test.py

import sys
from PyQt5 import QtCore, QtGui
from PyQt5.QtWidgets import (QWidget, QLabel, QLineEdit, 
    QTextEdit, QGridLayout, QApplication, QMainWindow)
from PyQt5.QtCore import *
from PyQt5.QtGui import *


class MyMainWindow(QMainWindow):
    def __init__(self, parent = None):
        super(MyMainWindow, self).__init__(parent)

        self.main_widget = QWidget(self)
        ...
        self.form_widget = FormWidget(self) 
        #This is my UI widget

        self.main_layout = QVBoxLayout(self.main_widget)
        self.main_layout.sizeConstraint = QLayout.SetDefaultConstraint
        self.main_layout.addWidget(self.form_widget.main_widget) 
        #form_widget has its own main_widget where I put all other widgets onto

        self.main_widget.setLayout(self.main_layout)
        self.setCentralWidget(self.main_widget)