text: import os
  import time
  import tempfile
  
  from latex import latex
  
  def preview(expr, output='png', viewer=None, euler=True):
      """View expression in PNG, DVI, PostScript or PDF form.
  
         This will generate LaTeX representation of the given expression
         and compile it using available TeX distribution. Then it will
         run appropriate viewer for the given output format or use the
         user defined one. If you prefer not to use external viewer
         then you can use combination of 'png' output and 'pyglet'
         viewer. By default png output is generated.
  
         By default pretty Euler fonts are used for typesetting (they
         were used to typeset the well known "Concrete Mathematics"
         book). If you prefer default AMS fonts or your system lacks
         'eulervm' LaTeX package then unset 'euler' keyword argument.
  
         To use viewer auto-detection, lets say for 'png' output, issue::
  
             >> from sympy import *
             >> x, y = symbols("xy")
  
             >> preview(x + y, output='png')
  
         This will choose 'pyglet by default. To select different one::
  
             >> preview(x + y, output='png', viewer='gimp')
  
         The 'png' format is considered special. For all other formats
         the rules are slightly different. As an example we will take
         'dvi' output format. If you would run::
  
             >> preview(x + y, output='dvi')
  
         then 'view' will look for available 'dvi' viewers on your
         system (predefined in the function, so it will try evince,
         first, then kdvi and xdvi). If nothing is found you will
         need to set the viewer explicitly::
  
             >> preview(x + y, output='dvi', viewer='superior-dvi-viewer')
  
         This will skip auto-detection and will run user specified
         'superior-dvi-viewer'. If 'view' fails to find it on
         your system it will gracefully raise an exception.
  
         Currently this depends on pexpect, which is not available for windows.
      """
  
      # we don't want to depend on anything not in the
      # standard library with SymPy by default
      import pexpect
  
      special = [ 'pyglet' ]
  
      if viewer is None:
          if output == "png":
              viewer = "pyglet"
          else:
              # sorted in order from most pretty to most ugly
              # very discussable, but indeed 'gv' looks awful :)
              candidates = {
                  "dvi" : [ "evince", "okular", "kdvi", "xdvi" ],
                  "ps"  : [ "evince", "okular", "gsview", "gv" ],
                  "pdf" : [ "evince", "okular", "kpdf", "acroread", "xpdf", "gv" ],
              }
  
              try:
                  for candidate in candidates[output]:
                      if pexpect.which(candidate):
                          viewer = candidate
                          break
                  else:
                      raise SystemError("No viewers found for '%s' output format." % output)
              except KeyError:
                  raise SystemError("Invalid output format: %s" % output)
      else:
          if viewer not in special and not pexpect.which(viewer):
              raise SystemError("Unrecognized viewer: %s" % viewer)
  
      if not euler:
          format = r"""\documentclass[12pt]{article}
                       \usepackage{amsmath}
                       \begin{document}
                       \pagestyle{empty}
                       %s
                       \vfill
                       \end{document}
                   """
      else:
          format = r"""\documentclass[12pt]{article}
                       \usepackage{amsmath}
                       \usepackage{eulervm}
                       \begin{document}
                       \pagestyle{empty}
                       %s
                       \vfill
                       \end{document}
                   """
  
      if viewer == "pyglet":
          # import pyglet before we change the current dir, because after that it
          # would fail:
          from sympy.thirdparty import import_thirdparty
          pyglet = import_thirdparty("pyglet")
      tmp = tempfile.mktemp()
  
      tex = open(tmp + ".tex", "w")
      tex.write(format % latex(expr, inline=True))
      tex.close()
  
      cwd = os.getcwd()
      os.chdir(tempfile.gettempdir())
  
      if os.system("latex -halt-on-error %s.tex" % tmp) != 0:
          raise SystemError("Failed to generate DVI output.")
  
      os.remove(tmp + ".tex")
      os.remove(tmp + ".aux")
      os.remove(tmp + ".log")
  
      if output != "dvi":
          command = {
              "ps"  : "dvips -o %s.ps %s.dvi",
              "pdf" : "dvipdf %s.dvi %s.pdf",
              "png" : "dvipng -T tight -z 9 " + \
                      "--truecolor -o %s.png %s.dvi",
          }
  
          try:
              if os.system(command[output] % (tmp, tmp)) != 0:
                  raise SystemError("Failed to generate '%s' output." % output)
              else:
                  os.remove(tmp + ".dvi")
          except KeyError:
              raise SystemError("Invalid output format: %s" % output)
  
      src = "%s.%s" % (tmp, output)
  
      if viewer == "pyglet":
          from pyglet import window, image, gl
          from pyglet.window import key
  
          if output == "png":
              from pyglet.image.codecs.png import PNGImageDecoder
              img = image.load(src, decoder=PNGImageDecoder())
          else:
              raise SystemError("pyglet preview works only for 'png' files.")
  
          offset = 25
  
          win = window.Window(
              width = img.width + 2*offset,
              height = img.height + 2*offset,
              caption = "sympy",
              resizable = False
          )
  
          win.set_vsync(False)
  
          try:
              def on_close():
                  win.has_exit = True
  
              win.on_close = on_close
  
              def on_key_press(symbol, modifiers):
                  if symbol in [key.Q, key.ESCAPE]:
                      on_close()
  
              win.on_key_press = on_key_press
  
              def on_expose():
                  gl.glClearColor(1.0, 1.0, 1.0, 1.0)
                  gl.glClear(gl.GL_COLOR_BUFFER_BIT)
  
                  img.blit(
                      (win.width - img.width) / 2,
                      (win.height - img.height) / 2
                  )
  
              win.on_expose = on_expose
  
              while not win.has_exit:
                  win.dispatch_events()
                  win.flip()
          except KeyboardInterrupt:
              pass
  
          win.close()
      else:
          os.system("%s %s &> /dev/null &" % (viewer, src))
          time.sleep(2) # wait for the viewer to read data
  
      os.remove(src)
      os.chdir(cwd)
  