pythongeneratorpeek

How to look ahead one element (peek) in a Python generator?

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?

See also: Resetting generator object in Python

Solution

  • 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 and prepend the element:

    import itertools

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