week_8.txt
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Week 8, Nov 29

 Lecture 1
  Review classes and objects
  Atom exercise - due last week
  Bag exercise - due this week

 Lab 1
  In-class exercise on classes and inheritance
  Speakers try out projector, etc.
  Return Atom exercise

 Lightning talks

 Lecture 2
  Decorators
  Debugger (if there is time)
  Explain next exercise

 Lab 2
  Get started on next exercise



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Atom exercise - due last week

 Requires more invention than previous exercises

 Good strategy: first write skeleton (with placeholders) from problem statement
  placeholders can be empty class/method bodies (just a pass statement)
  placeholders can be comments 

 Once again, read entire problem statement, implement everything
  placeholders at least! they show you understood the problem

 Every requirement in my problem statement affects the solution
   (real-life problem statements are less clear, usually include some redundancy)
  unique identification for atoms - all atoms must be distinct, not aliases
  printing ids checks for aliases
  some atoms were present but not bonded - checks for unintended side effects
  Could do more - problem statement didn't ask to test attempting too many bonds

 Understand the whole problem
  Tension: get partial solution working quickly 
   BUT anticipate path to full solution







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Atom exercise

 Design variations (just a few, many others are possible too)

  Separate bond method vs. optional list of atoms to bond in __init__
  Bond method bonds both ends (call once)
  Bond method bonds just one end (must call both atom's bond methods)
  Bond method bonds one end then calls other end's bond method (just call first)
  Bond method takes multiple atoms to bind at once, using *args
  Can use __add__ for bond method, so you can write o0 + h0 to make bond - neat!

  Error checking -- exceed maxbonds -- how to indicate?  
   Printing not recommended, better to use return value 

  ... and more ...









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Atom exercise - Inheritance

 Inheritance really works - no kidding!  
  No need to duplicate __init__ or anything else - that's the whole point!
  If method def is missing from subclass, automatically use superclass method
  If method def is present in subclass, it *overrides* (replaces) superclass method
  To use both - use base class method *also* - call base class explicitly as in:
  
   class H(atom):
      def __init__(...):
        Atom.__init_(...) 
        ... more code specific to subclass ...


  Multiple inheritance, here C inherits from both A and B

    class C(A, B):
      ....

  Method Resolution Order 
    What if superclasses A and B both define a method m ?
    Resolve in left-to-right order, use A.m not B.m
     so C(A,B) behaves differently from C(B,A)






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Bag exercise

 The dictionary *encodes* the bag: not [a,a,b,b,b] but { a:2, b:3 }
  Encodings like this are pervasive in computing

 bag inherits dict so it still has all the familiar dictionary operations
  we just added + , the bag union operator.  dict has no + operator.