The code attached produces weird behaviour on macOS (Catalina at least) on Python3.8 / Python3.9. Issue has been reproduced on at least one other mac belonging to a friend. Does not occur on Linux as far as I have tested. I have simplified the code as much as I can, so the example attached does not actually need to use parallelism.

Code description: Read text from a file, change directory using `os.chdir`, spawn 5 processes and pass the `print` function as the handler along with the text.

Expectation: File contents should be printed 5 times

Result: Code tries to load (again) the file in each thread, but fails since the directory was changed.


Precondition: Have `file.txt` in the same directory containing some text

Output when run on Python3.7, I get the following (expected output):

```
Reading file...
Text loaded was: 'file contents'
file contents
file contents
file contents
file contents
file contents
```

When running on 3.8 / 3.9 (on a mac) I get the following:

```
Reading file...
Text loaded was: 'file contents'
Error: /Users/mustafa/dev/ptest/test_dir/file.txt does not exist.
Error: /Users/mustafa/dev/ptest/test_dir/file.txt does not exist.
Error: /Users/mustafa/dev/ptest/test_dir/file.txt does not exist.
```

It seems to me that even opened `file.txt` is loaded in the main thread and text is read from it, this value isn't passed into the spawned processes and they are calling `load()` again (which is where the error message is produced).