Issue #28485: Merge single-threading fix from 3.5 into 3.6

Lib/compileall.py

@@ -68,13 +68,13 @@ def compile_dir(dir, maxlevels=10, ddir=None, force=False, rx=None,
     optimize:  optimization level or -1 for level of the interpreter
     workers:   maximum number of parallel workers
     """
+    if workers is not None and workers < 0:
+        raise ValueError('workers must be greater or equal to 0')
+
     files = _walk_dir(dir, quiet=quiet, maxlevels=maxlevels,
                       ddir=ddir)
     success = True
     if workers is not None and workers != 1 and ProcessPoolExecutor is not None:
-        if workers < 0:
-            raise ValueError('workers must be greater or equal to 0')
-
         workers = workers or None
         with ProcessPoolExecutor(max_workers=workers) as executor:
             results = executor.map(partial(compile_file,

Misc/NEWS

@@ -573,6 +573,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #28485: Always raise ValueError for negative
+  compileall.compile_dir(workers=...) parameter, even when multithreading is
+  unavailable.
+
 - Issue #28037: Use sqlite3_get_autocommit() instead of setting
   Connection->inTransaction manually.