PyQt bug: UI files loading/parsing is not reentrant (PyQt 5.4.2 nightly d5b00e76067f)

main.py

import sys

from PyQt5 import uic
from PyQt5.QtWidgets import QWidget, QApplication

# Problem: the ui loader/parser is not reentrant. If another UI load is requested
# during loading of UI file, final layout may be corrupted.
#
# In this example "main.ui" file requests promoted widget from "widget.py", and
# on module level of "widget.py" UI file "widget.ui" is being loaded.

# A workaround is to force loading of all UI files independently, e.g. by loading
# "widget.ui" first by importing "widget.py" module below.
#import widget


class MainWidget(QWidget):
    def __init__(self):
        super().__init__()

        self._ui = uic.loadUi("main.ui", self)


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

    w = MainWidget()
    w.resize(200, 100)
    w.show()

    sys.exit(app.exec_())

if __name__ == "__main__":
    main()

main.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>Widget</class>
 <widget class="QWidget" name="Widget">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>870</width>
    <height>578</height>
   </rect>
  </property>
  <property name="sizePolicy">
   <sizepolicy hsizetype="Expanding" vsizetype="Preferred">
    <horstretch>0</horstretch>
    <verstretch>0</verstretch>
   </sizepolicy>
  </property>
  <property name="windowTitle">
   <string notr="true">Form</string>
  </property>
  <property name="childrenCollapsible" stdset="0">
   <bool>false</bool>
  </property>
  <layout class="QGridLayout" name="gridLayout">
   <property name="leftMargin">
    <number>0</number>
   </property>
   <property name="topMargin">
    <number>0</number>
   </property>
   <property name="rightMargin">
    <number>0</number>
   </property>
   <property name="bottomMargin">
    <number>0</number>
   </property>
   <property name="spacing">
    <number>0</number>
   </property>
   <item row="0" column="0">
    <widget class="QLabel" name="label">
     <property name="text">
      <string>1. Main label</string>
     </property>
    </widget>
   </item>
   <item row="1" column="0">
    <widget class="QPushButton" name="pushButton">
     <property name="text">
      <string>2. Main button, Widget should be below this button</string>
     </property>
    </widget>
   </item>
   <item row="2" column="0">
    <widget class="MyWidget" name="widget" native="true"/>
   </item>
  </layout>
 </widget>
 <customwidgets>
  <customwidget>
   <class>MyWidget</class>
   <extends>QWidget</extends>
   <header>widget.h</header>
   <container>1</container>
  </customwidget>
 </customwidgets>
 <resources/>
 <connections/>
</ui>

screenshot_bug.png

screenshot_workaround.png

widget.py

from PyQt5 import uic


_Ui_Widget, _WidgetBase = uic.loadUiType("widget.ui")


class MyWidget(_WidgetBase):
    def __init__(self, parent=None):
        super().__init__(parent)

        self._ui = _Ui_Widget()
        self._ui.setupUi(self)

widget.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MyWidget</class>
 <widget class="QWidget" name="MyWidget">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>442</width>
    <height>286</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Unrecognized</string>
  </property>
  <layout class="QVBoxLayout" name="verticalLayout" stretch="0,0,0">
   <property name="spacing">
    <number>0</number>
   </property>
   <item>
    <widget class="QPushButton" name="pushButton">
     <property name="text">
      <string>3. Widget button</string>
     </property>
    </widget>
   </item>
   <item>
    <widget class="QLabel" name="label">
     <property name="text">
      <string>4. Widget label</string>
     </property>
    </widget>
   </item>
   <item>
    <widget class="QPushButton" name="pushButton_2">
     <property name="text">
      <string>5. Widget button</string>
     </property>
    </widget>
   </item>
  </layout>
 </widget>
 <resources/>
 <connections/>
</ui>