from PyQt4 import QtCore, QtGui
 
 
class CustomCombo(QtGui.QComboBox):
    def __init__(self, parent=None, button=None):
        QtGui.QComboBox.__init__(self, parent)
        self.button = None
        if button is not None:
            self.set_button(button)
         
    def resizeEvent(self, event):
         if self.button is None:
            return
         sz = self.button.sizeHint()
         frame_width = self.style().pixelMetric(QtGui.QStyle.PM_ComboBoxFrameWidth)
         self.button.move(self.rect().right() - frame_width - sz.width() - 17, 
                         (self.rect().bottom() + 1 - sz.height())/2)
 
    def set_button(self, button):
        self.button = button
        button.setParent(self)
        frame_width = self.style().pixelMetric(QtGui.QStyle.PM_DefaultFrameWidth)
        msz = self.minimumSizeHint()
        self.setMinimumSize(
            max(msz.width(), button.sizeHint().width() + frame_width * 2 + 2),
            max(msz.height(), button.sizeHint().height() + frame_width * 2 + 2)
        )
 
 
class MainWidget(QtGui.QWidget):
    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)
        layout = QtGui.QVBoxLayout()
        combo = CustomCombo(parent=self, button=QtGui.QPushButton('test'))
        # or combo.set_button(self.some_existing_button)
        combo.addItems(map(unicode, range(5)))
        layout.addWidget(combo)
        self.setLayout(layout)
         
 
if __name__ == "__main__":
    import sys
    app = QtGui.QApplication(sys.argv)
    widget = MainWidget()
    widget.resize(640, 480)
    widget.show()
    sys.exit(app.exec_())

class CustomWidget(QtGui.QWidget):
    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)
        layout = QtGui.QHBoxLayout()
        self.button = QtGui.QPushButton(maximumWidth=100)
        self.combo = QtGui.QComboBox()
        layout.addWidget(self.combo)
        layout.addWidget(self.button)
        self.setLayout(layout)
    def __getattr__(self, attr):
        return getattr(self.combo, attr)