OGR CreateLayer returns TypeError

Question

I'm trying to create a shapefile layer based on the spatial projection of an already existing raster. (Ultimately, this will be used in gdal.Polygonize to get a shape from my raster data.) However, I am encountering an error when trying to use CreateLayer.

• raster_uri- location of the raster file that I want to make into a shapefile. I know this exists.
• out_uri- Location I want the new shapefile to print to.
• layer_name- String layer name input.
• filed_name- String field name input.

This is the code that I was using.

raster = gdal.Open(raster_uri)
driver = ogr.GetDriverByName("ESRI Shapefile")
ds = driver.CreateDataSource(out_uri)

spat_ref = osr.SpatialReference()
proj = raster.GetProjectionRef()
spat_ref.ImportFromWkt(proj)

layer = ds.CreateLayer(layer_name, spat_ref, ogr.wkbPolygon)

It yields this error message:

TypeError: in method 'DataSource_CreateLayer', argument 2 of type 'char const *'

Outputing the type of spat_ref shows up as <class 'osgeo.osr.SpatialReference'> so I'm unsure what char const * it's complaining about.

Interestingly, if I try to do this all from a python shell using the same file, layer creates just fine. Any ideas on what might be the issue?

Answer 1

As it turns out, creating a layer name on an ESRI shapefile can't be done using a unicode string. Since way back in my file system, I was dynamically pulling layer names from a set of file names, I was allowing for unicode, but it is unsupported with that CreateLayer(). The issue was (sort of fixed) by doing the following:

layer_name = layer_name.encode('utf-8')
layer = ds.CreateLayer(layer_name, spat_ref, ogr.wkbPolygon)

Now it runs just fine, though with a slightly smaller potential character set.

Answer 2

Digging deeper on this issue I found this SO answer that provided a solution to the problem I was having.

It seems that, in your case here, your layer_name contains some Unicode characters, which can't be converted in a trivial manner to a 'char const *', and the reason why you get the exception.

This is why the solution proposed in the accepted answer works, as it changes it's encoding to a UTF-8 byte string, which can be easily converted to 'char const*' (coincidentally, that was also suggested in another answer on the same SO post).

What worked for me as an alternative is to simply cast your layer_name to string by doing:

layer = ds.CreateLayer(str(layer_name), spat_ref, ogr.wkbPolygon)