gh-145242: Add early validation for target in Thread and Process

Author: gourijain029-del

Lib/multiprocessing/process.py

@@ -79,7 +79,10 @@ def _Popen(self):
 
     def __init__(self, group=None, target=None, name=None, args=(), kwargs=None,
                  *, daemon=None):
-        assert group is None, 'group argument must be None for now'
+        if group is not None:
+            raise TypeError("group argument must be None for now")
+        if target is not None and not callable(target):
+            raise TypeError("target must be callable")
         count = next(_process_counter)
         self._identity = _current_process._identity + (count,)
         self._config = _current_process._config.copy()

Lib/test/_test_multiprocessing.py

@@ -326,6 +326,12 @@ def test_current(self):
         self.assertEqual(current.ident, os.getpid())
         self.assertEqual(current.exitcode, None)
 
+    def test_invalid_args(self):
+        with self.assertRaisesRegex(TypeError, "group argument must be None"):
+            self.Process(group="not None")
+        with self.assertRaisesRegex(TypeError, "target must be callable"):
+            self.Process(target=123)
+
     @warnings_helper.ignore_fork_in_thread_deprecation_warnings()
     def test_set_executable(self):
         if self.TYPE == 'threads':

Lib/test/test_threading.py

@@ -151,6 +151,12 @@ def func(): pass
             thread = threading.Thread(target=func)
             self.assertEqual(thread.name, "Thread-5 (func)")
 
+    def test_invalid_args(self):
+        with self.assertRaisesRegex(TypeError, "group argument must be None"):
+            threading.Thread(group="not None")
+        with self.assertRaisesRegex(TypeError, "target must be callable"):
+            threading.Thread(target=123)
+
     def test_args_argument(self):
         # bpo-45735: Using list or tuple as *args* in constructor could
         # achieve the same effect.

Lib/threading.py

@@ -901,7 +901,10 @@ class is implemented.
         else to the thread.
 
         """
-        assert group is None, "group argument must be None for now"
+        if group is not None:
+            raise TypeError("group argument must be None for now")
+        if target is not None and not callable(target):
+            raise TypeError("target must be callable")
         if kwargs is None:
             kwargs = {}
         if name:

Misc/NEWS.d/next/Library/2026-02-26-09-30-00.gh-issue-145242.N7hA6L.rst

@@ -0,0 +1 @@
+Added early validation of the ``target`` argument in :class:`threading.Thread` and :class:`multiprocessing.Process` constructors. If ``target`` is not callable, a :exc:`TypeError` is now raised immediately in the calling thread, resulting in a clearer traceback. Optimized the validation of the reserved ``group`` argument by replacing internal assertions with :exc:`TypeError`.