generator.txt
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Generators

  Computation that you can pause, get results, and resume later
  Like a function that remembers where it was when it last returned
  Can perform "lazy" computations where items are generated only when needed
  Can provide "infinite" data structures 

 Motivation
  Demo range vs. xrange, show delay (or worse!) with range but not xrange
   (sizes chosen to demo memory/speed limitations, different on your computer)
  range is eager - computes all the elements in the range first
  xrange is lazy - computes each element as needed (to execute loop body once).

 Coding a generator
  Like a function, but with "yield" instead of "return"
  Resumes after yield when you next call it
  Remembers values of local variables between calls 
   calling the generator again resumes the calculation where it left off
  Raise StopIteration to indicate generator is done - no more elements 
  May NEVER raise StopIteration - can simulate infinite data structures!





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 Generator expression

  Shorthand for some generators
  Notice how sum works with generator as well as list
  'sum' (and many other functions), like 'for', work with any iterable 
  Iterables include sequences (str, list, tuple, dict) and now, generators