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(I think you should bring this up in the OpenGl community; if the with-statement supports the usage I don't see why not. GvR)

(Doing so. -- Andrew Dalke)

OpenGL programmers have complained about using Python because the code indentation doesn't follow the display tree. For an example pulled from one of my (Andrew Dalke) projects

        glBegin(GL_QUAD_STRIP)
        glColor3f(1.0,1.0,1.0) #corner 1
        glNormal3f(0.57735027, 0.57735027, 0.57735027)
        glVertex3f(0.5, 0.5, 0.5)
        glColor3f(1.0,0.0,1.0) #corner 2
        glNormal3f(0.57735027, -0.57735027, 0.57735027)
        glVertex3f(0.5, -0.5, 0.5)
        ...
        glEnd()

Some people write this as some variant of

        glBegin(GL_QUAD_STRIP)
        if 1:
          glColor3f(1.0,1.0,1.0) #corner 1
          glNormal3f(0.57735027, 0.57735027, 0.57735027)
          glVertex3f(0.5, 0.5, 0.5)
          glColor3f(1.0,0.0,1.0) #corner 2
          glNormal3f(0.57735027, -0.57735027, 0.57735027)
          glVertex3f(0.5, -0.5, 0.5)
          ...
        glEnd()

and sometimes using try/finally so that errors don't cause the gl state to become corrupted.

Would an appropriate use of this proposal be to allow

  with QUAD_STRIP:
      glColor3f(1.0,1.0,1.0) #corner 1
      glNormal3f(0.57735027, 0.57735027, 0.57735027)
      glVertex3f(0.5, 0.5, 0.5)
      glColor3f(1.0,0.0,1.0) #corner 2
      glNormal3f(0.57735027, -0.57735027, 0.57735027)
      glVertex3f(0.5, -0.5, 0.5)
      ....

where there are a bunch of small classes for each of the possible glBegins, such as

class QUAD_STRIP:
  @staticmethod
  def __enter__():
    glBegin(GL_QUAD_STRIP)
  @staticmethod
  def __exit__(*args):
    glEnd()

If so, I rather like that ability as it makes the graphics programmer's intent clearer and prevents problems balancing glBegin and glEnd - even in the face of code errors in the actual code block! -- Andrew Dalke


I'm (Andrew Dalke) often one to resist change to Python. The WithStatement is a counter-example. I really like how it cleans up some OpenGL programming. Consider this example from OpenGLContext's indexlineset.py

           dl = displaylist.DisplayList()
           ...
           # compile the color-friendly ILS
           dl.start()
           try:
               glEnable( GL_COLOR_MATERIAL )
               for polyline, color in zip(indices, colorIndices):
                   if type(color) == int:
                       glColor3d( *colors[color] )
                       glBegin( GL_LINE_STRIP )
                       try:
                           for i in polyline:
                               glVertex3f(*points[i])
                       finally:
                           glEnd()
                    else:
                       glBegin( GL_LINE_STRIP )
                       try:
                           for i,c in zip(polyline, color):
                               glColor3d( *colors[c] )
                               glVertex3f(*points[i])
                       finally:
                            glEnd()
               glDisable( GL_COLOR_MATERIAL )
           finally:
               dl.end()

Assuming a few minor helper classes and a couple of new __enter__/__exit__ methods and it can be rewritten using 'with' statements like this:

           dl = displaylist.DisplayList()
           ...
           with dl:
               with GLEnable( GL_COLOR_MATERIAL ):
                   for polyline, color in zip(indices, colorIndices):
                       if type(color) == int:
                           glColor3d( *colors[color] )
                       with GLGeometry( GL_LINE_STRIP ):
                           for i in polyline:
                               glVertex3f(*points[i])
                    else:
                       with GLBegin( GL_LINE_STRIP ):
                           for i,c in zip(polyline, color):
                               glColor3d( *colors[c] )
                               glVertex3f(*points[i])

Shorter, cleaner, less error-prone, easier to read and maintain. +1 from me! -- Andrew Dalke

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