week_9.txt
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Week 9 - Dec 6 

 Annoucements
  Seattle Python Interest Group (SeaPIG) Next Thurs 12/15 Lighting Talks

 Lecture 1
  Review bag exercise - lots to discuss!
  Review decorator/generator exercise

 Lab 1
  Lab exercise: is-a, has-a relations in object-oriented programming
  Speakers check equipment
  Return bag exercise

 Lightning talks - Maryam Afkarian, Guillermo Rivera, Dmitri Verkolashin

 Lecture 2
  Python ecosystem: Packages, Installation, Distribution, Other languages

 Lab 2
  Start on assignment: find and install a package
















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Bag exercise - turned in last week

1. Inheritance really works, no kidding!  Let it do the heavy lifting for you.

    You don't need an __init__ method - but many of you did something like this:
    
       class bag(dict):
          def __init__(self, mydict):   # mydict is argument to constructor
              self.mydict = mydict      # now self has a dict
   
    This is does not use inheritance!  self already is a dictionary, due to 
    inheritance - you already have all the dictionary machinery for self 
    available in the body of the bag class, you don't need to add self.mydict

     You don't need to do this:           You can simply do this:

       def __add__(self, other)              def __add__(self, other)
          union = copy(self.mydict)             union = copy(self) # self is a dict
          ... union[k] = ...                    ... union[k] = ...
    
    Philosophy / ontology: inheritance creates "is a" relation with superclass,
      assigning an attribute creates "has a" relation.

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Bag exercise

 2.  b1 + b2  should return a new bag, should not alter b1

    This will alter b1 (self):

     def __add_(self, other):
        ...
          self[k] = ...
        ...
        return self
          
    So will this:
        ...
         union = self         # union is an alias for self
         ... union[k] = ...   # changes to union also change self
        return union

    You have to do this:
         union = copy(self)
    or this:
         union = dict(self)
 
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Bag exercise

  3.  Logic - Be sure to iterate over ALL elements

     If you copy self to union first, then you only have to iterate over other 

     def __add__(self, other):
       union = copy(self)
       for k in other:
       	  if k in union:
 	      union[k] += other[k]
           else
              union[k] = other[k]
   
      BUT if you start with empty union, you have to iterate over both:
       ...
        union = bag()
        for k in self:
           ...
        for k in other: 
           ...



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Bag exercise

 Updating a dictionary - must handle case where key is not already in dict

  if k in union:
    union[k] += other[k]
  else:
    union[k] = other[k]

 Or use try...except... 

  try:
    union[k] += other[k]
  except:
    union[k] = other[k]

 Or use get... method, second argument is value to return if key not in dict:

  for k in set(self) + set(other):                 # set union on keys
     union[k] = self.get(k,0) + other.get(k,0)     # dig the symmetry!