pushback iterator

Simon Forman sajmikins at gmail.com
Fri May 22 19:33:41 EDT 2009

On May 17, 10:39 am, Matus <mat... at gmail.com> wrote:
> Hallo pylist,
> I searches web and python documentation for implementation of pushback
> iterator but found none in stdlib.
> problem:
> ========
> when you parse a file, often you have to read a line from parsed file
> before you can decide if you want that line it or not. if not, it would
> be a nice feature to be able po push the line back into the iterator, so
> nest time when you pull from iterator you get this 'unused' line.
> solution:
> =========
> I found a nice and fast solution somewhere on the net:
> ---------------------------------------------------------------------
> class Pushback_wrapper( object ):
>         def __init__( self, it ):
>                 self.it = it
>                 self.pushed_back = [ ]
>                 self.nextfn = it.next
>         def __iter__( self ):
>                 return self
>         def __nonzero__( self ):
>                 if self.pushed_back:
>                         return True
>                 try:
>                         self.pushback( self.nextfn( ) )
>                 except StopIteration:
>                         return False
>                 else:
>                         return True
>         def popfn( self ):
>                 lst = self.pushed_back
>                 res = lst.pop( )
>                 if not lst:
>                         self.nextfn = self.it.next
>                 return res
>         def next( self ):
>                 return self.nextfn( )
>         def pushback( self, item ):
>                 self.pushed_back.append( item )
>                 self.nextfn = self.popfn
> ---------------------------------------------------------------------
> proposal:
> =========
> as this is (as I suppose) common problem, would it be possible to extend
> the stdlib of python (ie itertools module) with a similar solution so
> one do not have to reinvent the wheel every time pushback is needed?
> thx, Matus

By coincidence I had need of just such a pushback iterator just the
other day. I was playing with queues and generators after reading
David Beazley's "Generator Tricks For Systems Programmers" [1] and I
needed an iterator that could be used with this generator function:

def pushQueueNonblocking(queue, iterator):
    for item in iterator:
            queue.put(item, False)
        except Queue.Full:
            raise Queue.Full(item)

If the queue is full I pass item taken from the iterator but not yet
pushed onto the queue up to the caller in the Full exception so it
doesn't get lost. I wrote a wrapper function that retried putting
items on the queue after sleeping for a certain time if the queue is

def pushQueueRetry(queue, iterator, retry_delay=1):
    iterator = IteratorWithPushback(iterator)
    while True:
            pushQueueNonblocking(queue, iterator)
        except Queue.Full, err:
            if retry_delay >= 0:

Originally I used an itertools.chain() solution, but when I tried the
function with sleep of 0 (the queue was being get() from once a second
in another thread) I hit a funny error.

Because the put() thread was running, and retrying, much faster than
the get() thread, a huge "linked list" of itertools.chain generator
objects was stored up in memory.  When the get() thread finally
unblocked the queue and the call to put() succeeded, the
pushQueueNonblocking() would loop and call next() on the chain object,
which would call next() on the next chain object, which called next()
on the next next chain object, etc...

That linked list of chain iterators was longer than

Anyway, FWIW here's the IteratorWithPushback class that I whanged up:

class IteratorWithPushback:

    def __init__(self, initial_iterator):
        self.iter = iter(initial_iterator)
        self.last = None
        self.has_last = False

    def __iter__(self):
        return self

    def next(self):
        if self.has_last:
            item = self.last
            self.has_last = False
            self.last = None # Don't hang on to it.
            item = self.iter.next()
        return item

    def pushback(self, item):
        if self.has_last:
            raise Exception("I'm full!")
        self.last = item
        self.has_last = True

As for putting something like this in the standard library, I'm not
sure. I've never needed one before and it's easy enough to write.


[1] http://www.dabeaz.com/generators/Generators.pdf

More information about the Python-list mailing list