diff --git a/sdks/python/apache_beam/transforms/core.py b/sdks/python/apache_beam/transforms/core.py
index d7415e8d8135..a3762adac0cb 100644
--- a/sdks/python/apache_beam/transforms/core.py
+++ b/sdks/python/apache_beam/transforms/core.py
@@ -1462,8 +1462,14 @@ def partition_for(self, element, num_partitions, *args, **kwargs):
 def _get_function_body_without_inners(func):
   source_lines = inspect.getsourcelines(func)[0]
   source_lines = dropwhile(lambda x: x.startswith("@"), source_lines)
-  def_line = next(source_lines).strip()
-  if def_line.startswith("def ") and def_line.endswith(":"):
+  first_def_line = next(source_lines).strip()
+  if first_def_line.startswith("def "):
+    last_def_line_without_comment = first_def_line.split("#")[0] \
+        .split("\"\"\"")[0]
+    while not last_def_line_without_comment.strip().endswith(":"):
+      last_def_line_without_comment = next(source_lines).split("#")[0] \
+        .split("\"\"\"")[0]
+
     first_line = next(source_lines)
     indentation = len(first_line) - len(first_line.lstrip())
     final_lines = [first_line[indentation:]]
@@ -1487,7 +1493,7 @@ def _get_function_body_without_inners(func):
 
     return "".join(final_lines)
   else:
-    return def_line.rsplit(":")[-1].strip()
+    return first_def_line.rsplit(":")[-1].strip()
 
 
 def _check_fn_use_yield_and_return(fn):
@@ -1497,15 +1503,26 @@ def _check_fn_use_yield_and_return(fn):
     source_code = _get_function_body_without_inners(fn)
     has_yield = False
     has_return = False
+    return_none_warning = (
+        "No iterator is returned by the process method in %s.",
+        fn.__self__.__class__)
     for line in source_code.split("\n"):
-      if line.lstrip().startswith("yield ") or line.lstrip().startswith(
+      lstripped_line = line.lstrip()
+      if lstripped_line.startswith("yield ") or lstripped_line.startswith(
           "yield("):
         has_yield = True
-      if line.lstrip().startswith("return ") or line.lstrip().startswith(
+      if lstripped_line.startswith("return ") or lstripped_line.startswith(
           "return("):
         has_return = True
+        if lstripped_line.startswith(
+            "return None") or lstripped_line.rstrip() == "return":
+          _LOGGER.warning(return_none_warning)
       if has_yield and has_return:
         return True
+
+    if not has_yield and not has_return:
+      _LOGGER.warning(return_none_warning)
+
     return False
   except Exception as e:
     _LOGGER.debug(str(e))
diff --git a/sdks/python/apache_beam/transforms/core_test.py b/sdks/python/apache_beam/transforms/core_test.py
index b492ab0938cc..54afb365d2d8 100644
--- a/sdks/python/apache_beam/transforms/core_test.py
+++ b/sdks/python/apache_beam/transforms/core_test.py
@@ -30,6 +30,8 @@
 from apache_beam.testing.util import equal_to
 from apache_beam.transforms.window import FixedWindows
 
+RETURN_NONE_PARTIAL_WARNING = "No iterator is returned"
+
 
 class TestDoFn1(beam.DoFn):
   def process(self, element):
@@ -96,6 +98,24 @@ def process(self, element):
     yield element
 
 
+class TestDoFn10(beam.DoFn):
+  """test process returning None explicitly"""
+  def process(self, element):
+    return None
+
+
+class TestDoFn11(beam.DoFn):
+  """test process returning None (no return and no yield)"""
+  def process(self, element):
+    pass
+
+
+class TestDoFn12(beam.DoFn):
+  """test process returning None (return statement without a value)"""
+  def process(self, element):
+    return
+
+
 class CreateTest(unittest.TestCase):
   @pytest.fixture(autouse=True)
   def inject_fixtures(self, caplog):
@@ -119,6 +139,24 @@ def test_dofn_with_yield_and_return(self):
       beam.ParDo(TestDoFn3())
       assert warning_text in self._caplog.text
 
+  def test_dofn_with_explicit_return_none(self):
+    with self._caplog.at_level(logging.WARNING):
+      beam.ParDo(TestDoFn10())
+      assert RETURN_NONE_PARTIAL_WARNING in self._caplog.text
+      assert str(TestDoFn10) in self._caplog.text
+
+  def test_dofn_with_implicit_return_none_missing_return_and_yield(self):
+    with self._caplog.at_level(logging.WARNING):
+      beam.ParDo(TestDoFn11())
+      assert RETURN_NONE_PARTIAL_WARNING in self._caplog.text
+      assert str(TestDoFn11) in self._caplog.text
+
+  def test_dofn_with_implicit_return_none_return_without_value(self):
+    with self._caplog.at_level(logging.WARNING):
+      beam.ParDo(TestDoFn12())
+      assert RETURN_NONE_PARTIAL_WARNING in self._caplog.text
+      assert str(TestDoFn12) in self._caplog.text
+
 
 class PartitionTest(unittest.TestCase):
   def test_partition_boundedness(self):