Validation

import arcpy
class ToolValidator(object):
  """Class for validating a tool's parameter values and controlling
  the behavior of the tool's dialog."""

  def __init__(self):
    """Setup arcpy and the list of tool parameters."""
    self.params = arcpy.GetParameterInfo()

  def initializeParameters(self):
    """Refine the properties of a tool's parameters.  This method is
    called when the tool is opened."""
    return

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

    if self.params[0].value == 'Single Camera':

    	self.params[1].enabled = 1
	self.params[2].enabled = 0
    	self.params[3].enabled = 0
	self.params[4].enabled = 0
 	self.params[6].enabled = 0
	self.params[7].enabled = 0


    else:
    	self.params[1].enabled = 0
	self.params[2].enabled = 1
    	self.params[3].enabled = 1
	self.params[4].enabled = 1
 	self.params[6].enabled = 1
	self.params[7].enabled = 1

    return

  def updateMessages(self):
    """Modify the messages created by internal validation for each tool
    parameter.  This method is called after internal validation."""

    if self.params[0].value == 'Single Camera':

    	self.params[1].setErrorMessage("Photo folder required")
    	self.params[5].setErrorMessage("Staging Geodatabase required")
    	self.params[8].setErrorMessage("Survey question file required")

     else:

    	self.params[2].setErrorMessage("Passenger side photos required")
    	self.params[3].setErrorMessage("Driver side photos required")
    	self.params[5].setErrorMessage("Staging Geodatabase required")
    	self.params[6].setErrorMessage("Parcel (polygon) feature class required")
    	self.params[7].setErrorMessage("Parcel Identification field required")
    	self.params[8].setErrorMessage("Survey question file required")

     return