Python-сообщество

 import sys
from form_main import *
from form_day import *
from PyQt5 import QtCore, QtGui, QtWidgets
class FormMain(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        QtWidgets.QWidget.__init__(self, parent)
        self.ui = Ui_form_main()
        self.ui.setupUi(self)
        self.ui.ButtonDay.clicked.connect(self.show_form_day)
    def show_form_day(self): 
        self.form_day = FormDay(self) 
        self.form_day.show()
class FormDay(QtWidgets.QMainWindow): 
    def __init__(self, parent=None):
        QtWidgets.QWidget.__init__(self, parent)
        self.form_day = Ui_form_day()
        self.form_day.setupUi(self)
	#... настройки
        form_main.hide()
if __name__=="__main__":
    app = QtWidgets.QApplication(sys.argv)
    form_main = FormMain()
    form_main.show()
    sys.exit(app.exec_())

 from PyQt5 import QtCore, QtGui, QtWidgets
class Ui_form_day(object):
    def setupUi(self, form_day):
        form_day.setObjectName("form_day")
        form_day.resize(1024, 600)
        self.centralwidget = QtWidgets.QWidget(form_day)
	self.ButtonCancel.setFont(font)
	self.ButtonCancel.setStyleSheet("")
	self.ButtonCancel.setObjectName("ButtonCancel")
	#....
	self.ButtonCancel.clicked.connect(self.push_button_cancel)
	self.ButtonApply.clicked.connect(self.push_button_apply)
    def push_button_cancel(self):
        print('cancel')
        form_main.show()
        self.hide()

 form_main.hide()
....
form_main.show()
...