File: tree_widget.py

# -*- coding: utf-8 -*-
from PyQt5 import QtCore, QtGui, QtWidgets


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(800, 600)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.gridLayout = QtWidgets.QGridLayout(self.centralwidget)
        self.gridLayout.setObjectName("gridLayout")
        self.main_player_list_treeWidget = QtWidgets.QTreeWidget(self.centralwidget)
        self.main_player_list_treeWidget.setFocusPolicy(QtCore.Qt.NoFocus)
        self.main_player_list_treeWidget.setAcceptDrops(True)
        self.main_player_list_treeWidget.setStyleSheet("")
        self.main_player_list_treeWidget.setDragEnabled(True)
        self.main_player_list_treeWidget.setDragDropOverwriteMode(False)
        self.main_player_list_treeWidget.setDragDropMode(QtWidgets.QAbstractItemView.InternalMove)
        self.main_player_list_treeWidget.setDefaultDropAction(QtCore.Qt.MoveAction)
        self.main_player_list_treeWidget.setAlternatingRowColors(False)
        self.main_player_list_treeWidget.setSelectionMode(QtWidgets.QAbstractItemView.SingleSelection)
        self.main_player_list_treeWidget.setSelectionBehavior(QtWidgets.QAbstractItemView.SelectRows)
        self.main_player_list_treeWidget.setAnimated(True)
        self.main_player_list_treeWidget.setExpandsOnDoubleClick(False)
        self.main_player_list_treeWidget.setObjectName("main_player_list_treeWidget")
        self.main_player_list_treeWidget.header().setCascadingSectionResizes(False)
        self.gridLayout.addWidget(self.main_player_list_treeWidget, 0, 0, 1, 1)
        MainWindow.setCentralWidget(self.centralwidget)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.main_player_list_treeWidget.setStatusTip(_translate("MainWindow", "Δενδρική λίστα με τα περιεχόμενα της κεντρικής λίστας αναπαραγωγής"))
        self.main_player_list_treeWidget.headerItem().setText(0, _translate("MainWindow", "Α/Α"))
        self.main_player_list_treeWidget.headerItem().setText(1, _translate("MainWindow", "Είδος"))
        self.main_player_list_treeWidget.headerItem().setText(2, _translate("MainWindow", "Τίτλος"))
        self.main_player_list_treeWidget.headerItem().setText(3, _translate("MainWindow", "Διάρκεια"))
        self.main_player_list_treeWidget.headerItem().setText(4, _translate("MainWindow", "Πλήθος επαναλήψεων"))
        self.main_player_list_treeWidget.headerItem().setText(5, _translate("MainWindow", "Υπόλοιπο επαναλήψεων"))

File: main.py

from PyQt5 import QtWidgets, QtGui, QtCore
from tree_widget import Ui_MainWindow
import sys

class Example:

    def __init__(self):
        self.app = QtWidgets.QApplication(sys.argv)
        self.MainWindow = QtWidgets.QMainWindow()
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self.MainWindow)
        self.MainWindow.show()
        
        self.fill_tree_widget()
        
        sys.exit(self.app.exec_())
    
    def fill_tree_widget(self):
        parent = self.ui.main_player_list_treeWidget
        self.ui.main_player_list_treeWidget.setStyleSheet("QTreeWidget::item{background-color: white;}QTreeWidget::branch::!has-children:selected,QTreeWidget::item:selected {background-color: rgb(17, 130, 220);}")
        self.tree_items = []
        self.ui.main_player_list_treeWidget.clear()
        
        position = 0
        for i in range(0,400):
            position += 1
            self.tree_items.append(QtWidgets.QTreeWidgetItem(parent))
            item = self.tree_items[-1]
            
            related_data = QtCore.QVariant((item, str(position)))
            item.setData(0, QtCore.Qt.UserRole, related_data)
            
            self.tree_items[-1].setText(0,str(i+1))
            self.tree_items[-1].setText(1,str("fddfs"))
            self.tree_items[-1].setText(2,str("fddfs"))
            self.tree_items[-1].setText(3,str("fddfs"))
            self.tree_items[-1].setText(4,str("fddfs"))
            self.tree_items[-1].setText(5,str("fddfs"))


if __name__ == "__main__":
    program = Example()

Try to comment this two lines in main.py and drag and drop works.

related_data = QtCore.QVariant((item, str(position)))
item.setData(0, QtCore.Qt.UserRole, related_data)

With this two lines this error appear while drag and drop:

TypeError: cannot pickle 'QTreeWidgetItem' object