how to have a directory dialog

Just as simple as that:

folderpath = QtWidgets.QFileDialog.getExistingDirectory(self, 'Select Folder')

Here, self represents the parent window usually the QMainWindow object.

Similarly for File dialog:

filepath = QtWidgets.QFileDialog.getOpenFileName(self, 'Hey! Select a File')

From inside your QDialog/QWidget class, you should be able to do:

file = str(QFileDialog.getExistingDirectory(self, "Select Directory"))