Issue #12285: multiprocessing.Pool() raises a ValueError if the number of

processes if negative or null.

Lib/multiprocessing/pool.py

@@ -125,6 +125,8 @@ class Pool(object):
                 processes = cpu_count()
             except NotImplementedError:
                 processes = 1
+        if processes < 1:
+            raise ValueError("Number of processes must be at least 1")
 
         if initializer is not None and not hasattr(initializer, '__call__'):
             raise TypeError('initializer must be a callable')