使用pyscreenshot截取特定区域时触发RuntimeError问题求助

嘿，我看你在Windows上用pyscreenshot截取指定区域时碰到了多进程相关的错误，这是Windows Python特有的小坑，咱们一步步来解决它。

你的代码 #

import pyscreenshot as ImageGrab
im=ImageGrab.grab(bbox=(10,10,500,500))
im.save('im.png')
if __name__ == '__main__':
    pass

你遇到的报错 #

================================================================================
Traceback (most recent call last):
File "", line 1, in
File "C:\Python27\lib\multiprocessing\forking.py", line 380, in main
prepare(preparation_data)
File "C:\Python27\lib\multiprocessing\forking.py", line 509, in prepare
' parents_main ', file, path_name, etc
File "C:\harsh\CodeForAutomation\latest_25jan2019\aha-gui-fvt\pytesseract\pytes\test_pyscreenshot_localised.py", line 9, in
im=ImageGrab.grab(bbox=(10,10,500,500))
File "build\bdist.win32\egg\pyscreenshot__init__.py", line 67, in grab
File "build\bdist.win32\egg\pyscreenshot_init_.py", line 46, in _grab
File "build\bdist.win32\egg\pyscreenshot\procutil.py", line 31, in run_in_childprocess
File "C:\Python27\lib\multiprocessing\process.py", line 130, in start
self._popen = Popen(self)
File "C:\Python27\lib\multiprocessing\forking.py", line 258, in init
cmd = get_command_line() + [rhandle]
File "C:\Python27\lib\multiprocessing\forking.py", line 358, in get_command_line
is not going to be frozen to produce a Windows executable.''')
RuntimeError: Attempt to start a new process before the current process has finished its bootstrapping phase.
This probably means that you are on Windows and you have forgotten to use the proper idiom in the main module:
if name == 'main':
freeze_support()
...
The "freeze_support()" line can be omitted if the program is not going to be frozen to produce a Windows executable.

问题出在哪？ #

Windows的Python多进程机制和Linux/macOS不一样，它是通过生成新进程并重新导入主模块来创建子进程的。你现在把截图、保存这些操作写在了模块的顶层（也就是if __name__ == '__main__':块外面），这样子进程启动时会重复执行这些代码，进而触发进程启动的循环，就报了这个错。

你代码里虽然有if __name__ == '__main__':块，但关键逻辑都没放进去，等于没起到保护作用。

怎么修复？ #

只需要把所有要执行的逻辑都放进if __name__ == '__main__':这个代码块里，这样只有主进程会执行这些代码，子进程启动时就不会重复跑了：

import pyscreenshot as ImageGrab

if __name__ == '__main__':
    im = ImageGrab.grab(bbox=(10,10,500,500))
    im.save('im.png')

要是之后你需要把这个脚本打包成Windows可执行文件（比如用PyInstaller），记得加上freeze_support()，它能帮打包后的程序正确处理多进程：

import pyscreenshot as ImageGrab
from multiprocessing import freeze_support

if __name__ == '__main__':
    freeze_support()
    im = ImageGrab.grab(bbox=(10,10,500,500))
    im.save('im.png')

其实报错信息里已经明确提示了解决方法，核心就是用if __name__ == '__main__':把主逻辑包裹起来，避免子进程重复执行启动代码。

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