Question

I am going through a generator, whats the Pythonic way of determining if the current element is the first or last element of a generator, given that they need special care?

thanks

basically generating tags, so i have items like

<div class="first">1</div>
<div>...</div>
<div class="last">n</div>

so i would like to keep the last item in loop?

Answer 1

Here’s an enumerate-like generator that skips ahead one; it returns -1 for the last element.

>>> def annotate(gen):
...     prev_i, prev_val = 0, gen.next()
...     for i, val in enumerate(gen, start=1):
...         yield prev_i, prev_val
...         prev_i, prev_val = i, val
...     yield '-1', prev_val
>>> for i, val in annotate(iter(range(4))):
...     print i, val
... 
0 0
1 1
2 2
-1 3

It can’t tell whether the generator passed to it is “fresh” or not, but it still tells you when the end is nigh:

>>> used_iter = iter(range(5))
>>> used_iter.next()
0
>>> for i, val in annotate(used_iter):
...     print i, val
... 
0 1
1 2
2 3
-1 4

Once an iterator is used up, it raises StopIteration as usual.

>>> annotate(used_iter).next()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in annotate
StopIteration