I'm looking for a way to "page through" a Python iterator. That is, I would like to wrap a given iterator iter and page_size with another iterator that would would return the items from iter as a series of "pages". Each page would itself be an iterator with up to page_size iterations.
I looked through itertools and the closest thing I saw is itertools.islice. In some ways, what I'd like is the opposite of itertools.chain -- instead of chaining a series of iterators together into one iterator, I'd like to break an iterator up into a series of smaller iterators. I was expecting to find a paging function in itertools but couldn't locate one.
I came up with the following pager class and demonstration.
class pager(object):
"""
takes the iterable iter and page_size to create an iterator that "pages through" iter. That is, pager returns a series of page iterators,
each returning up to page_size items from iter.
"""
def __init__(self,iter, page_size):
self.iter = iter
self.page_size = page_size
def __iter__(self):
return self
def next(self):
# if self.iter has not been exhausted, return the next slice
# I'm using a technique from
# https://stackoverflow.com/questions/1264319/need-to-add-an-element-at-the-start-of-an-iterator-in-python
# to check for iterator completion by cloning self.iter into 3 copies:
# 1) self.iter gets advanced to the next page
# 2) peek is used to check on whether self.iter is done
# 3) iter_for_return is to create an independent page of the iterator to be used by caller of pager
self.iter, peek, iter_for_return = itertools.tee(self.iter, 3)
try:
next_v = next(peek)
except StopIteration: # catch the exception and then raise it
raise StopIteration
else:
# consume the page from the iterator so that the next page is up in the next iteration
# is there a better way to do this?
#
for i in itertools.islice(self.iter,self.page_size): pass
return itertools.islice(iter_for_return,self.page_size)
iterator_size = 10
page_size = 3
my_pager = pager(xrange(iterator_size),page_size)
# skip a page, then print out rest, and then show the first page
page1 = my_pager.next()
for page in my_pager:
for i in page:
print i
print "----"
print "skipped first page: " , list(page1)
I'm looking for some feedback and have the following questions:
Thanks! -Raymond
Why aren't you using this?
def grouper( page_size, iterable ):
page= []
for item in iterable:
page.append( item )
if len(page) == page_size:
yield page
page= []
yield page
"Each page would itself be an iterator with up to page_size" items. Each page is a simple list of items, which is iterable. You could use yield iter(page)
to yield the iterator instead of the object, but I don't see how that improves anything.
It throws a standard StopIteration
at the end.
What more would you want?