arcpy - CalculateField_management, python Toolbox

I am trying to fill a field and I am using arcpy's CalculateField_management function.

I want to fill the new field with the name of the shapefile, so I am using arcpy.Describe. But when I run the script I get this error: ExecuteError: ERROR 000539: Error running expression: PRUEBAS Traceback (most recent call last): File "", line 1, in NameError: name 'PRUEBAS' is not defined. 'Pruebas' is the name of the shapefile, I obtain it from desc.baseName

The code is:

import arcpy, os, sys
from arcpy import env
from arcpy import Describe 


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

    desc = arcpy.Describe(parameters[0])
    a=arcpy.AddField_management(a, b, "TEXT", "", "", "", "", "NULLABLE", "NON_REQUIRED", "")

    a=arcpy.CalculateField_management(a, b, desc.baseName,"PYTHON_9.3",code_block="")

    return a

`

Answer

In the CalculateField_management you are giving the desc.baseName as the expression parameter. It assumes the value of that variable is the name of a function that is not defined.

This should work:

arcpy.CalculateField_management(a, b, '"%s"' % desc.baseName)