I am trying to iteratively interpolate a shapefile using the QGIS IDW interpolator.  What I need to do is take a point shapefile with time series data in the columns associated with each point, interpolate that column of data, then move to the next column.

The process I've generally been following is outlined here:
https://gis.stackexchange.com/questions/100188/how-to-compute-an-interpolation-raster-from-the-python-console-in-qgis/233322#233322

I've modified a few things and have been working on a test case (since the input data hasn't been provided yet - format should be similar except ~300 iterations), but I can't get the interpolator to move past the first, 0-indexed, column. 

Code runs without error, outputs the 4 test raster interpolation .asc files, and adds them to the QGIS layers panel, but they are all the same.  You can see in the layers panel snippet below that the raster cell bounds are all the same and visually if I toggle the layers on/off, they are all the same.

Code below (only change is that I modified actual folder paths to dummy variables):

    import qgis.core
    import qgis.analysis
    import os
    
    
    pathToFile = r"input_folder\interp_test.shp"
    
    interp_layer = QgsVectorLayer(pathToFile, 'interp_test','ogr')
    
    #not used yet
    field_names = [field.name() for field in interp_layer.pendingFields()]
    
    #test first 4 columns
    for i in np.arange(0, 4):
    	layer_data = qgis.analysis.QgsInterpolator.LayerData()
    	layer_data.vectorLayer = interp_layer
    	layer_data.zCoordInterpolation=False
        
        #this line should change the interpolation attribute/column, but it doesn't
    	layer_data.InterpolationAttribute = i
    	layer_data.mInputType = 1 #points
    	
    	#pick interpolator engine
    	idw_interpolator = qgis.analysis.QgsIDWInterpolator([layer_data])
    	
    	export_path =r"output_folder\rasterout_{}.asc".format(i)
    	rect = interp_layer.extent()
    	res = 500
    	ncol = int( ( rect.xMaximum() - rect.xMinimum() ) / res )
    	nrows = int( (rect.yMaximum() - rect.yMinimum() ) / res)
    
    	output = qgis.analysis.QgsGridFileWriter(idw_interpolator,export_path,rect,ncol, nrows,res,res)
    	output.writeFile(True)  
    	iface.addRasterLayer(export_path, "interpolation_output_{}.asc".format(i))


Input test attribute table:
[![Attribute table of interpolation input][1]][1]

Output to QGIS console:
[![Output interpolated raster, duplicated 4x][2]][2]

Any help would be much appreciated.  Running 64-bit QGIS 2.18.3 on windows, using the built-in python console.


  [1]: https://i.sstatic.net/gvCCI.png
  [2]: https://i.sstatic.net/Qj2nt.png