Interprocess Communication between two python scripts without STDOUT

Normally, you want to change the child process to work like a typical Unix userland tool: the logging and other side-band information goes to stderr (or to a file, or syslog, etc.), and only the actual output goes to stdout.

Then, the problem is easy: just capture stdout to a PIPE that you process, and either capture stderr to a different PIPE, or pass it through to real stderr.

If that's not appropriate for some reason, you need to come up with some other mechanism for IPC: Unix or Windows named pipes, anonymous pipes that you pass by leaking the file descriptor across the fork/exec and then pass the fd as an argument, Unix-domain sockets, TCP or UDP localhost sockets, a higher-level protocol like a web service on top of TCP sockets, mmapped files, anonymous mmaps or pipes that you pass between processes via a Unix-domain socket or Windows API calls, …

As you can see, there are a huge number of options. Without knowing anything about your problem other than that you want "custom messages", it's impossible to tell you which one you want.

While we're at it: If you can rewrite your code around multiprocessing rather than subprocess, there are nice high-level abstractions built in to that module. For example, you can use a Queue that automatically manages synchronization and blocking, and also manages pickling/unpickling so you can just pass any (picklable) object rather than having to worry about serializing to text and parsing the text. Or you can create shared memory holding arrays of int32 objects, or NumPy arrays, or arbitrary structures that you define with ctypes. And so on. Of course you could build the same abstractions yourself, without needing to use multiprocessing, but it's a lot easier when they're there out of the box.

Finally, while your question is tagged ipc and pipe, and titled "Interprocess Communication", your description refers to threads, not processes. If you actually are using a bunch of threads in a single process, you don't need any of this.

You can just stick your results on a queue.Queue, or store them in a list or deque with a Lock around it, or pass in a callback to be called with each new result, or use a higher-level abstraction like concurrent.futures.ThreadPoolExecutor and return a Future object or an iterator of Futures, etc.