How to prompt user to open a file with python3? (pygame)

Pygame is a low-level library, so it doesn't have the kind of built-in dialogs you're after. You can create such, but it will be quicker to use the tkinter module which is distributed with Python and provides an interface to the TK GUI libraries. I'd recommend reading the documentation, but here's a function that will pop up a file selection dialog and then return the path selected:

def prompt_file():
    """Create a Tk file dialog and cleanup when finished"""
    top = tkinter.Tk()
    top.withdraw()  # hide window
    file_name = tkinter.filedialog.askopenfilename(parent=top)
    top.destroy()
    return file_name

Here's a small example incorporating this function, pressing Spacebar will popup the dialog:

import tkinter
import tkinter.filedialog
import pygame

WIDTH = 640
HEIGHT = 480
FPS = 30

def prompt_file():
    """Create a Tk file dialog and cleanup when finished"""
    top = tkinter.Tk()
    top.withdraw()  # hide window
    file_name = tkinter.filedialog.askopenfilename(parent=top)
    top.destroy()
    return file_name

pygame.init()
window = pygame.display.set_mode((WIDTH, HEIGHT))
clock = pygame.time.Clock()

f = "<No File Selected>"
frames = 0
running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        elif event.type == pygame.KEYUP:
            if event.key == pygame.K_SPACE:
                f = prompt_file()

    # draw surface - fill background
    window.fill(pygame.color.Color("grey"))
    ## update title to show filename
    pygame.display.set_caption(f"Frames: {frames:10}, File: {f}")
    # show surface
    pygame.display.update()
    # limit frames
    clock.tick(FPS)
    frames += 1
pygame.quit()

Notes: This will pause your game loop, as indicated by the frame counter but as events are being handled by the window manager, this shouldn't be an issue. I'm not sure why I needed the explicit import tkinter.filedialog, but I get an AttributeError if I don't.

As for string entry in pygame, you might want to do this natively, in which case you'd be handling KEYUP events for letter keys to build the string and perhaps finishing when the user presses Enter or your own drawn button. You could continue down the tk path, in which case you'll want to use something like tkinter.simpledialog.askstring(…)

It is possible to make a file dialogue (though not using the native file explorer) using the pygame_gui module.

Simply create an instance of UIFileDialog and grab the path when the user hits 'ok':

file_selection = UIFileDialog(rect=Rect(0, 0, 300, 300), manager=manager, initial_file_path='C:\\')
if event.ui_element == file_selection.ok_button:
    file_path = file_selection.current_file_path

If you want to allow selecting a directory set allow_picking_directories to True, but note that it does not allow picking an initial_file_path.

file_selection = UIFileDialog(rect=Rect(0, 0, 300, 300), manager=manager, allow_picking_directories=True)

Here's the above code in a simple program that allows you to pick a file when the button is clicked:

#!/usr/bin/env python
import pygame
import pygame_gui
from pygame_gui.windows.ui_file_dialog import UIFileDialog
from pygame_gui.elements.ui_button import UIButton
from pygame.rect import Rect

pygame.init()

window_surface = pygame.display.set_mode((800, 600))

background = pygame.Surface((800, 600))
background.fill(pygame.Color('#000000'))

manager = pygame_gui.UIManager((800, 600))
clock = pygame.time.Clock()

file_selection_button = UIButton(relative_rect=Rect(350, 250, 100, 100),
                                 manager=manager, text='Select File')

while 1:
    time_delta = clock.tick(60) / 1000.0

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            quit()

        if event.type == pygame.USEREVENT:
            if event.user_type == pygame_gui.UI_BUTTON_PRESSED:
                if event.ui_element == file_selection_button:
                    file_selection = UIFileDialog(rect=Rect(0, 0, 300, 300), manager=manager, allow_picking_directories=True)

                if event.ui_element == file_selection.ok_button:
                    print(file_selection.current_file_path)

        manager.process_events(event)

    manager.update(time_delta)
    window_surface.blit(background, (0, 0))
    manager.draw_ui(window_surface)

    pygame.display.update()