Attempting to repro mapnik-support #7 (https://github.com/mapnik/mapnik-support/issues/7).

mapnik-support-7.py

#!/usr/bin/env python

import io
import mapnik

from shapely.geometry import box, MultiPoint

class SimpleDatasource(mapnik.PythonDatasource):

    def __init__(self):
        self.points = MultiPoint([(-1.0,-1.0), (-1.0,1.0), (1.0,1.0), (1.0,-1.0)])
        def extend (delta, x):
            if x > 0: return x + delta
            else: return x - delta
        self.envelope = mapnik.Box2d(*[extend(0.1,x) for x in self.points.bounds])
        super(SimpleDatasource, self).__init__(
            envelope=self.envelope,
        )

    def features(self, query):
        b = box(query.bbox.minx,query.bbox.miny,query.bbox.maxx,query.bbox.maxy)
        return mapnik.PythonDatasource.wkb_features(
            keys = (),
            features = [(x.wkb, {}) for x in self.points if b.contains(x)]
        )

def init_map():
    m = mapnik.Map(256,256)
    m.background_color = mapnik.Color('white')
    s = mapnik.Style()
    r = mapnik.Rule()
    r.symbols.append(mapnik.PointSymbolizer())
    s.rules.append(r)
    m.append_style('point_style', s)
    ds = mapnik.Python(factory='SimpleDatasource')
    l = mapnik.Layer('points')
    l.datasource = ds
    l.styles.append('point_style')
    m.layers.append(l)
    return m

if __name__=='__main__':
    map = init_map()
    map.zoom_all()
    mapnik.render_to_file(map, 'mapnik_rendered.png')
    im = mapnik.Image(256,256)
    mapnik.render(map,im)
    im.save('im_rendered.png', 'png')
    buf = im.tostring('png')
    with open('manually.png', 'wb') as f:
        f.write(buf)