How to use .xml file generated by openstreetmap-carto with mapnik and postgis

Hi,

I'm struggling for 2 weeks now to render a map using mapnik, mapnik-python, postigs and carto. I'm able to render a map from a local .osm file and I've compiled and installed the latest mapnik library and python bidings.

I've exported a .osm file to a local postgis DB using osm2pgsql and using QGIS Browser (a windows postgis client) I can validate that the export was successful. In order to render a map from this DB I generated a .xml stylesheet using carto -a "3.0.0" ../openstreetmap-carto/project.mml > mapnik2.xml and I modified project.mml and added my DB credentials like this:

  osm2pgsql: &osm2pgsql
  type: "postgis"
  dbname: "gis"
  port: "5432"
host: "localhost"
user: "maps"
password: "asdfe"
key_field: ""
geometry_field: "way"
extent: "-20037508,-20037508,20037508,20037508"

I checked the generated mapnik2.xml file and all DB queries now have information about my DB credentials and it looks like this (snapshot from mapnik2.xml):

</Style>
<Layer name="cliffs"
maximum-scale-denominator="100000"
srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
<StyleName>cliffs</StyleName>
<StyleName>cliffs-man_made</StyleName>
<Datasource>
<Parameter name="type"><![CDATA[postgis]]></Parameter>
<Parameter name="dbname"><![CDATA[gis]]></Parameter>
<Parameter name="port"><![CDATA[5432]]></Parameter>
<Parameter name="host"><![CDATA[localhost]]></Parameter>
<Parameter name="user"><![CDATA[maps]]></Parameter>
<Parameter name="password"><![CDATA[asdfe]]></Parameter>
<Parameter name="key_field"><![CDATA[]]></Parameter>
<Parameter name="geometry_field"><![CDATA[way]]></Parameter>
<Parameter name="extent"><![CDATA[-20037508,-20037508,20037508,20037508]]></Parameter>
<Parameter name="table"><![CDATA[(SELECT
way, "natural", man_made
FROM planet_osm_line
WHERE "natural" = 'cliff' OR man_made = 'embankment'
) AS cliffs]]></Parameter>
</Datasource>
</Layer>

This is the python script: #!/usr/bin/env python

import mapnik
from mapnik import *

mapnik.logger.set_severity(mapnik.severity_type.Debug)

mapfile = 'mapnik2.xml'
map_output = 'mymap33.png'

m = Map(1024, 1024)
load_map(m, mapfile)
bbox=(Box2d(-6.5, 49.5, 2.1, 59))

m.zoom_to_box(bbox) 
render_to_file(m, map_output)

The output image is always blank and I see no errors in the logs. I also tried to use nik4.py with the same result, blank image. Is this because mapnik can't connect to my local postgis DB? Did I miss a step ?