A snippet with an implementation of arrow graphics item in Qt4.

arrow.py

class Arrow(QGraphicsItem):

    def __init__(self, parent, position, angle, size = 10.0):
        QGraphicsItem.__init__(self)

        self._parent = parent

        self.setFlag(QGraphicsItem.ItemIsSelectable, False)
        self.setFlag(QGraphicsItem.ItemIsMovable, False)
        self.position = self.mapToScene(position)
        self.angle = angle
        self.size = size
        self.selected = False
        self.inner_offset = size / 2.0
        self.setZValue(20)
        
        self.adjust()


    def type(self):
        return self._parent.type()


    def diagramObject(self):
        return self._parent


    def parent(self):
        return self._parent


    def polygon(self):
        return self._polygon


    def setPolygon(self, polygon):
        self._polygon = polygon


    def updateParams(self, pos = None, angle = None):
        if (pos is None and angle is None):
            Debug.error("`Graphics.Items.Arrow`: No new parameters provided!")

        if (pos):
            self.position = pos

        if (angle):
            self.angle = angle

        self.adjust()


    def adjust(self):
        "Adjusts the arrow position and angle."
        
        fp = self.position                  # Arrow's fixing point on the spline.
        angle = math.radians(self.angle)    # Arrow's direction angle (in radians).

        # Arrow's shape points calculation (side points: p1, p2 and inner point: ip).
        p1 = fp + \
             QPointF(math.sin(angle + math.pi + math.pi/3) * self.size,
                     math.cos(angle + math.pi + math.pi/3) * self.size)

        p2 = fp + \
             QPointF(math.sin(angle - math.pi/3) * self.size,
                     math.cos(angle - math.pi/3) * self.size)

        ip = fp + \
             QPointF(math.sin(math.pi + math.pi/2 + angle) * self.inner_offset,
                     math.cos(math.pi + math.pi/2 + angle) * self.inner_offset)

        # Create and set the new polygon for the arrow item.
        arrow_polygon = QPolygonF([fp, p1, ip, p2])
        self.setPolygon(arrow_polygon)


    def paint(self, painter, option, widget):

        if (self.polygon() is None):
            print self, "- Error: No polygon is set for drawing! Aborting painting."
            return

        painter.setClipRect(option.exposedRect)

        pen = QPen()
        pen.setColor(QColor(150,150,150))
        pen.setStyle(Qt.DotLine)
        painter.setPen(pen)

        painter.setBrush(Qt.black)
        painter.drawPolygon(self.polygon())


    def boundingRect(self):
        return self.polygon().boundingRect()


    def shape(self):
        path = QPainterPath()
        path.addPolygon(self.polygon())
        return path