RuntimeError triggered by pymc3 for figure 5.24

[bsipocz]

at the time of opening this issue I suspect this is a local issue on my laptop, but either case having the issue doesn't hurt.

I now run into pymc3 issues a few times with pycharm mostly when examples are embended in notebooks, but this now consistently appears on the command line, too. I only see the error using python3.8, while it works as expected with identical numpy and pymc3 versions on python3.7.

python book_figures/chapter5/fig_model_comparison_mcmc.py 
Auto-assigning NUTS sampler...
Initializing NUTS using jitter+adapt_diag...
Multiprocess sampling (4 chains in 4 jobs)
NUTS: [M1_log_sigma, M1_mu]
Auto-assigning NUTS sampler...
Initializing NUTS using jitter+adapt_diag...
Multiprocess sampling (4 chains in 4 jobs)
NUTS: [M1_log_sigma, M1_mu]
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/multiprocessing/spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/multiprocessing/spawn.py", line 125, in _main
    prepare(preparation_data)
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/multiprocessing/spawn.py", line 236, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/multiprocessing/spawn.py", line 287, in _fixup_main_from_path
    main_content = runpy.run_path(main_path,
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/runpy.py", line 262, in run_path
    return _run_module_code(code, init_globals, run_name,
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/runpy.py", line 95, in _run_module_code
    _run_code(code, mod_globals, init_globals,
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/Users/bsipocz/munka/devel/worktrees/astroML_figures/giant_figure_generating_branch_ed2/book_figures/chapter5/fig_model_comparison_mcmc.py", line 87, in <module>
    trace1 = pm.sample(draws=2500, tune=100)
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/site-packages/pymc3/sampling.py", line 469, in sample
    trace = _mp_sample(**sample_args)
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/site-packages/pymc3/sampling.py", line 1053, in _mp_sample
    sampler = ps.ParallelSampler(
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/site-packages/pymc3/parallel_sampling.py", line 355, in __init__
    self._samplers = [
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/site-packages/pymc3/parallel_sampling.py", line 356, in <listcomp>
    ProcessAdapter(
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/site-packages/pymc3/parallel_sampling.py", line 242, in __init__
    self._process.start()
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/multiprocessing/context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/multiprocessing/context.py", line 283, in _Popen
    return Popen(process_obj)
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/multiprocessing/popen_spawn_posix.py", line 42, in _launch
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/multiprocessing/spawn.py", line 154, in get_preparation_data
    _check_not_importing_main()
  File "/Users/bsipocz/.pyenv/versions/3.8.0/lib/python3.8/multiprocessing/spawn.py", line 134, in _check_not_importing_main
    raise RuntimeError('''
RuntimeError: 
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.