Small modifications to the default file created by the Python Add-In assistant to be able to include and use a custom toolbox and tool as part of an add-in.

python-addin-button-call-tool.py

import arcpy
import pythonaddins
import os


class addDefinitionQueryLayers(object):
"""Implementation for arcpyMappingAddin_addin.button (Button)"""

  def __init__(self):
    self.enabled = True
    self.checked = False

  def onClick(self):
    # name of toolbox without tbx extension
    toolboxName = "Mapping"
    
    # name of tool to be executed
    toolName = "AddDefinitionQueryLayers"
    
    # create string with path to toolbox
    toolboxPath = os.path.join(os.path.dirname(__file__), toolboxName + ".tbx")

    # call geoprocessing tool
    pythonaddins.GPToolDialog(toolboxPath, toolName)