I've just been reviewing a really useful PR (https://github.com/matplotlib/matplotlib/pull/1531) from Pierre Haessig which speeds up the drawing of non-visible artists by bringing the following line to the top of the LineArtist's draw method:

    if self.get_visible() is False:
        return

This *does* fix the problem (and will fix the problem for all other artists if applied in the same way), but it relies on a developer remembering the rule of thumb that they must always start their draw method with these two (simple) lines. Additionally, testing this functionality is actually quite hard without resorting to timing the execution.

It made we wonder if there was a better approach to fixing this. Having a decorator to do this for you is a good idea, except that a developer would need to remember to decorate their subclass' draw method, so the next level up is to use a metaclass to *always* wrap the draw method with the "if visible" lines.

An example of implementing this (apologies if the code doesn't come out well in the email):


class ArtistMeta(type):
    def __new__(cls, classname, bases, class_dict):
        # replace the draw method with one which short-circuits if self.visible is False
        draw_method = class_dict['draw']
        def draw(self, *args, **kwargs):
            if self.visible is False:
                print 'draw **not** called with visible={}'.format(self.visible)
                return
            else:
                return draw_method(self, *args, **kwargs)
        class_dict['draw'] = draw
           
        return  type.__new__(cls, classname, bases, class_dict)


class Artist(object):
    __metaclass__ = ArtistMeta
   
    def __init__(self, visible=True):
        self.visible = visible
       
    def draw(self, renderer=None):
        print 'draw called with visible={}'.format(self.visible)
        return 'foobar'


class SubArtist(Artist):
    def draw(self, renderer=None):
        print "subclass' draw method"
        return Artist.draw(self, renderer=renderer)




With the following results:


>>> a = Artist().draw('wibble')
draw called with visible=True

>>> b = Artist(False).draw('wibble')
draw **not** called with visible=False

>>> c = SubArtist(True).draw('wibble')
subclass' draw method
draw called with visible=True

>>> d = SubArtist(False).draw('wibble')
draw **not** called with visible=False



In my eyes this makes testing the functionality possible without timing (and is therefore an improvement), but I wanted to know how others felt about the approach, and in particular, using more metaclasses in matplotlib (a simple tutorial which I found useful: http://www.voidspace.org.uk/python/articles/metaclasses.shtml).


Cheers,

Phil