Detailed explanation of python3+PyQt5 custom view

#!/usr/bin/env python3

import gzip
import os
import platform
import sys
from PyQt5.QtCore import (QAbstractTableModel, QDateTime, QModelIndex,
    QSize, QTimer, QVariant, Qt,pyqtSignal)
from PyQt5.QtGui import ( QColor, QCursor, QFont,
    QFontDatabase, QFontMetrics, QPainter, QPalette, QPixmap)
from PyQt5.QtWidgets import QApplication,QDialog,QHBoxLayout, QLabel, QMessageBox,QScrollArea, QSplitter, QTableView,QWidget


(TIMESTAMP, TEMPERATURE, INLETFLOW, TURBIDITY, CONDUCTIVITY,
 COAGULATION, RAWPH, FLOCCULATEDPH) = range(8)

TIMESTAMPFORMAT = "yyyy-MM-dd hh:mm"


class WaterQualityModel(QAbstractTableModel):

  def __init__(self, filename):
    super(WaterQualityModel, self).__init__()
    self.filename = filename
    self.results = []


  def load(self):
    self.beginResetModel()
    exception = None
    fh = None
    try:
      if not self.filename:
        raise IOError("no filename specified for loading")
      self.results = []
      line_data = gzip.open(self.filename).read()
      for line in line_data.decode("utf8").splitlines():
        parts = line.rstrip().split(",")
        date = QDateTime.fromString(parts[0] + ":00",
                      Qt.ISODate)

        result = [date]
        for part in parts[1:]:
          result.append(float(part))
        self.results.append(result)

    except (IOError, ValueError) as e:
      exception = e
    finally:
      if fh is not None:
        fh.close()
      self.endResetModel()
      if exception is not None:
        raise exception


  def data(self, index, role=Qt.DisplayRole):
    if (not index.isValid() or
      not (0 <= index.row() < len(self.results))):
      return QVariant()
    column = index.column()
    result = self.results[index.row()]
    if role == Qt.DisplayRole:
      item = result[column]
      if column == TIMESTAMP:
        #item = item.toString(TIMESTAMPFORMAT)
        item=item
      else:
        #item = QString("%1").arg(item, 0, "f", 2)
        item = "{0:.2f}".format(item)
      return item
    elif role == Qt.TextAlignmentRole:
      if column != TIMESTAMP:
        return QVariant(int(Qt.AlignRight|Qt.AlignVCenter))
      return QVariant(int(Qt.AlignLeft|Qt.AlignVCenter))
    elif role == Qt.TextColorRole and column == INLETFLOW:
      if result[column] < 0:
        return QVariant(QColor(Qt.red))
    elif (role == Qt.TextColorRole and
       column in (RAWPH, FLOCCULATEDPH)):
      ph = result[column]
      if ph < 7:
        return QVariant(QColor(Qt.red))
      elif ph >= 8:
        return QVariant(QColor(Qt.blue))
      else:
        return QVariant(QColor(Qt.darkGreen))
    return QVariant()


  def headerData(self, section, orientation, role=Qt.DisplayRole):
    if role == Qt.TextAlignmentRole:
      if orientation == Qt.Horizontal:
        return QVariant(int(Qt.AlignCenter))
      return QVariant(int(Qt.AlignRight|Qt.AlignVCenter))
    if role != Qt.DisplayRole:
      return QVariant()
    if orientation == Qt.Horizontal:
      if section == TIMESTAMP:
        return "Timestamp"
      elif section == TEMPERATURE:
        return "\u00B0" +"C"
      elif section == INLETFLOW:
        return "Inflow"
      elif section == TURBIDITY:
        return "NTU"
      elif section == CONDUCTIVITY:
        return "\u03BCS/cm"
      elif section == COAGULATION:
        return "mg/L"
      elif section == RAWPH:
        return "Raw Ph"
      elif section == FLOCCULATEDPH:
        return "Floc Ph"
    return int(section + 1)


  def rowCount(self, index=QModelIndex()):
    return len(self.results)


  def columnCount(self, index=QModelIndex()):
    return 8


class WaterQualityView(QWidget):
  clicked = pyqtSignal(QModelIndex)
  FLOWCHARS = (chr(0x21DC), chr(0x21DD), chr(0x21C9))

  def __init__(self, parent=None):
    super(WaterQualityView, self).__init__(parent)
    self.scrollarea = None
    self.model = None
    self.setFocusPolicy(Qt.StrongFocus)
    self.selectedRow = -1
    self.flowfont = self.font()
    size = self.font().pointSize()
    if platform.system() == "Windows":
      fontDb = QFontDatabase()
      for face in [face.toLower() for face in fontDb.families()]:
        if face.contains("unicode"):
          self.flowfont = QFont(face, size)
          break
      else:
        self.flowfont = QFont("symbol", size)
        WaterQualityView.FLOWCHARS = (chr(0xAC), chr(0xAE),
                       chr(0xDE))


  def setModel(self, model):
    self.model = model
    #self.connect(self.model,
    #    SIGNAL("dataChanged(QModelIndex,QModelIndex)"),
    #    self.setNewSize)
    self.model.dataChanged.connect(self.setNewSize)
    #self.connect(self.model, SIGNAL("modelReset()"), self.setNewSize)
    self.model.modelReset.connect(self.setNewSize)
    self.setNewSize()


  def setNewSize(self):
    self.resize(self.sizeHint())
    self.update()
    self.updateGeometry()


  def minimumSizeHint(self):
    size = self.sizeHint()
    fm = QFontMetrics(self.font())
    size.setHeight(fm.height() * 3)
    return size


  def sizeHint(self):
    fm = QFontMetrics(self.font())
    size = fm.height()
    return QSize(fm.width("9999-99-99 99:99 ") + (size * 4),
           (size / 4) + (size * self.model.rowCount()))


  def paintEvent(self, event):
    if self.model is None:
      return
    fm = QFontMetrics(self.font())
    timestampWidth = fm.width("9999-99-99 99:99 ")
    size = fm.height()
    indicatorSize = int(size * 0.8)
    offset = int(1.5 * (size - indicatorSize))
    minY = event.rect().y()
    maxY = minY + event.rect().height() + size
    minY -= size
    painter = QPainter(self)
    painter.setRenderHint(QPainter.Antialiasing)
    painter.setRenderHint(QPainter.TextAntialiasing)
    y = 0
    for row in range(self.model.rowCount()):
      x = 0
      if minY <= y <= maxY:
        painter.save()
        painter.setPen(self.palette().color(QPalette.Text))
        if row == self.selectedRow:
          painter.fillRect(x, y + (offset * 0.8),
              self.width(), size, self.palette().highlight())
          painter.setPen(self.palette().color(
              QPalette.HighlightedText))
        #timestamp = self.model.data(
            #self.model.index(row, TIMESTAMP)).toDateTime()
        timestamp = self.model.data(self.model.index(row, TIMESTAMP))    
        painter.drawText(x, y + size,
            timestamp.toString(TIMESTAMPFORMAT))
        #print(timestamp.toString(TIMESTAMPFORMAT))
        x += timestampWidth
        temperature = self.model.data(
            self.model.index(row, TEMPERATURE))
        #temperature = temperature.toDouble()[0]
        temperature = float(temperature)
        if temperature < 20:
          color = QColor(0, 0,
              int(255 * (20 - temperature) / 20))
        elif temperature > 25:
          color = QColor(int(255 * temperature / 100), 0, 0)
        else:
          color = QColor(0, int(255 * temperature / 100), 0)
        painter.setPen(Qt.NoPen)
        painter.setBrush(color)
        painter.drawEllipse(x, y + offset, indicatorSize,
                  indicatorSize)
        x += size
        rawPh = self.model.data(self.model.index(row, RAWPH))
        #rawPh = rawPh.toDouble()[0]
        rawPh = float(rawPh)
        if rawPh < 7:
          color = QColor(int(255 * rawPh / 10), 0, 0)
        elif rawPh >= 8:
          color = QColor(0, 0, int(255 * rawPh / 10))
        else:
          color = QColor(0, int(255 * rawPh / 10), 0)
        painter.setBrush(color)
        painter.drawEllipse(x, y + offset, indicatorSize,
                  indicatorSize)
        x += size
        flocPh = self.model.data(
            self.model.index(row, FLOCCULATEDPH))
        #flocPh = flocPh.toDouble()[0]
        flocPh = float(flocPh)
        if flocPh < 7:
          color = QColor(int(255 * flocPh / 10), 0, 0)
        elif flocPh >= 8:
          color = QColor(0, 0, int(255 * flocPh / 10))
        else:
          color = QColor(0, int(255 * flocPh / 10), 0)
        painter.setBrush(color)
        painter.drawEllipse(x, y + offset, indicatorSize,
                  indicatorSize)
        painter.restore()
        painter.save()
        x += size
        flow = self.model.data(
            self.model.index(row, INLETFLOW))
        #flow = flow.toDouble()[0]
        flow = float(flow)
        char = None
        if flow <= 0:
          char = WaterQualityView.FLOWCHARS[0]
        elif flow < 3.6:
          char = WaterQualityView.FLOWCHARS[1]
        elif flow > 4.7:
          char = WaterQualityView.FLOWCHARS[2]
        if char is not None:
          painter.setFont(self.flowfont)
          painter.drawText(x, y + size, char)
        painter.restore()
      y += size
      if y > maxY:
        break


  def mousePressEvent(self, event):
    fm = QFontMetrics(self.font())
    self.selectedRow = event.y() // fm.height()
    self.update()
    #self.emit(SIGNAL("clicked(QModelIndex)"),
    #     self.model.index(self.selectedRow, 0))
    self.clicked.emit(self.model.index(self.selectedRow, 0))



  def keyPressEvent(self, event):
    if self.model is None:
      return
    row = -1
    if event.key() == Qt.Key_Up:
      row = max(0, self.selectedRow - 1)
    elif event.key() == Qt.Key_Down:
      row = min(self.selectedRow + 1, self.model.rowCount() - 1)
    if row != -1 and row != self.selectedRow:
      self.selectedRow = row
      if self.scrollarea is not None:
        fm = QFontMetrics(self.font())
        y = fm.height() * self.selectedRow
        print(y)
        self.scrollarea.ensureVisible(0, y)
      self.update()
      #self.emit(SIGNAL("clicked(QModelIndex)"),
      #     self.model.index(self.selectedRow, 0))
      self.clicked.emit(self.model.index(self.selectedRow, 0))
    else:
      QWidget.keyPressEvent(self, event)


class MainForm(QDialog):

  def __init__(self, parent=None):
    super(MainForm, self).__init__(parent)

    self.model = WaterQualityModel(os.path.join(
        os.path.dirname(__file__), "waterdata.csv.gz"))
    self.tableView = QTableView()
    self.tableView.setAlternatingRowColors(True)
    self.tableView.setModel(self.model)
    self.waterView = WaterQualityView()
    self.waterView.setModel(self.model)
    scrollArea = QScrollArea()
    scrollArea.setBackgroundRole(QPalette.Light)
    scrollArea.setWidget(self.waterView)
    self.waterView.scrollarea = scrollArea

    splitter = QSplitter(Qt.Horizontal)
    splitter.addWidget(self.tableView)
    splitter.addWidget(scrollArea)
    splitter.setSizes([600, 250])
    layout = QHBoxLayout()
    layout.addWidget(splitter)
    self.setLayout(layout)

    self.setWindowTitle("Water Quality Data")
    QTimer.singleShot(0, self.initialLoad)


  def initialLoad(self):
    QApplication.setOverrideCursor(QCursor(Qt.WaitCursor))
    splash = QLabel(self)
    pixmap = QPixmap(os.path.join(os.path.dirname(__file__),
        "iss013-e-14802.jpg"))
    #print(os.path.join(os.path.dirname(__file__),
    #    "iss013-e-14802.jpg"))
    splash.setPixmap(pixmap)
    splash.setWindowFlags(Qt.SplashScreen)
    splash.move(self.x() + ((self.width() - pixmap.width()) / 2),
          self.y() + ((self.height() - pixmap.height()) / 2))
    splash.show()
    QApplication.processEvents()
    try:
      self.model.load()
    except IOError as e:
      QMessageBox.warning(self, "Water Quality - Error", e)
    else:
      self.tableView.resizeColumnsToContents()
    splash.close()
    QApplication.processEvents()
    QApplication.restoreOverrideCursor()


app = QApplication(sys.argv)
form = MainForm()
form.resize(850, 620)
form.show()
app.exec_()