So, I'm using glReadPixels(...) to get data for a screenshot.  I would really like to be able to get data from any framebuffer.  Currently, though, I can only get data from the screen framebuffer (or GL_COLOR_ATTACHMENT0_EXT).  I looked at glReadBuffer.  I'm using PyGame and PyOpenGL, and glGetString(GL_READ_BUFFER) returns None.  glReadBuffer(GL_BACK) crashes.

What's going on?