ArcGIS Project Map Parameter

tool.py

    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].altered and self.params[0].value != "":
            projectPath = self.params[0].value
            aprx = arcpy.mp.ArcGISProject(projectPath)
            # Get list of maps in project
            map_list = [m.name for m in aprx.listMaps()]
            # Populate Map param
            self.params[1].filter.list = map_list