title: How to use this rewritten QLabel class in pyqt5?
body: <p>I rewritten the OLabel class naming roiLabel(), and I want to get the ROI in a frame to tracking object.</p>
  <pre><code>class roiLabel(QLabel):  # use roiLabel class to draw ROI
      def __init__(self):
          super().__init__()
          self.open_keyboard_flag = False
          self.open_mouse_flag = False
          self.draw_roi_flag = False
          self.select_roi_flag = False
          self.clear_flag = False
          self.x0 = 0
          self.y0 = 0
          self.x1 = 0
          self.y1 = 0
          self.rect = QRect()
          self.label_show = None
  
      # when clicking mouse, record the top left corner position.
      def mousePressEvent(self, event: QMouseEvent):
          if event.buttons() == Qt.LeftButton:
              if self.open_mouse_flag is True:
                  self.select_roi_flag = True
                  self.x0 = event.x()
                  self.y0 = event.y()
  
      # release the mouse
      def mouseReleaseEvent(self, event):
          self.select_roi_flag = False
  </code></pre>
  <p>And I instantiate this class to the <code>self.show_windows = roiLabel()</code>.
  I have the question that how to call the function <code>mousePressEvent()</code>:</p>
  <pre><code>    def mousePressEvent(self, event: QMouseEvent):
          if event.buttons() == Qt.LeftButton:
              if self.open_mouse_flag is True:
                  self.select_roi_flag = True
                  self.x0 = event.x()
                  self.y0 = event.y()
  </code></pre>
  <p><em>what should I fill this in (<strong>?</strong>)</em>:</p>
  <pre><code>self.show_windows = roiLabel()
  self.show_windows.mousePressEvent(?)
  </code></pre>
  <p>I have no idea how to fill using <strong>self</strong> and <strong>event: QMouseEvent</strong> two arguments....</p>
  <p>Thanks in advance!</p>