Hi and welcome to devnet,

Without a minimal code example showing what you do it's not possible to see what might be wrong.

@hcroope
For PyQt question you might ask at: https://riverbankcomputing.com/mailman/listinfo/pyqt

@asanka424 Thanks a lot! Your suggestion works. After moving the connect line into the constructor, I also needed to change this:

filename = QFileDialog.getSaveFileName(self, "Select output file ","", '*.txt')

to this:

filename = QFileDialog.getSaveFileName(None, "Select output file ","", '*.txt')