Derive (default) value in ValueTable based on values in ValueTable of Python Toolbox using ArcPy?

Question

I am writing a Python Toolbox which needs Shapefiles (or Featureclasses) as input and calculates an output for each of the input files. I would like to have the possibility to rename the files in order to avoid an overwriting or an exception. My approach is to use a toolbox parameter of type GPValueTable with 2 columns. The input files is given in the first column (by the user) and I want the 2. column to be automatically filled by the base name without extension of the input file.

Unfortunatelly, I am not able to fill the second column based on the first. I tried various ways already but here is a snipped of sample code:

    def getParameterInfo(self):
    """Define parameter definitions"""

    input_paths = arcpy.Parameter(
        displayName="Input files",
        name="in_paths",
        datatype="GPValueTable",
        parameterType="Required",
        direction="Input",
        multiValue=True)
    input_paths.columns = [['GPFeatureLayer','Eingabedatensatz'], ['GPString','Ausgabename']]

    output_path = arcpy.Parameter(
        displayName="Output path",
        name="out_path",
        datatype="DEFolder",
        parameterType="Required",
        direction="Input")

    params = [input_paths,output_path]
    return params

def isLicensed(self):
    """Set whether tool is licensed to execute."""
    return True

def updateParameters(self, parameters):
    """Modify the values and properties of parameters before internal
    validation is performed.  This method is called whenever a parameter
    has been changed."""
    if parameters[0].value:
        new_vt = []
        gp_value_table = parameters[0].value
        vt_length = len(gp_value_table)
        if  gp_value_table[vt_length-1][0] and not not gp_value_table[vt_length-1][1]:
                    new_vt = [[v[0],v[2]] for v in gp_value_table]
                    input_path = gp_value_table[vt_length-1][0]
                    new_vt.append([vt_length-1,arcpy.Describe(input_path).baseName])
                    parameters[0].value = new_vt
    return

In this case nothing happens and the 2. column remains emtpy. How can I - row by row - derive the default output name based on the input?

Answer 1

The only code you need to change is the code in your updateParameters()

def updateParameters(self, parameters):
    """Modify the values and properties of parameters before internal
    validation is performed.  This method is called whenever a parameter
    has been changed."""

    # Create a ValueTable object and load it with current values from parameter 
    # which itself is a ValueTable
    vtab = arcpy.ValueTable(len(parameters[0].columns))
    vtab.loadFromString(parameters[0].valueAsText)

    # Create a list of the values in the parameter, this will be a list of lists
    aList = parameters[0].values

    # Step through list, extract basename and update row in ValueTable object
    for i,v in enumerate(aList):
        fc = v[0]
        bn = arcpy.Describe(fc).baseName
        newvalue = "'" + str(fc) + "'" + " " + "'" + bn + "'"
        vtab.setRow(i, newvalue)

    # Write the contents of ValueTable object back into the Parameter which will overwrite everything
    parameters[0].value = vtab.exportToString()
    return