright © Software Carpentry 2010 This work is licensed under the Creative Commons Attribution License See http://software-carpentry.org/license.html for more information. created by Greg Wilson