merge pdf files

import flet as ft
import PyPDF2
from PyPDF2 import PdfWriter

merger = PdfWriter()


def main(page: ft.page):

    def close_banner(e):
        page.banner.open = False
        page.update()

    def show_banner(e):
        page.banner.open = True
        page.update()

    def merge_pdfs(e: ft.FilePickerResultEvent):

        # get file name and password from the corresponding textfields
        merge_file_name = textField_name.value
        file_password = textField_password1.value

        # show warning when no filename is provided
        if not merge_file_name or merge_file_name == ' ':
            # banner for when there is error in file name or file selection
            page.banner = ft.Banner(
                bgcolor=ft.colors.RED_500,
                leading=ft.Icon(ft.icons.WARNING_AMBER_ROUNDED,
                                color=ft.colors.AMBER, size=40),
                content=ft.Text("Please check the file name entered."),
                actions=[ft.TextButton("Dismiss", on_click=close_banner)])
            show_banner(e)
            return None

        # show warning if less than 2 files selected
        if not e.files or len(e.files) < 2:
            # banner for when there is error in file name or file selection
            page.banner = ft.Banner(
                bgcolor=ft.colors.RED_500,
                leading=ft.Icon(ft.icons.WARNING_AMBER_ROUNDED,
                                color=ft.colors.AMBER, size=40),
                content=ft.Text("Please select at least 2 files."),
                actions=[ft.TextButton("Dismiss", on_click=close_banner)])
            show_banner(e)
            return None

        # merge all selected pdf files
        for pdf_file in e.files:
            merger.append(pdf_file.name)

        # check if user provided password
        if file_password:
            merger.encrypt(file_password, use_128bit=True)

        # save new pdf file using value of entry input
        merger.write(merge_file_name + ".pdf")
        merger.close()

        # show messagebox merger is complete
        page.snack_bar = ft.SnackBar(ft.Text("PDF file merge complete."))
        page.snack_bar.open = True
        page.update()

    pick_files_dialog = ft.FilePicker(on_result=merge_pdfs)
    page.overlay.append(pick_files_dialog)

    page.title = "Merge PDF"
    page.icon = ft.icons.INFO
    page.window_width = 320
    page.window_height = 300
    page.bgcolor = "gray"
    page.window_center()
    page.window_resizable = False
    page.vertical_alignment = "center"
    page.horizontal_alignment = "center"

    textField_name = ft.TextField(
        label="File Name",
        autofocus=True,
        width=250,
        bgcolor="gray",
        color="yellow",
        border_color="yellow")

    textField_password1 = ft.TextField(
        label="Optional Password",
        password=True,
        can_reveal_password=True,
        autofocus=True,
        width=250,
        bgcolor="gray",
        color="yellow",
        border_color="yellow")

    select_merge_button = ft.ElevatedButton("Select and Merge",
                                            icon=ft.icons.UPLOAD_FILE,
                                            on_click=lambda _: pick_files_dialog.pick_files(
                                                allow_multiple=True),
                                            color="black",
                                            bgcolor="white",
                                            height=50,
                                            style=ft.ButtonStyle(
                                                shape=ft.RoundedRectangleBorder(
                                                    radius=5)))

    entriesCol = ft.Column(spacing=30,
                           controls=[textField_name,
                                     textField_password1,
                                     select_merge_button],
                           horizontal_alignment="center")

    page.add(entriesCol)


if __name__ == "__main__":

    # app as a desktop app
    ft.app(target=main)

    # app as a web app
    # ft.app(target=main, view=ft.WEB_BROWSER)