Re-allow None as argument in "parallel(num_threads=None)", seems to be explicitly allowed.

But at least detect and disallow duplicate keyword arguments to parallel() and prange().

Cython/Compiler/Nodes.py

@@ -8016,10 +8016,16 @@ class ParallelStatNode(StatNode, ParallelNode):
         if self.kwargs:
             # Try to find num_threads and chunksize keyword arguments
             pairs = []
+            seen = set()
             for dictitem in self.kwargs.key_value_pairs:
+                if dictitem.key.value in seen:
+                    error(self.pos, "Duplicate keyword argument found: %s" % dictitem.key.value)
+                seen.add(dictitem.key.value)
                 if dictitem.key.value == 'num_threads':
+                    if not dictitem.value.is_none:
                        self.num_threads = dictitem.value
                 elif self.is_prange and dictitem.key.value == 'chunksize':
+                    if not dictitem.value.is_none:
                         self.chunksize = dictitem.value
                 else:
                     pairs.append(dictitem)

tests/errors/e_cython_parallel.pyx

@@ -149,9 +149,6 @@ with nogil, cython.parallel.parallel():
     with cython.parallel.parallel():
         pass
 
-with nogil, cython.parallel.parallel(num_threads=None):
-    pass
-
 
 _ERRORS = u"""
 3:8: cython.parallel.parallel is not a module
@@ -187,6 +184,4 @@ _ERRORS = u"""
 139:62: Chunksize not valid for the schedule runtime
 145:70: Calling gil-requiring function not allowed without gil
 149:33: Nested parallel with blocks are disallowed
-152:49: Cannot assign None to int
-152:49: Coercion from Python not allowed without the GIL
 """

tests/errors/e_invalid_num_threads.pyx

@@ -9,12 +9,18 @@ with nogil, parallel(num_threads=None):
     pass
 
 # invalid
+with nogil, parallel(num_threads=None, num_threads=None):
+    pass
+
 with nogil, parallel(num_threads=0):
     pass
 
 with nogil, parallel(num_threads=i):
     pass
 
+with nogil, parallel(num_threads=2, num_threads=2):
+    pass
+
 with nogil, parallel(num_threads=2):
     for i in prange(10, num_threads=2):
         pass
@@ -28,7 +34,9 @@ for i in prange(10, nogil=True, num_threads=2):
     pass
 
 _ERRORS = u"""
-e_invalid_num_threads.pyx:12:20: argument to num_threads must be greater than 0
-e_invalid_num_threads.pyx:19:19: num_threads already declared in outer section
-e_invalid_num_threads.pyx:23:19: num_threads must be declared in the parent parallel section
+12:20: Duplicate keyword argument found: num_threads
+15:20: argument to num_threads must be greater than 0
+21:20: Duplicate keyword argument found: num_threads
+25:19: num_threads already declared in outer section
+29:19: num_threads must be declared in the parent parallel section
 """