> Python 3.7 added the support for the PROC_THREAD_ATTRIBUTE_HANDLE_LIST 
> in subprocess.STARTUPINFO: lpAttributeList['handle_list'] parameter.

The motivating reason to add support for the WinAPI handle list was to allow changing the default to close_fds=True regardless of the need to inherit standard handles. However, even when using the handle list, one still has to make each handle in the list inheritable. Thus concurrent calls to os.system() and os.spawn*() -- which are not implemented via subprocess.Popen(), but should be -- may leak the handles in the list. If the default is changed to close_fds=False, then by default concurrent Popen() calls may also leak temporarily inheritable handles when a handle list isn't used to constrain inheritance.

Background

Windows implicitly duplicates standard I/O handles from a parent process to a child process if they're both console applications and the child inherits the console session. However, subprocess.Popen() requires standard I/O inheritance to work consistently even without an inherited console session. It explicitly inherits standard handles in the STARTUPINFO record, which requires CreateProcessW to be called with bInheritHandles as TRUE. In 3.7+, Popen() also passes the standard-handle values in a PROC_THREAD_ATTRIBUTE_HANDLE_LIST attribute that constrains inheritance, but handles in the list still have to be made inheritable before calling CreateProcessW. Thus they may be leaked by concurrent CreateProcessW calls that inherit handles without a constraining handle list.