Running shell commands without a shell window
I imagine your observation is limited to Windows, since that, I believe, is the only platform on which you'll get that "console flash" issue. If so, then the docs offer the following semi-helpful paragraph:
The startupinfo and creationflags, if given, will be passed to the underlying CreateProcess() function. They can specify things such as appearance of the main window and priority for the new process. (Windows only)
Unfortunately the Python online docs do not reproduce the relevant portion of the Windows API docs, so you have to locate those elsewhere, e.g. starting here on MSDN which leads you here for the creationflags, and specifically to
CREATE_NO_WINDOW0x08000000The process is a console application that is being run without a console window. Therefore, the console handle for the application is not set.
So, adding creationflags=0x08000000 to your Popen call should help (unfortunately I have no Windows-running machine on which to try this out, so you'll have to try it yourself).