OSM rendering with mapnik and Python script

For the Python/mapnik part of the question take a look at nik4:

https://github.com/Zverik/Nik4

(or maybe one of the other tools mentioned in the readme there)

I am using this:

m = mapnik.Map(image_width,image_height)
mapnik.load_map(m, v['style'])
bbox=mapnik.Envelope( left, top, right, bottom )
m.zoom_to_box(bbox)
mapnik.render_to_file(m, v['out'],'jpeg')

Where v['style'] points to a mapnik XML style sheet. This is the older way to style mapnik, the newer being to use a CSS file that, I think, is used to generate mapnik XML. Some information to get you started is here: https://github.com/mapnik/mapnik/wiki/XMLConfigReference

The mapnik XML is not as well documented as I'd like and you need to be sure that the tags your style(s) need are actually inserted into the Postgresql database.

Here is the script I use to get OSM data and to load it into my database. The "style" file for osm2pgsql is actually in a column format listing the tagging on the objects that are to be loaded.

#! /bin/bash
  # Define defaults
  state_file="california-latest.osm.pbf"
  state_server="http://download.geofabrik.de/north-america/us/"
  cache_dir="osm/"
  osm_style="${cache_dir}osm2pgsql.style"
  if [ -e "${cache_dir}${state_file}" ] ; then
    mv -f "${cache_dir}${state_file}" "${cache_dir}${state_file}.bak"
  fi
  wget -O "${cache_dir}${state_file}" "${state_server}${state_file}"
  echo "Filling PostGIS database"
  cmd="osm2pgsql --verbose --create --multi-geometry --database osm --style ${osm_style} --hstore ${cache_dir}${state_file}"
  echo ${cmd}
  ${cmd}
  echo 'CREATE INDEX idx_planet_osm_point_tags ON planet_osm_point USING gist(tags);' > pgsql osm
  echo 'CREATE INDEX idx_planet_osm_polygon_tags ON planet_osm_polygon USING gist(tags);' > pgsql osm
  echo 'CREATE INDEX idx_planet_osm_line_tags ON planet_osm_line USING gist(tags);' > pgsql osm

For rendering, the mapnik XML has one part that lists what and how to render another part that gets the data. Here is how I render buildings and fences:

    <Style name="buildings">
    <Rule>
        <Filter>[building] != ''</Filter>
            <PolygonSymbolizer fill="rgb(0,0,0)" />
    </Rule>
</Style>

<Style name="fences">
    <Rule>
        <Filter>
            ([barrier] != '')
        </Filter>
        <LineSymbolizer
            stroke="rgb(0,0,0)"
            stroke-width="&pt_size_0_5;"
            stroke-dasharray="&pt_size_0_5;,&pt_size_0_5;"
        />
    </Rule>
</Style>

<Layer name="buildings" status="on" srs="&osm2pgsql_projection;">
    <StyleName>buildings</StyleName>
    <Datasource>
        <Parameter name="type">postgis</Parameter>
        <Parameter name="host">localhost</Parameter>
        <Parameter name="dbname">osm</Parameter>
        <Parameter name="user"></Parameter>
        <Parameter name="password"></Parameter>
        <Parameter name="table">
            (SELECT way, building
            from planet_osm_polygon where building is not null) as foo
        </Parameter>
    </Datasource>
</Layer>

<Layer name="fences" status="on" srs="&osm2pgsql_projection;">
    <StyleName>fences</StyleName>
    <Datasource>
        <Parameter name="type">postgis</Parameter>
        <Parameter name="host">localhost</Parameter>
        <Parameter name="dbname">osm</Parameter>
        <Parameter name="user"></Parameter>
        <Parameter name="password"></Parameter>
        <Parameter name="table">
            (SELECT way, barrier
            from planet_osm_line where (barrier = 'fence') or (barrier = 'gate')) as foo
        </Parameter>
    </Datasource>
</Layer>