Switch widget in QStackWidget from a button in another file

There is a lot going on here, but let's break it down.

The main problem seems to be juggling between modules. Eventhough it might seem appealing to import the modules back and forth, it doesn't really work. What you need to look for, is the built-in Signals module that you can utilize.

Another bigger problem is that you are re-assigning some attributes eventhough you really shouldn't. You also should revisit the condition you are using to assign the .setCurrentWidget. Currently the condition reads as if list1 doesn't exist, do this. Else, do the other. Also, switch_widget should be outside of the def __init__(self):.

I rewrote some parts of the code to make it work with signals as an example for you.

mainwindow.py

import sys
import os
import pathlib
from PySide2.QtWidgets import *
from PySide2 import *
from PySide2.QtCore import *
from PySide2.QtGui import *
from widget_test import widget



list1 = ["item1", "item2", "item3"]


class MainWindow(QWidget):
    def __init__(self):
        super(MainWindow, self).__init__()
        self.resize(400, 300)

        self.toolbar = QWidget()
        self.toolbar.setFixedHeight(30)
        self.toolbar.setStyleSheet("background: grey;")

        self.button = QPushButton("Click here!")
        t_layout = QHBoxLayout()
        t_layout.setMargin(0)
        t_layout.addWidget(self.button)
        self.toolbar.setLayout(t_layout)

        self.p1_label = QLabel("Such empty!")
        self.p1_label.setStyleSheet("font-size: 30px;")
        self.p1_label.setAlignment(Qt.AlignCenter)

        self.p2_widget = QWidget()
        self.p2_widget.setStyleSheet("background: orange;")

        self.p3 = None

        self.sw = QStackedWidget()
        self.sw.addWidget(self.p1_label)
        self.sw.addWidget(self.p2_widget)

        if not list1:
            self.sw.setCurrentWidget(self.p1_label)
        else:
           self.sw.setCurrentWidget(self.p2_widget)

        self.mw_layout = QVBoxLayout()
        self.mw_layout.addWidget(self.toolbar)
        self.mw_layout.addWidget(self.sw)
        self.setLayout(self.mw_layout)

        self.button.clicked.connect(self.switch_widget)

    def switch_widget(self):
        self.p3 = widget()
        self.p3.update_signal.connect(self.switch_back)
        self.sw.addWidget(self.p3)
        self.sw.setCurrentWidget(self.p3)


    def switch_back(self):
        self.sw.removeWidget(self.p3)
        if list1:
            self.sw.setCurrentWidget(self.p1_label)
        else:
            self.sw.setCurrentWidget(self.p2_widget)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    mw = MainWindow()
    mw.show()
    sys.exit(app.exec_())

widget.py

import sys
import os
import pathlib
import datetime
from PySide2.QtWidgets import *
from PySide2 import *
from PySide2.QtCore import *
from PySide2.QtGui import *
from PySide2.QtCore import Signal


class widget(QWidget):
    update_signal = Signal()
    def __init__(self):
        super(widget, self).__init__()
        self.setStyleSheet("background: teal;")

        widget_label = QLabel("fluid dynamics is cool")
        show_pop_up = QPushButton("show pop up")

        pop_up = QDialog(self)
        pop_up_label = QLabel("click below to, hopefully, get outta here")
        get_outta_here = QPushButton("get outta here")
        pop_up_layout = QVBoxLayout()
        pop_up_layout.addWidget(pop_up_label)
        pop_up_layout.addWidget(get_outta_here)
        pop_up.setLayout(pop_up_layout)

        def show_popup():
            pop_up.show()

        def get_out():
            self.update_signal.emit()
            pop_up.reject()

        get_outta_here.clicked.connect(get_out)

        show_pop_up.clicked.connect(show_popup)

        widget_layout = QVBoxLayout()
        widget_layout.addWidget(widget_label)
        widget_layout.addWidget(show_pop_up)
        self.setLayout(widget_layout)

Finally, check Python coding conventions for naming and other "minor" details.