Add and fill a field in Python Toolbox (pyt) in ArcMap

Question

I am using python toolbox in ArcMap, and I want to add a new field in a shapefile and after that I want to fill this field with the name of the shapefile.

I achieve to add a new field using arcpy.AddField_management function, but then I cannot fill this field. I am using arcpy.CalculateField_management function to fill it, however I think that this is a wrong way.

Any suggest is welcome.

The code is:

import arcpy, os, sys
from arcpy import env

class Toolbox(object):
    def __init__(self):
        """Define the toolbox (the name of the toolbox is the name of the
        .pyt file)."""
        self.label = "Toolbox"
        self.alias = ""

        # List of tool classes associated with this toolbox
        self.tools = [Tool]
class Tool(object):
    def __init__(self):
        """Define the tool (tool name is the name of the class)."""
        self.label = "FIELD"
        self.description = ""
        self.canRunInBackground = False

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

# First parameter

    param0 = arcpy.Parameter(
        displayName="Shape entrada",
        name="in_features",
        datatype="DEShapefile",
            parameterType="Required",
        direction="Input")

# Second parameter

param1 = arcpy.Parameter(
        displayName="Nuevo campo",
            name="field",
        datatype="Field",
            parameterType="Required",
        direction="Input")

    parameters = [param0, param1]
    return parameters

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

def updateParameters(self, parameters):return

def updateMessages(self, parameters):return

def execute(self, parameters, messages):
    """The source code of the tool."""
    a= parameters[0].valueAsText
b= parameters[1].valueAsText


a=arcpy.AddField_management(a, b, "TEXT", "", "", "", "", "NULLABLE", "NON_REQUIRED", "")

a=arcpy.CalculateField_management(a, b, "hola" ,"VB")

    return a

Answer 1

The reason you cannot calculate your field to contain the text hola is wrong string formatting in the expression parameter (look at the Calculate Field GP tool docs).

When using the VB parser, you need to supply a string inside double quotes. Look at the '"hola"'.

arcpy.CalculateField_management(
    in_table="U.S. Cities",
    field="CITY_NAME",
    expression='"hola"',
    expression_type="VB",
    code_block="")

You could alternatively use the Python parser. You would need to reverse the strings as in "'hola'":

arcpy.CalculateField_management(
    in_table="U.S. Cities",
    field="CITY_NAME",
    expression="'hola'",
    expression_type="PYTHON_9.3",
    code_block="")

As you are writing a Python tool, it makes sense to use the Python expression instead of the VB.