Mapnik python demo error

[darkblueb]

build 196; mapnik3 python;

run Jupyter, execute

​http://localhost:8883/notebooks/projects/FIONA/fiona-shapely-mapnik.ipynb

ArgumentError                             Traceback (most recent call last)
<ipython-input-7-dd353890fe83> in <module>()
      9 rule = mapnik.Rule()
     10 
---> 11 line_symbolizer = mapnik.LineSymbolizer(mapnik.Color('green'),0.4)
     12 rule.symbols.append(line_symbolizer)
     13 

ArgumentError: Python argument types in
    LineSymbolizer.__init__(LineSymbolizer, Color, float)
did not match C++ signature:
    __init__(_object*)

[darkblueb]

this error look like the same sort of error that occurred when the python bindings were out of synch with the mapnik.so. How to verify?

• looking in the build 196 log, install_osm.sh shows:

  Get:80 http://ppa.launchpad.net/osgeolive/nightly/ubuntu xenial/main amd64 python-mapnik amd64 1:0.0~20160531-f65767a-1~xenial0 [635 kB]
  

• python -v and import mapnik shows the full paths of all components loaded

  ...
  import mapnik._mapnik # dynamically loaded from /usr/lib/python2.7/dist-packages/mapnik/_mapnik.x86_64-linux-gnu.so
  ...
  

• using ​http://tracker.debian.org see versions and status ​here

• VCS entry for the package shows ​https://anonscm.debian.org/git/pkg-grass/python-mapnik.git

but, where does the _mapnik get built ? logs for that ?

[Bas Couwenberg]

To me it looks like the iPython notebook is still using the old python-mapnik API (from the Mapnik 2.x times, instead of the current Mapnik 3.x API), because it passes two parameters to LineSymbolizer() instead of an object.

Buildlogs for the python-mapnik package in OSGeo-Live are on Launchpad:

​https://launchpad.net/~osgeolive/+archive/ubuntu/nightly/+packages?field.name_filter=python-mapnik

Expand the line to show the 'Publishing details', click the 'amd64' link to the build details page which includes a link to the buildlog.

[Bas Couwenberg]

Based on the demo code included in Mapnik (​demo/python/rundemo.py), the notebook code should probably be rewritten to:

line_symbolizer = mapnik.LineSymbolizer()
line_symbolizer.stroke = mapnik.Color('green')
line_symbolizer.stroke_width = 0.4
rule.symbols.append(line_symbolizer)

[Bas Couwenberg]

Replacing source in the problematic cell with the following works on build 197:

import mapnik

BASE_FOLDER = '/home/user/data/north_carolina/shape'
c_shapefile = os.path.join(BASE_FOLDER, 'nc_state.shp')

m = mapnik.Map(600, 600)

layer = mapnik.Layer('contour')
layer.datasource = mapnik.Shapefile(file=c_shapefile)

style = mapnik.Style()
rule = mapnik.Rule()

line_symbolizer = mapnik.LineSymbolizer()
line_symbolizer.stroke = mapnik.Color('green')
line_symbolizer.stroke_width = 0.4
rule.symbols.append(line_symbolizer)

m.layers.append(layer)
style.rules.append(rule)
m.append_style('My Style', style)
layer.styles.append('My Style')

m.layers.append(layer)
m.zoom_all()
show_mapnik(m)

[darkblueb]

patch to ​OSGeoLive-Notebooks repo applied via ​GitHub-PR

[Bas Couwenberg]

Confirmed fixed in RC1.