"select by attribute" error, when using getParameter

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I'm developing an ArcGIS tool by Python. Below is a part of the inputs:

StudyArea = arcpy.GetParameterAsText(0)

LandCover = arcpy.GetParameterAsText(1)

gridcode = arcpy.GetParameterAsText(2)

gridcode is a field from LandCover.

gridcode aims to derive the field which contains land cover class code.

Then I try to select some land cover types (land cover code < 17 and = 21), and the code I write is:


arcpy.SelectLayerByAttribute_management("LC_test","NEW_SELECTION",'gridcode