[YAML] Make datetime available for jinja templatization.

CHANGES.md

@@ -31,6 +31,7 @@
 
 ## New Features / Improvements
 
+* The datetime module is now available for use in jinja templatization for yaml.
 * X feature added (Java/Python) ([#X](https://github.com/apache/beam/issues/X)).
 
 ## Breaking Changes

sdks/python/apache_beam/yaml/main_test.py

@@ -15,6 +15,7 @@
 # limitations under the License.
 #
 
+import datetime
 import glob
 import logging
 import os
@@ -100,6 +101,18 @@ def test_preparse_jinja_flags(self):
             'pos_arg',
         ])
 
+  def test_jinja_datetime(self):
+    with tempfile.TemporaryDirectory() as tmpdir:
+      out_path = os.path.join(tmpdir, 'out.txt')
+      main.run([
+          '--yaml_pipeline',
+          TEST_PIPELINE.replace('PATH', out_path).replace(
+              'ELEMENT', '"{{datetime.datetime.now().strftime("%Y-%m-%d")}}"'),
+      ])
+      with open(glob.glob(out_path + '*')[0], 'rt') as fin:
+        self.assertEqual(
+            fin.read().strip(), datetime.datetime.now().strftime("%Y-%m-%d"))
+
 
 if __name__ == '__main__':
   logging.getLogger().setLevel(logging.INFO)

sdks/python/apache_beam/yaml/yaml_transform.py

@@ -16,6 +16,7 @@
 #
 
 import collections
+import datetime
 import functools
 import json
 import logging
@@ -992,7 +993,7 @@ def expand_jinja(
       jinja2.Environment(
           undefined=jinja2.StrictUndefined, loader=_BeamFileIOLoader())
       .from_string(jinja_template)
-      .render(**jinja_variables))
+      .render(datetime=datetime, **jinja_variables))
 
 
 class YamlTransform(beam.PTransform):