Hovna

import sys

from PyQt5.QtCore import pyqtSlot
from PyQt5.QtWidgets import QApplication, QWidget, QComboBox, QVBoxLayout


class MainWindow(QWidget):
    def __init__(self, parent=None):
        super().__init__(parent)

        self._items = {
            "a": ["1", "2", "3"],
            "b": ["a", "b", "b"],
        }

        self._combobox_1 = QComboBox()
        self._combobox_2 = QComboBox()

        self._combobox_1.addItems(self._items.keys())

        layout = QVBoxLayout()
        layout.addWidget(self._combobox_1)
        layout.addWidget(self._combobox_2)
        self._handle_item_change()

        self.setLayout(layout)

        self._combobox_1.activated.connect(self._handle_item_change)

    @pyqtSlot()
    def _handle_item_change(self):
        self._combobox_2.clear()
        self._combobox_2.addItems(self._items[self._combobox_1.currentText()])


app = QApplication(sys.argv)

window = MainWindow()
window.show()

sys.exit(app.exec_())