Android设备中Kivy-Python应用文件读写权限申请失败致启动异常

问题描述 #

开发基于Kivy的Python Android应用时，尝试在外部存储创建文件夹并写入文件，首次启动虽弹出权限申请，但仍触发权限拒绝错误：

02-22 10:23:22.339 29320 29348 I python  :  PermissionError: [Errno 13] Permission denied: '/storage/emulated/0/folder'

用户代码：

import kivy
from kivy.app import App
from kivy.uix.label import Label
from kivy.utils import platform
from pathlib import Path

class MyApp(App):
    def build(self):
        text = "1"+App.get_running_app().user_data_dir + '\n'
        self.label = Label(text=text)
        if platform == "android":
            from android.storage import app_storage_path,primary_external_storage_path
            from android.permissions import request_permissions, check_permission, Permission
            self.label.text += primary_external_storage_path() +'\n'
            folder_path = Path(primary_external_storage_path(),"folder")
            request_permissions([Permission.READ_EXTERNAL_STORAGE, Permission.WRITE_EXTERNAL_STORAGE])
            folder_path.mkdir(parents=True, exist_ok=True)
            file = Path(folder_path,"file.txt")
        else:
            file = Path(Path.home(),"file.txt")
        with open(file,"w") as tfile:
            tfile.write("test test")
        return self.label

if __name__ == '__main__':
    MyApp().run()

buildozer.spec配置：

# (list) Permissions
# (See https://python-for-android.readthedocs.io/en/latest/buildoptions/#build-options-1 for all the supported syntaxes and properties)
android.permissions = android.permission.WRITE_EXTERNAL_STORAGE, android.permission.READ_EXTERNAL_STORAGE, android.permission.INTERNET

错误原因 #

1. 权限申请异步性：request_permissions是异步调用，调用后不会立即获得权限，代码中直接执行文件夹创建操作时，权限尚未通过，导致拒绝。
2. Scoped Storage限制：Android 10（API 29）及以上版本引入Scoped Storage机制，即使申请了读写权限，直接操作外部存储根目录的文件夹也会被系统限制。

解决方案 #

1. 异步处理权限申请结果 #

将文件操作逻辑放到权限申请成功的回调函数中，确保只有在权限通过后才执行：

import kivy
from kivy.app import App
from kivy.uix.label import Label
from kivy.utils import platform
from pathlib import Path

class MyApp(App):
    def build(self):
        self.text = "1" + App.get_running_app().user_data_dir + '\n'
        self.label = Label(text=self.text)
        if platform == "android":
            from android.storage import primary_external_storage_path
            from android.permissions import request_permissions, check_permission, Permission
            
            self.label.text += primary_external_storage_path() + '\n'
            self.folder_path = Path(primary_external_storage_path(), "folder")
            
            # 先检查权限状态，未授权则申请
            if not all([check_permission(Permission.READ_EXTERNAL_STORAGE), check_permission(Permission.WRITE_EXTERNAL_STORAGE)]):
                request_permissions([Permission.READ_EXTERNAL_STORAGE, Permission.WRITE_EXTERNAL_STORAGE], self.on_permission_result)
            else:
                self.perform_file_operations()
        else:
            self.file = Path(Path.home(), "file.txt")
            self.perform_file_operations()
        return self.label

    def on_permission_result(self, permissions, grants):
        # 确认所有权限都被授权
        if all(grants):
            self.perform_file_operations()
        else:
            self.label.text += "\n权限申请被拒绝，无法执行文件操作"

    def perform_file_operations(self):
        if platform == "android":
            self.folder_path.mkdir(parents=True, exist_ok=True)
            self.file = Path(self.folder_path, "file.txt")
        with open(self.file, "w") as tfile:
            tfile.write("test test")
        self.label.text += "\n文件操作完成"

if __name__ == '__main__':
    MyApp().run()

2. 适配Scoped Storage #

对于Android 10+设备，优先使用以下合规路径：

• 应用专属外部存储：使用app_storage_path()，无需额外权限，卸载应用时数据会被清理：

  from android.storage import app_storage_path
  folder_path = Path(app_storage_path(), "folder")
  

• 公共媒体/文档目录：如果需要创建用户可见的公共文件夹，选择Documents等系统允许的目录：

  from android.storage import primary_external_storage_path
  docs_path = Path(primary_external_storage_path(), "Documents")
  folder_path = docs_path / "my_app_folder"
  

3. 优化buildozer配置 #

根据目标Android版本调整权限：

• 针对API 29以下，保留原有读写权限即可；
• 针对API 30+，若确实需要访问所有外部文件，可添加MANAGE_EXTERNAL_STORAGE权限（不推荐，仅特殊场景使用）：

android.permissions = android.permission.WRITE_EXTERNAL_STORAGE, android.permission.READ_EXTERNAL_STORAGE
# API 30+全存储访问权限（可选）
# android.permissions = android.permission.WRITE_EXTERNAL_STORAGE, android.permission.READ_EXTERNAL_STORAGE, android.permission.MANAGE_EXTERNAL_STORAGE

4. 增加权限前置检查 #

在每次执行文件操作前，先检查权限状态，避免因用户后续取消权限导致报错。

内容的提问来源于stack exchange，提问作者user23447338