An S57 file is a complex thing that can have multiple layers. I downloaded a sample from https://web.archive.org/web/20130730111701/http://home.gdal.org/projects/s57/index.html for testing.
If you create a layer with just the path, you get a valid layer but it has no geometry - this is a layer that is just a table with some metadata in one row:
>>> layer_p = QgsVectorLayer(path, "pathonly","ogr")
When added to QGIS this shows as a tabular data layer in the legend, but has no geometry so does not appear on the map.
You can get the geometry type from this:
100 is code for "no geometry".
To get a layer from an S57 file you have to add a
layername= parameter when creating the layer:
>>> layer_w = QgsVectorLayer(path, "withlayername","ogr")
Now I have a valid layer, and its geometry type is...
which is a real geometry. Add this to the QGIS legend and you should get something on the map.
So your code needs to iterate over the valid layers and load all or some in depending on user input. This is what QGIS does - a dialog is presented of all 52 layers in the test file and the user can select the ones they want. Note this is not done by
QgsVectorLayer - QGIS must be querying the metadata on the file when you ask to open a file in the QGIS application to produce a layer list, then calls
QgsVectorLayer on the selected layers with the
layername= parameter added. You can see the full layer source URI in the properties dialog of these layers once you've loaded them.
I am not sure how to get a list of layers in a data source given a path - this is a more common operation than just for S57 files. If you point OGR (and hence QGIS) at a folder, and that folder has a load of Shapefiles in it, OGR will consider it as a single data source with multiple layers just like the demo S57 file.