gridwidget.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

from __future__ import (division, absolute_import, print_function,
                        unicode_literals)

import os
import sys

from PyQt4 import QtCore, QtGui, uic


class GridWidget(QtGui.QTableWidget):
    def resizeEvent(self, event):
        total_width = self.viewport().width()
        nb_columns = self.columnCount()

        for column in range(nb_columns):
            self.horizontalHeader().resizeSection(column,
                                                  total_width / nb_columns)


TestPanelUi = uic.loadUiType(os.path.join(os.path.dirname(__file__),
                                          'gridwidget.ui'))[0]


class TestPanel(QtGui.QMainWindow, TestPanelUi):
    def __init__(self):
        super(TestPanel, self).__init__()
        self.setupUi(self)
        self.pushButton.clicked.connect(self.addWidget)

        self.counter = 0

    @QtCore.pyqtSlot()
    def addWidget(self):
        widget = QtGui.QLabel(self.gridWidget)

        self.counter += 1
        widget.setText('Widget {}'.format(self.counter))

        next_empty_cell = (-1, -1)
        for row in range(self.gridWidget.rowCount()):
            for column in range(self.gridWidget.columnCount()):
                if not self.gridWidget.cellWidget(row, column):
                    next_empty_cell = (row, column)
                    break

            if next_empty_cell != (-1, -1):
                break

        if next_empty_cell == (-1, -1):
            self.gridWidget.insertRow(self.gridWidget.rowCount())
            next_empty_cell = (self.gridWidget.rowCount() - 1, 0)

        self.gridWidget.setCellWidget(next_empty_cell[0],
                                      next_empty_cell[1], widget)


def main():
    app = QtGui.QApplication(sys.argv)

    panel = TestPanel()
    panel.show()

    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

gridwidget.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>800</width>
    <height>600</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralwidget">
   <layout class="QHBoxLayout" name="horizontalLayout">
    <item>
     <widget class="QPushButton" name="pushButton">
      <property name="text">
       <string>PushButton</string>
      </property>
     </widget>
    </item>
    <item>
     <widget class="GridWidget" name="gridWidget">
      <property name="dragEnabled">
       <bool>true</bool>
      </property>
      <property name="dragDropOverwriteMode">
       <bool>false</bool>
      </property>
      <property name="dragDropMode">
       <enum>QAbstractItemView::InternalMove</enum>
      </property>
      <property name="rowCount">
       <number>3</number>
      </property>
      <property name="columnCount">
       <number>3</number>
      </property>
      <attribute name="horizontalHeaderVisible">
       <bool>false</bool>
      </attribute>
      <attribute name="verticalHeaderVisible">
       <bool>false</bool>
      </attribute>
      <row/>
      <row/>
      <row/>
      <column/>
      <column/>
      <column/>
     </widget>
    </item>
   </layout>
  </widget>
 </widget>
 <customwidgets>
  <customwidget>
   <class>GridWidget</class>
   <extends>QTableWidget</extends>
   <header>gridwidget</header>
  </customwidget>
 </customwidgets>
 <resources/>
 <connections/>
</ui>