Generators are very useful and can help simplify your code and improve its performance.

Examples

For example, the RangeGenerator can be used to iterate over a large number of values, without creating a massive list (like range would)

   1 #the for loop will generate each i (i.e. 1,2,3,4,5, ...), add it to sum,  and throw it away
   2 #before the next i is generated.  This is opposed to iterating through range(...), which creates
   3 #a potentially massive list and then iterates through it.
   4 for i in irange(1000000):
   5    sum = sum+i

Generators can be composed. Here we create a generator on the squares of consecutive integers.

   1 #square is a generator 
   2 square = (i*i for i in irange(1000000))
   3 #add the squares
   4 for i in square:
   5    sum = sum +i

Here, we compose a square generator with the takewhile generator, to generate squares less than 100

   1 #add squares less than 100
   2 square = (i*i for i in count())
   3 bounded_squares = takewhile(lambda x : x< 100, square)
   4 for i in bounded_squares:
   5    sum += i

to be written: Generators made from classes?

Links

• [http://www.python.org/peps/pep-0255.html PEP-255: Simple Iterators] -- the original

• [http://www-106.ibm.com/developerworks/library/l-pycon.html?n-l-9271 Iterators and Simple Generators]

• [http://www-106.ibm.com/developerworks/linux/library/l-cpyiter.html combinatorial functions in itertools]

• [http://linuxgazette.net/100/pramode.html Python Generator Tricks] -- various infinite sequences, recursions, ...

• [http://www-106.ibm.com/developerworks/linux/library/l-pythrd.html "weightless threads"] -- simulating threads using generators

• [http://www-106.ibm.com/developerworks/library/x-tipgenr.html XML processing] -- yes, using generators

• [http://c2.com/cgi/wiki?GeneratorsAreNotCoroutines C2:GeneratorsAreNotCoroutines] -- particulars on generators, coroutines, and continuations

Discussion

I once saw MikeOrr demonstrate Before and After examples. But, I forget how they worked.

Can someone demonstrate here?

He did something like: Show how a normal list operation could be written to use generators. Something like:

   1 def double(L):
   2     return [x*2 for x in L]
   3 
   4 eggs = double([1, 2, 3, 4, 5])

...he showed how that, or something like that, could be rewritten using iterators, generators.

It's been a while since I've seen it, I may be getting this all wrong.

-- LionKimbro DateTime(2005-04-02T19:12:19Z)