Warn when DoFn.process uses both yield and return

sdks/python/apache_beam/transforms/core.py

@@ -28,6 +28,7 @@
 import traceback
 import types
 import typing
+from itertools import dropwhile
 
 from apache_beam import coders
 from apache_beam import pvalue
@@ -1387,6 +1388,56 @@ def partition_for(self, element, num_partitions, *args, **kwargs):
     return self._fn(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_line = next(source_lines)
+    indentation = len(first_line) - len(first_line.lstrip())
+    final_lines = [first_line[indentation:]]
+
+    skip_inner_def = False
+    if first_line[indentation:].startswith("def "):
+      skip_inner_def = True
+    for line in source_lines:
+      line_indentation = len(line) - len(line.lstrip())
+
+      if line[indentation:].startswith("def "):
+        skip_inner_def = True
+        continue
+
+      if skip_inner_def and line_indentation == indentation:
+        skip_inner_def = False
+
+      if skip_inner_def and line_indentation > indentation:
+        continue
+      final_lines.append(line[indentation:])
+
+    return "".join(final_lines)
+  else:
+    return def_line.rsplit(":")[-1].strip()
+
+
+def _check_fn_use_yield_and_return(fn):
+  if isinstance(fn, types.BuiltinFunctionType):
+    return False
+  try:
+    source_code = _get_function_body_without_inners(fn)
+    has_yield = False
+    has_return = False
+    for line in source_code.split("\n"):
+      if line.lstrip().startswith("yield"):
+        has_yield = True
+      if line.lstrip().startswith("return"):
+        has_return = True
+      if has_yield and has_return:
+        return True
+    return False
+  except TypeError:
+    return False
+
+
 class ParDo(PTransformWithSideInputs):
   """A :class:`ParDo` transform.
 
@@ -1427,6 +1478,16 @@ def __init__(self, fn, *args, **kwargs):
     if not isinstance(self.fn, DoFn):
       raise TypeError('ParDo must be called with a DoFn instance.')
 
+    # DoFn.process cannot allow both return and yield
+    if _check_fn_use_yield_and_return(self.fn.process):
+      _LOGGER.warning(
+          'The yield and return statements in the process method '
+          'of %s can not be mixed.'
+          'We recommend to use `yield` for emitting individual '
+          ' elements and `yield from` for emitting the content '
+          'of entire iterables.',
+          self.fn.__class__)
+
     # Validate the DoFn by creating a DoFnSignature
     from apache_beam.runners.common import DoFnSignature
     self._signature = DoFnSignature(self.fn)
@@ -2663,6 +2724,7 @@ def from_runner_api_parameter(unused_ptransform, combine_payload, context):
 
 class CombineValuesDoFn(DoFn):
   """DoFn for performing per-key Combine transforms."""
+
   def __init__(
       self,
       input_pcoll_type,
@@ -2725,6 +2787,7 @@ def default_type_hints(self):
 
 
 class _CombinePerKeyWithHotKeyFanout(PTransform):
+
   def __init__(
       self,
       combine_fn,  # type: CombineFn
@@ -2939,11 +3002,12 @@ class GroupBy(PTransform):
   The GroupBy operation can be made into an aggregating operation by invoking
   its `aggregate_field` method.
   """
+
   def __init__(
       self,
       *fields,  # type: typing.Union[str, typing.Callable]
       **kwargs  # type: typing.Union[str, typing.Callable]
-    ):
+  ):
     if len(fields) == 1 and not kwargs:
       self._force_tuple_keys = False
       name = fields[0] if isinstance(fields[0], str) else 'key'
@@ -2966,7 +3030,7 @@ def aggregate_field(
       field,  # type: typing.Union[str, typing.Callable]
       combine_fn,  # type: typing.Union[typing.Callable, CombineFn]
       dest,  # type: str
-    ):
+  ):
     """Returns a grouping operation that also aggregates grouped values.
 
     Args:
@@ -3054,7 +3118,7 @@ def aggregate_field(
       field,  # type: typing.Union[str, typing.Callable]
       combine_fn,  # type: typing.Union[typing.Callable, CombineFn]
       dest,  # type: str
-      ):
+  ):
     field = _expr_to_callable(field, 0)
     return _GroupAndAggregate(
         self._grouping, list(self._aggregations) + [(field, combine_fn, dest)])
@@ -3096,10 +3160,12 @@ class Select(PTransform):
 
       pcoll | beam.Map(lambda x: beam.Row(a=x.a, b=foo(x)))
   """
-  def __init__(self,
-               *args,  # type: typing.Union[str, typing.Callable]
-               **kwargs  # type: typing.Union[str, typing.Callable]
-               ):
+
+  def __init__(
+      self,
+      *args,  # type: typing.Union[str, typing.Callable]
+      **kwargs  # type: typing.Union[str, typing.Callable]
+  ):
     self._fields = [(
         expr if isinstance(expr, str) else 'arg%02d' % ix,
         _expr_to_callable(expr, ix)) for (ix, expr) in enumerate(args)
@@ -3161,14 +3227,16 @@ def expand(self, pcoll):
 
 
 class Windowing(object):
-  def __init__(self,
-               windowfn,  # type: WindowFn
-               triggerfn=None,  # type: typing.Optional[TriggerFn]
-               accumulation_mode=None,  # type: typing.Optional[beam_runner_api_pb2.AccumulationMode.Enum]
-               timestamp_combiner=None,  # type: typing.Optional[beam_runner_api_pb2.OutputTime.Enum]
-               allowed_lateness=0, # type: typing.Union[int, float]
-               environment_id=None, # type: typing.Optional[str]
-               ):
+
+  def __init__(
+      self,
+      windowfn,  # type: WindowFn
+      triggerfn=None,  # type: typing.Optional[TriggerFn]
+      accumulation_mode=None,  # type: typing.Optional[beam_runner_api_pb2.AccumulationMode.Enum]
+      timestamp_combiner=None,  # type: typing.Optional[beam_runner_api_pb2.OutputTime.Enum]
+      allowed_lateness=0,  # type: typing.Union[int, float]
+      environment_id=None,  # type: typing.Optional[str]
+  ):
     """Class representing the window strategy.
 
     Args:

sdks/python/apache_beam/transforms/core_test.py

@@ -0,0 +1,82 @@
+#
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You under the Apache License, Version 2.0
+# (the "License"); you may not use this file except in compliance with
+# the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+"""Unit tests for the core python file."""
+# pytype: skip-file
+
+import logging
+import unittest
+
+import pytest
+
+import apache_beam as beam
+
+
+class TestDoFn1(beam.DoFn):
+  def process(self, element):
+    yield element
+
+
+class TestDoFn2(beam.DoFn):
+  def process(self, element):
+    def inner_func(x):
+      yield x
+
+    return inner_func(element)
+
+
+class TestDoFn3(beam.DoFn):
+  """mixing return and yield is not allowed"""
+  def process(self, element):
+    if not element:
+      return -1
+    yield element
+
+
+class TestDoFn4(beam.DoFn):
+  """test the variable name containing return"""
+  def process(self, element):
+    my_return = element
+    yield my_return
+
+
+class TestDoFn5(beam.DoFn):
+  """test the variable name containing yield"""
+  def process(self, element):
+    my_yield = element
+    return my_yield
+
+
+class CreateTest(unittest.TestCase):
+  @pytest.fixture(autouse=True)
+  def inject_fixtures(self, caplog):
+    self._caplog = caplog
+
+  def test_dofn_with_yield_and_return(self):
+    assert beam.ParDo(sum)
+    assert beam.ParDo(TestDoFn1())
+    assert beam.ParDo(TestDoFn2())
+    assert beam.ParDo(TestDoFn4())
+    assert beam.ParDo(TestDoFn5())
+    with self._caplog.at_level(logging.WARNING):
+      beam.ParDo(TestDoFn3())
+      assert 'The yield and return statements in' in self._caplog.text
+
+
+if __name__ == '__main__':
+  logging.getLogger().setLevel(logging.INFO)
+  unittest.main()