python3+PyQt5 implements custom fluid mixing widgets

#!/usr/bin/env python3

from PyQt5.QtCore import (QPointF, QSize, Qt,pyqtSignal)
from PyQt5.QtWidgets import (QApplication,
  QFrame, QLabel,
  QSizePolicy, QSpinBox, QWidget)
from PyQt5.QtGui import QColor,QPainter,QFontMetricsF,QBrush,QLinearGradient,QPolygon,QPolygonF

class YPipeWidget(QWidget):
 signal_valuechanged = pyqtSignal(int,int)
 def __init__(self, leftFlow=0, rightFlow=0, maxFlow=100,
     parent=None):
  super(YPipeWidget, self).__init__(parent)

  self.leftSpinBox = QSpinBox(self)
  self.leftSpinBox.setRange(0, maxFlow)
  self.leftSpinBox.setValue(leftFlow)
  self.leftSpinBox.setSuffix(" l/s")
  self.leftSpinBox.setAlignment(Qt.AlignRight|Qt.AlignVCenter)
  self.leftSpinBox.valueChanged.connect(self.valueChanged)
  self.rightSpinBox = QSpinBox(self)
  self.rightSpinBox.setRange(0, maxFlow)
  self.rightSpinBox.setValue(rightFlow)
  self.rightSpinBox.setSuffix(" l/s")
  self.rightSpinBox.setAlignment(Qt.AlignRight|Qt.AlignVCenter)
  self.rightSpinBox.valueChanged.connect(self.valueChanged)

  self.label = QLabel(self)
  self.label.setFrameStyle(QFrame.StyledPanel|QFrame.Sunken)
  self.label.setAlignment(Qt.AlignCenter)
  fm = QFontMetricsF(self.font())
  self.label.setMinimumWidth(fm.width(" 999 l/s "))

  self.setSizePolicy(QSizePolicy(QSizePolicy.Expanding,
          QSizePolicy.Expanding))
  self.setMinimumSize(self.minimumSizeHint())
  self.valueChanged()


 def valueChanged(self):
  a = self.leftSpinBox.value()
  b = self.rightSpinBox.value()
  self.label.setText("{0} l/s".format(a + b))
  self.signal_valuechanged.emit(a,b)
  self.update()


 def values(self):
  return self.leftSpinBox.value(), self.rightSpinBox.value()


 def minimumSizeHint(self):
  return QSize(self.leftSpinBox.width() * 3,
      self.leftSpinBox.height() * 5)


 def resizeEvent(self, event=None):
  fm = QFontMetricsF(self.font())
  x = (self.width() - self.label.width()) / 2
  y = self.height() - (fm.height() * 1.5)
  self.label.move(x, y)
  y = self.height() / 60.0
  x = (self.width() / 4.0) - self.leftSpinBox.width()
  self.leftSpinBox.move(x, y)
  x = self.width() - (self.width() / 4.0)
  self.rightSpinBox.move(x, y)


 def paintEvent(self, event=None):
  LogicalSize = 100.0

  def logicalFromPhysical(length, side):
   return (length / side) * LogicalSize

  fm = QFontMetricsF(self.font())
  ymargin = ((LogicalSize / 30.0) +
     logicalFromPhysical(self.leftSpinBox.height(),
          self.height()))
  ymax = (LogicalSize -
    logicalFromPhysical(fm.height() * 2, self.height()))
  width = LogicalSize / 4.0
  cx, cy = LogicalSize / 2.0, LogicalSize / 3.0
  ax, ay = cx - (2 * width), ymargin
  bx, by = cx - width, ay
  dx, dy = cx + width, ay
  ex, ey = cx + (2 * width), ymargin
  fx, fy = cx + (width / 2), cx + (LogicalSize / 24.0)
  gx, gy = fx, ymax
  hx, hy = cx - (width / 2), ymax
  ix, iy = hx, fy

  painter = QPainter(self)
  painter.setRenderHint(QPainter.Antialiasing)
  side = min(self.width(), self.height())
  painter.setViewport((self.width() - side) / 2,
       (self.height() - side) / 2, side, side)
  painter.setWindow(0, 0, LogicalSize, LogicalSize)

  painter.setPen(Qt.NoPen)

  gradient = QLinearGradient(QPointF(0, 0),
           QPointF(0, 100))
  gradient.setColorAt(0, Qt.white)
  a = self.leftSpinBox.value()
  gradient.setColorAt(1, (Qt.red if a != 0 else Qt.white))
  painter.setBrush(QBrush(gradient))
  painter.drawPolygon(QPolygonF([QPointF(ax, ay), QPointF(bx, by), QPointF(cx, cy), QPointF(ix, iy)]))

  gradient = QLinearGradient(QPointF(0, 0), QPointF(0, 100))
  gradient.setColorAt(0, Qt.white)
  b = self.rightSpinBox.value()
  gradient.setColorAt(1, (Qt.blue if b != 0
        else Qt.white))
  painter.setBrush(QBrush(gradient))
  painter.drawPolygon(QPolygonF([QPointF(cx, cy), QPointF(dx, dy),QPointF(ex, ey),QPointF(fx, fy)]))

  if (a + b) == 0:
   color = QColor(Qt.white)
  else:
   ashare = (a / (a + b)) * 255.0
   bshare = 255.0 - ashare
   color = QColor(ashare, 0, bshare)
  gradient = QLinearGradient(QPointF(0, 0), QPointF(0, 100))
  gradient.setColorAt(0, Qt.white)
  gradient.setColorAt(1, color)
  painter.setBrush(QBrush(gradient))
  painter.drawPolygon(QPolygonF(
    [QPointF(cx, cy),QPointF(fx, fy),QPointF(gx, gy), QPointF(hx, hy),QPointF(ix, iy)]))

  painter.setPen(Qt.black)
  painter.drawPolyline(QPolygonF([QPointF(ax, ay), QPointF(ix, iy),QPointF(hx, hy)]))
  painter.drawPolyline(QPolygonF([QPointF(gx, gy), QPointF(fx, fy), QPointF(ex, ey)]))
  painter.drawPolyline(QPolygonF([QPointF(bx, by), QPointF(cx, cy), QPointF(dx, dy)]))

if __name__ == "__main__":
 import sys

 def valueChanged(a, b):
  print(a, b)

 app = QApplication(sys.argv)
 form = YPipeWidget()
 form.signal_valuechanged.connect(valueChanged)
 form.setWindowTitle("YPipe")
 form.move(0, 0)
 form.show()
 form.resize(400, 400)
 app.exec_()