code pattern for process communication

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many programs, especially GUI apps, when a 2nd one is started, tell the 1st one to open the window and the 2nd on exits.  i am looking for a suggested code pattern for this kind of communication setup where the process that is the 2nd instance of an executable can find and set up brief communications with the 1st instance of the same executable.  in particular, i am curious how to avoid finding a process of a different executable that is also being interpreted by python.  in a typical GUI case, this communication would send what X display the 2nd instance was started on and any specified parameters to the 1st instance.

this is not a case of existing code that fails because things have not reached that level yet, so there is no (failing) code to show.  i am still thinking through many potential projects and looking for a pattern for this.