Trying to view a video one frame at a time. It works now that I make sure a reference to the buffer string always exists, via containing it in the deque.

vidview.py

#!/usr/bin/env python

import sys, os
import pygst
pygst.require("0.10")
import gst
import collections

import glib
import inspect

glib.threads_init()

avifile = gst.parse_launch("filesrc location=cap.avi ! avidemux ! appsink name=sink max-buffers=1")
vidout = gst.parse_launch("appsrc emit-signals=true name=src caps=video/x-raw-yuv,format=(fourcc)I420,framerate=(fraction)30/1,width=(int)1024,height=(int)768 ! xvimagesink")
avifile.set_state(gst.STATE_PLAYING)

framesink = avifile.get_by_name("sink")
vidsrc = vidout.get_by_name("src")

buf = framesink.emit("pull-buffer").make_metadata_writable()
avifile.set_state(gst.STATE_PAUSED)
ncmd = 'n'

pics = collections.deque([buf.data], 1)

def need_data(appsrc, tag):
    appsrc.emit("push-buffer", gst.Buffer(pics[0]))


vidsrc.connect("need-data", need_data)
vidout.set_state(gst.STATE_PLAYING)
while True:
    #buf.timestamp = 0
    cmd = raw_input()
    if cmd:
        ncmd = cmd
    if 'n' == ncmd:
        avifile.set_state(gst.STATE_PLAYING)
        pics.append(framesink.emit("pull-buffer").data)
        avifile.set_state(gst.STATE_PAUSED)
    if 'b' == ncmd:
        pos = avifile.query_position(gst.FORMAT_TIME, None)[0]
        print "pos: %s" % `pos`
        pos -= (100000000)
        print "pos: %s" % `pos`
        avifile.set_state(gst.STATE_PLAYING)
        avifile.seek_simple(gst.FORMAT_TIME, gst.SEEK_FLAG_FLUSH, pos)
        pics.append(framesink.emit("pull-buffer").data)
        avifile.set_state(gst.STATE_PAUSED)
    if 'w' == ncmd:
        outfile = gst.parse_launch("appsrc name=src caps=video/x-raw-yuv,format=(fourcc)I420,framerate=(fraction)1/4,width=(int)1024,height=(int)768 ! avimux ! filesink location=outimg.avi")
        outfile.set_state(gst.STATE_PLAYING)
        outfile.get_by_name("src").emit("push-buffer", gst.Buffer(pics[0]))
        outfile.get_by_name("src").emit("end-of-stream")
        outfile.set_state(gst.STATE_PAUSED)
        outfile.get_state()
        outfile.set_state(gst.STATE_NULL)
        outfile.get_state()
        del outfile
    if 'q' == ncmd:
        break