Enable the usage of generator functions in nodes

Makefile

@@ -19,6 +19,9 @@ test:
 test-no-spark:
 	pytest tests --no-cov --ignore tests/extras/datasets/spark --numprocesses 4 --dist loadfile
 
+test-no-datasets:
+	pytest tests --no-cov --ignore tests/extras/datasets/ --numprocesses 4 --dist loadfile
+
 e2e-tests:
 	behave
 

RELEASE.md

@@ -14,6 +14,9 @@
 * Added new `OmegaConfLoader` which uses `OmegaConf` for loading and merging configuration.
 * Added the `--conf-source` option to `kedro run`, allowing users to specify a source for project configuration for the run.
 * Added `omegaconf` syntax as option for `--params`. Keys and values can now be separated by colons or equals signs.
+* Added support for generator functions as nodes, i.e. using `yield` instead of return.
+  * Enable chunk-wise processing in nodes with generator functions.
+  * Save node outputs after every `yield` before proceeding with next chunk.
 
 ## Bug fixes and other changes
 * Fix bug where `micropkg` manifest section in `pyproject.toml` isn't recognised as allowed configuration.

dependency/requirements.txt

@@ -10,6 +10,7 @@ importlib-metadata>=3.6; python_version >= '3.8'
 importlib_metadata>=3.6, <5.0; python_version < '3.8'  # The "selectable" entry points were introduced in `importlib_metadata` 3.6 and Python 3.10. Bandit on Python 3.7 relies on a library with `importlib_metadata` < 5.0
 importlib_resources>=1.3  # The `files()` API was introduced in `importlib_resources` 1.3 and Python 3.9.
 jmespath>=0.9.5, <1.0
+more_itertools~=9.0
 omegaconf~=2.3
 pip-tools~=6.12
 pluggy~=1.0.0

kedro/pipeline/node.py

@@ -9,6 +9,8 @@
 from typing import Any, Callable, Dict, Iterable, List, Optional, Set, Union
 from warnings import warn
 
+from more_itertools import spy, unzip
+
 
 class Node:
     """``Node`` is an auxiliary class facilitating the operations required to
@@ -397,38 +399,57 @@ def _run_with_dict(self, inputs: Dict[str, Any], node_inputs: Dict[str, str]):
 
     def _outputs_to_dictionary(self, outputs):
         def _from_dict():
-            if set(self._outputs.keys()) != set(outputs.keys()):
+            result, iterator = outputs, None
+            # generator functions are lazy and we need a peek into their first output
+            if inspect.isgenerator(outputs):
+                (result,), iterator = spy(outputs)
+
+            keys = list(self._outputs.keys())
+            names = list(self._outputs.values())
+            if not isinstance(result, dict):
+                raise ValueError(
+                    f"Failed to save outputs of node {self}.\n"
+                    f"The node output is a dictionary, whereas the "
+                    f"function output is {type(result)}."
+                )
+            if set(keys) != set(result.keys()):
                 raise ValueError(
                     f"Failed to save outputs of node {str(self)}.\n"
-                    f"The node's output keys {set(outputs.keys())} do not match with "
-                    f"the returned output's keys {set(self._outputs.keys())}."
+                    f"The node's output keys {set(result.keys())} "
+                    f"do not match with the returned output's keys {set(keys)}."
                 )
-            return {name: outputs[key] for key, name in self._outputs.items()}
+            if iterator:
+                exploded = map(lambda x: tuple(x[k] for k in keys), iterator)
+                result = unzip(exploded)
+            else:
+                # evaluate this eagerly so we can reuse variable name
+                result = tuple(result[k] for k in keys)
+            return dict(zip(names, result))
 
         def _from_list():
-            if not isinstance(outputs, (list, tuple)):
+            result, iterator = outputs, None
+            # generator functions are lazy and we need a peek into their first output
+            if inspect.isgenerator(outputs):
+                (result,), iterator = spy(outputs)
+
+            if not isinstance(result, (list, tuple)):
                 raise ValueError(
                     f"Failed to save outputs of node {str(self)}.\n"
                     f"The node definition contains a list of "
                     f"outputs {self._outputs}, whereas the node function "
-                    f"returned a '{type(outputs).__name__}'."
+                    f"returned a '{type(result).__name__}'."
                 )
-            if len(outputs) != len(self._outputs):
+            if len(result) != len(self._outputs):
                 raise ValueError(
                     f"Failed to save outputs of node {str(self)}.\n"
-                    f"The node function returned {len(outputs)} output(s), "
+                    f"The node function returned {len(result)} output(s), "
                     f"whereas the node definition contains {len(self._outputs)} "
                     f"output(s)."
                 )
 
-            return dict(zip(self._outputs, outputs))
-
-        if isinstance(self._outputs, dict) and not isinstance(outputs, dict):
-            raise ValueError(
-                f"Failed to save outputs of node {self}.\n"
-                f"The node output is a dictionary, whereas the "
-                f"function output is not."
-            )
+            if iterator:
+                result = unzip(iterator)
+            return dict(zip(self._outputs, result))
 
         if self._outputs is None:
             return {}

kedro/runner/runner.py

@@ -2,6 +2,8 @@
 implementations.
 """
 
+import inspect
+import itertools as it
 import logging
 from abc import ABC, abstractmethod
 from collections import deque
@@ -12,8 +14,9 @@
     as_completed,
     wait,
 )
-from typing import Any, Dict, Iterable, List, Set
+from typing import Any, Dict, Iterable, Iterator, List, Set
 
+from more_itertools import interleave
 from pluggy import PluginManager
 
 from kedro.framework.hooks.manager import _NullPluginManager
@@ -294,10 +297,22 @@ def run_node(
             asynchronously with threads. Defaults to False.
         session_id: The session id of the pipeline run.
 
+    Raises:
+        ValueError: Raised if is_async is set to True for nodes wrapping
+        generator functions.
+
     Returns:
         The node argument.
 
     """
+    if is_async and inspect.isgeneratorfunction(node.func):
+        raise ValueError(
+            f"Async data loading and saving does not work with "
+            f"nodes wrapping generator functions. Please make "
+            f"sure you don't use `yield` anywhere "
+            f"in node {str(node)}."
+        )
+
     if is_async:
         node = _run_node_async(node, catalog, hook_manager, session_id)
     else:
@@ -399,7 +414,21 @@ def _run_node_sequential(
         node, catalog, inputs, is_async, hook_manager, session_id=session_id
     )
 
-    for name, data in outputs.items():
+    items: Iterable = outputs.items()
+    # if all outputs are iterators, then the node is a generator node
+    if all(isinstance(d, Iterator) for d in outputs.values()):
+        # Python dictionaries are ordered so we are sure
+        # the keys and the chunk streams are in the same order
+        # [a, b, c]
+        keys = list(outputs.keys())
+        # [Iterator[chunk_a], Iterator[chunk_b], Iterator[chunk_c]]
+        streams = list(outputs.values())
+        # zip an endless cycle of the keys
+        # with an interleaved iterator of the streams
+        # [(a, chunk_a), (b, chunk_b), ...] until all outputs complete
+        items = zip(it.cycle(keys), interleave(*streams))
+
+    for name, data in items:
         hook_manager.hook.before_dataset_saved(dataset_name=name, data=data)
         catalog.save(name, data)
         hook_manager.hook.after_dataset_saved(dataset_name=name, data=data)

tests/pipeline/test_node_run.py

@@ -109,8 +109,8 @@ def test_run_dict_diff_size(self, mocked_dataset):
 
 class TestNodeRunInvalidOutput:
     def test_miss_matching_output_types(self, mocked_dataset):
-        pattern = r"The node output is a dictionary, whereas the function "
-        pattern += r"output is not\."
+        pattern = "The node output is a dictionary, whereas the function "
+        pattern += "output is <class 'kedro.io.lambda_dataset.LambdaDataSet'>."
         with pytest.raises(ValueError, match=pattern):
             node(one_in_one_out, "ds1", dict(a="ds")).run(dict(ds1=mocked_dataset))
 

tests/runner/test_run_node.py

@@ -0,0 +1,89 @@
+import pytest
+
+from kedro.framework.hooks.manager import _NullPluginManager
+from kedro.pipeline import node
+from kedro.runner import run_node
+
+
+def generate_one():
+    yield from range(10)
+
+
+def generate_tuple():
+    for i in range(10):
+        yield i, i * i
+
+
+def generate_list():
+    for i in range(10):
+        yield [i, i * i]
+
+
+def generate_dict():
+    for i in range(10):
+        yield dict(idx=i, square=i * i)
+
+
+class TestRunGeneratorNode:
+    def test_generator_fail_async(self, mocker, catalog):
+        fake_dataset = mocker.Mock()
+        catalog.add("result", fake_dataset)
+        n = node(generate_one, inputs=None, outputs="result")
+
+        with pytest.raises(Exception, match="nodes wrapping generator functions"):
+            run_node(n, catalog, _NullPluginManager(), is_async=True)
+
+    def test_generator_node_one(self, mocker, catalog):
+        fake_dataset = mocker.Mock()
+        catalog.add("result", fake_dataset)
+        n = node(generate_one, inputs=None, outputs="result")
+        run_node(n, catalog, _NullPluginManager())
+
+        expected = [((i,),) for i in range(10)]
+        assert 10 == fake_dataset.save.call_count
+        assert fake_dataset.save.call_args_list == expected
+
+    def test_generator_node_tuple(self, mocker, catalog):
+        left = mocker.Mock()
+        right = mocker.Mock()
+        catalog.add("left", left)
+        catalog.add("right", right)
+        n = node(generate_tuple, inputs=None, outputs=["left", "right"])
+        run_node(n, catalog, _NullPluginManager())
+
+        expected_left = [((i,),) for i in range(10)]
+        expected_right = [((i * i,),) for i in range(10)]
+        assert 10 == left.save.call_count
+        assert left.save.call_args_list == expected_left
+        assert 10 == right.save.call_count
+        assert right.save.call_args_list == expected_right
+
+    def test_generator_node_list(self, mocker, catalog):
+        left = mocker.Mock()
+        right = mocker.Mock()
+        catalog.add("left", left)
+        catalog.add("right", right)
+        n = node(generate_list, inputs=None, outputs=["left", "right"])
+        run_node(n, catalog, _NullPluginManager())
+
+        expected_left = [((i,),) for i in range(10)]
+        expected_right = [((i * i,),) for i in range(10)]
+        assert 10 == left.save.call_count
+        assert left.save.call_args_list == expected_left
+        assert 10 == right.save.call_count
+        assert right.save.call_args_list == expected_right
+
+    def test_generator_node_dict(self, mocker, catalog):
+        left = mocker.Mock()
+        right = mocker.Mock()
+        catalog.add("left", left)
+        catalog.add("right", right)
+        n = node(generate_dict, inputs=None, outputs=dict(idx="left", square="right"))
+        run_node(n, catalog, _NullPluginManager())
+
+        expected_left = [((i,),) for i in range(10)]
+        expected_right = [((i * i,),) for i in range(10)]
+        assert 10 == left.save.call_count
+        assert left.save.call_args_list == expected_left
+        assert 10 == right.save.call_count
+        assert right.save.call_args_list == expected_right