def on_browse_input_crs_pressed(self):
     """
     define input crs
     :return:
     """
     crs_selector = QgsGenericProjectionSelector()
     crs_selector.show()
     crs_selector.exec_()
     self.input_authid = str(crs_selector.selectedAuthId())
     selected_crs = QgsCoordinateReferenceSystem()
     selected_crs.createFromString(self.input_authid)
     self.input_crs.setText(selected_crs.description() + "    (" + self.input_authid+")")
# coding: utf-8
from qgis.gui import QgsGenericProjectionSelector

generic_projection_selector = QgsGenericProjectionSelector()
generic_projection_selector.show()