"""Helper module to factorize the conditional multiprocessing import logic

We use a distinct module to simplify import statements and avoid introducing
circular dependencies (for instance for the assert_spawning name).
"""
import os
import warnings


# Obtain possible configuration from the environment, assuming 1 (on)
# by default, upon 0 set to None. Should instructively fail if some non
# 0/1 value is set.
mp = int(os.environ.get('JOBLIB_MULTIPROCESSING', 1)) or None
if mp:
    try:
        import multiprocessing as mp
        import multiprocessing.pool
    except ImportError:
        mp = None

# 2nd stage: validate that locking is available on the system and
#            issue a warning if not
if mp is not None:
    try:
        _sem = mp.Semaphore()
        del _sem # cleanup
    except (ImportError, OSError) as e:
        mp = None
        warnings.warn('%s.  joblib will operate in serial mode' % (e,))


# 3rd stage: backward compat for the assert_spawning helper
if mp is not None:
    try:
        # Python 3.4+
        from multiprocessing.context import assert_spawning
    except ImportError:
        from multiprocessing.forking import assert_spawning
else:
    assert_spawning = None