How to look ahead one element in a Python generator?
Question: I can't figure out how to look ahead one element in a Python generator. As
soon as I look it's gone.

Here is what I mean:

    
    
    gen = iter([1,2,3])
    next_value = gen.next()  # okay, I looked forward and see that next_value = 1
    # but now:
    list(gen)  # is [2, 3]  -- the first value is gone!
    

Here is a more real example:

    
    
    gen = element_generator()
    if gen.next_value() == 'STOP':
      quit_application()
    else:
      process(gen.next())
    

Can anyone help me write a generator that you can look one element forward?

Thanks, Boda Cydo.


Answer: The Python generator API is one way: You can't push back elements you've read.
But you can create a new iterator using the [itertools
module](http://docs.python.org/library/itertools.html) and prepend the
element:

    
    
    import itertools
    
    gen = iter([1,2,3])
    peek = gen.next()
    print list(itertools.chain([peek], gen))
    

