Posts: How to Set the Default Program to Open Files on Windows Using Python

Introduction

I am the class-appointed IT committee member (a burdensome duty), due to the teaching needs of the class, I need to switch to a specific player on a particular day.

I wanted to use Python to automate the setting of the default program for MP4 files, and then I ran the code generated by DeepSeek and other assistants. However, I was stuck by various strange errors. In desperation, I looked for related code on Github.

Fortunately, I really found a Python-written "File Default Master" software on Github, thank you to the original author.

https://github.com/3089464667/default-app

I then read through the code and extracted the core code for setting the default program.

Code Explanation

This code is a Python tool for setting the default program for file types on Windows systems. It modifies the association through command line and registry, and is applicable to Windows 7/10/11.

Core Features

• Set default application: The set_default_app() function specifies the file extension and the corresponding application path, and can automatically configure system associations.
• Compatibility handling: Prioritizes using assoc and ftype command line settings, directly modifies the registry, and handles the user selection verification mechanism in Windows 10/11.
• Verification function: The check_default_app() can check whether the current default application is the specified one.

Usage Example

set_default_app(".txt", "C:\\Windows\\notepad.exe")
if check_default_app(".txt", "C:\\Windows\\notepad.exe"):
    print("设置成功")

Full Code

import winreg
import subprocess
import os
import time
import hashlib

def set_default_app_cmd(file_extension, app_path):
    """
    使用 Windows 命令行工具设置默认打开程序，兼容 Windows 10/11
    """
    try:
        ext = file_extension if file_extension.startswith('.') else '.' + file_extension
        prog_id = ext[1:].upper() + "File"
        subprocess.run(f'assoc {ext}={prog_id}', shell=True, check=True)
        subprocess.run(f'ftype {prog_id}="{app_path}" "%1"', shell=True, check=True)
        return True, ""
    except Exception as e:
        return False, str(e)

def set_default_app(file_extension, app_path, icon_path=None):
    """
    设置文件扩展名的默认打开程序
    参数:
        file_extension: 文件扩展名（如 ".txt" 或 "txt"）
        app_path: 应用程序完整路径
        icon_path: 可选图标路径
    返回: 无
    """
    # 先用命令行设置
    ok, msg = set_default_app_cmd(file_extension, app_path)
    if not ok:
        print(f"命令行设置失败: {msg}")
    
    try:
        # 确保扩展名格式正确
        ext = file_extension if file_extension.startswith('.') else '.' + file_extension
        
        # 创建或打开扩展名对应的注册表键
        with winreg.CreateKey(winreg.HKEY_CLASSES_ROOT, ext) as key:
            prog_id = winreg.QueryValue(key, None)
            if not prog_id:
                prog_id = ext[1:].upper() + "File"
                winreg.SetValue(key, None, winreg.REG_SZ, prog_id)
            
            # 设置打开命令
            with winreg.CreateKey(winreg.HKEY_CLASSES_ROOT, f"{prog_id}\\shell\\open\\command") as cmd_key:
                winreg.SetValue(cmd_key, None, winreg.REG_SZ, f"\"{app_path}\" \"%1\"")
        
        # 处理用户选择（Windows 10/11 需要）
        user_choice_path = f"Software\\Microsoft\\Windows\\CurrentVersion\\Explorer\\FileExts\\{ext}\\"
        try:
            with winreg.OpenKey(winreg.HKEY_CURRENT_USER, user_choice_path, 0, winreg.KEY_ALL_ACCESS) as key:
                winreg.DeleteKey(key, "UserChoice")
        except WindowsError:
            pass
        
        # 生成用户选择哈希（Windows 10/11 验证机制）
        try:
            user_sid = os.getlogin()
        except Exception:
            user_sid = "unknown"
        
        timestamp = int(time.time())
        hash_input = f"{prog_id}{user_sid}{timestamp}".encode('utf-16le')
        hash_value = hashlib.sha256(hash_input).hexdigest()
        
        with winreg.CreateKey(winreg.HKEY_CURRENT_USER, user_choice_path + "UserChoice") as key:
            winreg.SetValueEx(key, "ProgId", 0, winreg.REG_SZ, prog_id)
            winreg.SetValueEx(key, "Hash", 0, winreg.REG_SZ, hash_value)
        
        print(f"成功设置 {ext} 的默认打开程序为: {app_path}")
        
    except Exception as e:
        print(f"设置默认程序失败: {e}")

def check_default_app(file_extension, app_path):
    """
    检查当前扩展名的默认打开程序是否为指定程序
    参数:
        file_extension: 文件扩展名
        app_path: 应用程序路径
    返回: True/False
    """
    ext = file_extension if file_extension.startswith('.') else '.' + file_extension
    try:
        output = subprocess.check_output(f"assoc {ext}", shell=True, encoding="gbk", errors="ignore")
        if "=" not in output:
            return False
        prog_id = output.strip().split("=")[-1]
        output2 = subprocess.check_output(f"ftype {prog_id}", shell=True, encoding="gbk", errors="ignore")
        if app_path.lower() in output2.lower():
            return True
    except Exception:
        pass
    return False

# 使用示例
if __name__ == "__main__":
    # 示例1：设置.txt文件用记事本打开
    set_default_app(".txt", "C:\\Windows\\notepad.exe")
    
    # 示例2：设置.jpg文件用照片查看器打开
    set_default_app("jpg", "C:\\Windows\\System32\\rundll32.exe", "C:\\Windows\\System32\\photo_viewer.dll")
    
    # 检查设置是否成功
    if check_default_app(".txt", "C:\\Windows\\notepad.exe"):
        print("设置成功！")
    else:
        print("设置失败！")