[jackie_w]

I remember going through this exact same learning process. Here's some minimal boiler-plate code for running a modal QProgressDialog with a Cancel button in a calibre User Interface plugin:

Spoiler:

from PyQt5.Qt import (QProgressDialog, QTimer)

class MyProgressDialog(QProgressDialog):

    def __init__(self, gui, book_ids, arg1, arg2, ...):
        self.total_books = len(book_ids)
        QProgressDialog.__init__(self, '', 'Cancel', 0, self.total_books, gui)
        self.gui = gui
        self.book_ids = book_ids
        self.db = self.gui.current_db.new_api
        # ... etc etc ...
        
        self.i = 0
        
        self.setWindowTitle('blah, blah')
        icon = get_icons('images/plugin_icon.png')
        self.setWindowIcon(icon)
        
        QTimer.singleShot(0, self.do_book_action)
        self.exec_()

    def do_book_action(self):
        # Process a single book 
        if self.wasCanceled():
            return self.do_close()
        if self.i >= self.total_books:
            return self.do_close()
        
        # get data for current book, e.g.
        book_id = self.book_ids[self.i]
        title = self.db.field_for('title', book_id)
        pathtoepub = self.db.format(book_id, 'EPUB', as_path=True)
        # ... do stuff ...
        
        self.i += 1
        self.setLabelText('%d of %d' % (self.i, self.total_books))
        self.setValue(self.i)
        QTimer.singleShot(0, self.do_book_action)

    def do_close(self):
        self.hide()
        self.gui = None

Then to run the QProgressDialog from your main process:

dlg = MyProgressDialog(self.gui, self.book_ids, arg1, arg2, ...)
if dlg.wasCanceled():
    # do cancelled stuff
else:
    # do ran-to-completion stuff