Minimize size of Qt widget

Question

Following .ui file, created with Qt 5.7 Designer, shows a text box and a spacer:

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>Form</class>
 <widget class="QWidget" name="Form">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>310</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Form</string>
  </property>
  <layout class="QVBoxLayout" name="verticalLayout">
   <item>
    <widget class="QPlainTextEdit" name="plainTextEdit">
     <property name="sizePolicy">
      <sizepolicy hsizetype="Expanding" vsizetype="MinimumExpanding">
       <horstretch>0</horstretch>
       <verstretch>0</verstretch>
      </sizepolicy>
     </property>
     <property name="plainText">
      <string>abc</string>
     </property>
    </widget>
   </item>
   <item>
    <spacer name="verticalSpacer">
     <property name="orientation">
      <enum>Qt::Vertical</enum>
     </property>
     <property name="sizeHint" stdset="0">
      <size>
       <width>20</width>
       <height>20</height>
      </size>
     </property>
    </spacer>
   </item>
  </layout>
 </widget>
 <resources/>
 <connections/>
</ui>

For example, if the text field has "abc", it should be one line high; if it has two lines (say "abc\nabc"), OR one long line that requires two lines to display, then the text box should be two lines high.

Minimum height is 0. I tried changing the sizing policy of the text box so that the widget is as small as possible, but none do this.

Equivalent Python the code to do this is:

app=QApplication([])

widget = QWidget()
widget.setLayout(QVBoxLayout())

text_widget = QPlainTextEdit('abc')
# text_widget.setSizePolicy(QSizePolicy.Expanding, QSizePolicy.MinimumExpanding)
widget.layout().addWidget(text_widget)

spacer = QSpacerItem(20, 40, vPolicy=QSizePolicy.Expanding)
widget.layout().addSpacerItem(spacer)

widget.show()
app.exec_()