Clarify pendulum use in timezone cases (apache#21646)

It is important to use Pendulum in case timezone is used - because
there are a number of limitations coming from using stdlib
timezone implementation.

However our documentation was not very clear about it, especially
some examples shown using standard datetime in DAGs which could
mislead our users to continue using datetime if they use timezone.

This PR clarifies and stresses the use of pendulum is necessary
when timezone is used. Also it points to the documentation
in case serialization throws error about not using Pendulum
so that the users can learn about the reasoning.

This is the first part of the change - the follow up will be
changing all provider examples to also use timezone and
pendulum explicitly.

See also apache#20070

CONTRIBUTING.rst

@@ -1438,7 +1438,7 @@ We are using certain prefixes for email subjects for different purposes. Start y
 Voting is governed by the rules described in `Voting <https://www.apache.org/foundation/voting.html>`_
 
 We are all devoting our time for community as individuals who except for being active in Apache Airflow have
-families, daily jobs, right for vacation. Sometimes we are in different time zones or simply are
+families, daily jobs, right for vacation. Sometimes we are in different timezones or simply are
 busy with day-to-day duties that our response time might be delayed. For us it's crucial
 to remember to respect each other in the project with no formal structure.
 There are no managers, departments, most of us is autonomous in our opinions, decisions.

UPDATING.md

@@ -245,7 +245,7 @@ Similarly, `DAG.concurrency` has been renamed to `DAG.max_active_tasks`.
 ```python
 dag = DAG(
     dag_id="example_dag",
-    start_date=datetime(2021, 1, 1),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
     concurrency=3,
 )
@@ -256,7 +256,7 @@ dag = DAG(
 ```python
 dag = DAG(
     dag_id="example_dag",
-    start_date=datetime(2021, 1, 1),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
     max_active_tasks=3,
 )
@@ -3329,7 +3329,7 @@ Type "help", "copyright", "credits" or "license" for more information.
 >>> from airflow.models.dag import DAG
 >>> from airflow.operators.dummy import DummyOperator
 >>>
->>> dag = DAG('simple_dag', start_date=datetime(2017, 9, 1))
+>>> dag = DAG('simple_dag', start_date=pendulum.datetime(2017, 9, 1, tz="UTC"))
 >>>
 >>> task = DummyOperator(task_id='task_1', dag=dag)
 >>>

airflow/example_dags/example_bash_operator.py

@@ -18,7 +18,9 @@
 
 """Example DAG demonstrating the usage of the BashOperator."""
 
-from datetime import datetime, timedelta
+import datetime
+
+import pendulum
 
 from airflow import DAG
 from airflow.operators.bash import BashOperator
@@ -27,9 +29,9 @@
 with DAG(
     dag_id='example_bash_operator',
     schedule_interval='0 0 * * *',
-    start_date=datetime(2021, 1, 1),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
-    dagrun_timeout=timedelta(minutes=60),
+    dagrun_timeout=datetime.timedelta(minutes=60),
     tags=['example', 'example2'],
     params={"example_key": "example_value"},
 ) as dag:

airflow/example_dags/example_branch_datetime_operator.py

@@ -20,15 +20,15 @@
 Example DAG demonstrating the usage of DateTimeBranchOperator with datetime as well as time objects as
 targets.
 """
-import datetime
+import pendulum
 
 from airflow import DAG
 from airflow.operators.datetime import BranchDateTimeOperator
 from airflow.operators.dummy import DummyOperator
 
 dag = DAG(
     dag_id="example_branch_datetime_operator",
-    start_date=datetime.datetime(2021, 1, 1),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
     tags=["example"],
     schedule_interval="@daily",
@@ -42,8 +42,8 @@
     task_id='datetime_branch',
     follow_task_ids_if_true=['date_in_range'],
     follow_task_ids_if_false=['date_outside_range'],
-    target_upper=datetime.datetime(2020, 10, 10, 15, 0, 0),
-    target_lower=datetime.datetime(2020, 10, 10, 14, 0, 0),
+    target_upper=pendulum.datetime(2020, 10, 10, 15, 0, 0),
+    target_lower=pendulum.datetime(2020, 10, 10, 14, 0, 0),
     dag=dag,
 )
 
@@ -54,7 +54,7 @@
 
 dag = DAG(
     dag_id="example_branch_datetime_operator_2",
-    start_date=datetime.datetime(2021, 1, 1),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
     tags=["example"],
     schedule_interval="@daily",
@@ -67,8 +67,8 @@
     task_id='datetime_branch',
     follow_task_ids_if_true=['date_in_range'],
     follow_task_ids_if_false=['date_outside_range'],
-    target_upper=datetime.time(0, 0, 0),
-    target_lower=datetime.time(15, 0, 0),
+    target_upper=pendulum.time(0, 0, 0),
+    target_lower=pendulum.time(15, 0, 0),
     dag=dag,
 )
 

airflow/example_dags/example_branch_day_of_week_operator.py

@@ -19,15 +19,15 @@
 """
 Example DAG demonstrating the usage of BranchDayOfWeekOperator.
 """
-from datetime import datetime
+import pendulum
 
 from airflow import DAG
 from airflow.operators.dummy import DummyOperator
 from airflow.operators.weekday import BranchDayOfWeekOperator
 
 with DAG(
     dag_id="example_weekday_branch_operator",
-    start_date=datetime(2021, 1, 1),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
     tags=["example"],
     schedule_interval="@daily",

airflow/example_dags/example_branch_labels.py

@@ -19,14 +19,17 @@
 """
 Example DAG demonstrating the usage of labels with different branches.
 """
-from datetime import datetime
+import pendulum
 
 from airflow import DAG
 from airflow.operators.dummy import DummyOperator
 from airflow.utils.edgemodifier import Label
 
 with DAG(
-    "example_branch_labels", schedule_interval="@daily", start_date=datetime(2021, 1, 1), catchup=False
+    "example_branch_labels",
+    schedule_interval="@daily",
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
+    catchup=False,
 ) as dag:
     ingest = DummyOperator(task_id="ingest")
     analyse = DummyOperator(task_id="analyze")

airflow/example_dags/example_branch_operator.py

@@ -19,7 +19,8 @@
 """Example DAG demonstrating the usage of the BranchPythonOperator."""
 
 import random
-from datetime import datetime
+
+import pendulum
 
 from airflow import DAG
 from airflow.operators.dummy import DummyOperator
@@ -29,7 +30,7 @@
 
 with DAG(
     dag_id='example_branch_operator',
-    start_date=datetime(2021, 1, 1),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
     schedule_interval="@daily",
     tags=['example', 'example2'],

airflow/example_dags/example_branch_python_dop_operator_3.py

@@ -20,7 +20,7 @@
 Example DAG demonstrating the usage of BranchPythonOperator with depends_on_past=True, where tasks may be run
 or skipped on alternating runs.
 """
-from datetime import datetime
+import pendulum
 
 from airflow import DAG
 from airflow.operators.dummy import DummyOperator
@@ -49,7 +49,7 @@ def should_run(**kwargs):
 with DAG(
     dag_id='example_branch_dop_operator_v3',
     schedule_interval='*/1 * * * *',
-    start_date=datetime(2021, 1, 1),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
     default_args={'depends_on_past': True},
     tags=['example'],

airflow/example_dags/example_complex.py

@@ -19,7 +19,7 @@
 """
 Example Airflow DAG that shows the complex DAG structure.
 """
-from datetime import datetime
+import pendulum
 
 from airflow import models
 from airflow.models.baseoperator import chain
@@ -28,7 +28,7 @@
 with models.DAG(
     dag_id="example_complex",
     schedule_interval=None,
-    start_date=datetime(2021, 1, 1),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
     tags=['example', 'example2', 'example3'],
 ) as dag:

airflow/example_dags/example_dag_decorator.py

@@ -15,10 +15,10 @@
 # KIND, either express or implied.  See the License for the
 # specific language governing permissions and limitations
 # under the License.
-from datetime import datetime
 from typing import Any, Dict
 
 import httpx
+import pendulum
 
 from airflow.decorators import dag, task
 from airflow.models.baseoperator import BaseOperator
@@ -38,7 +38,12 @@ def execute(self, context: Context):
 
 
 # [START dag_decorator_usage]
-@dag(schedule_interval=None, start_date=datetime(2021, 1, 1), catchup=False, tags=['example'])
+@dag(
+    schedule_interval=None,
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
+    catchup=False,
+    tags=['example'],
+)
 def example_dag_decorator(email: str = '[email protected]'):
     """
     DAG to send server IP to email.

airflow/example_dags/example_external_task_marker_dag.py

@@ -37,13 +37,13 @@
     exception
 """
 
-import datetime
+import pendulum
 
 from airflow import DAG
 from airflow.operators.dummy import DummyOperator
 from airflow.sensors.external_task import ExternalTaskMarker, ExternalTaskSensor
 
-start_date = datetime.datetime(2021, 1, 1)
+start_date = pendulum.datetime(2021, 1, 1, tz="UTC")
 
 with DAG(
     dag_id="example_external_task_marker_parent",

airflow/example_dags/example_kubernetes_executor.py

@@ -20,7 +20,8 @@
 """
 import logging
 import os
-from datetime import datetime
+
+import pendulum
 
 from airflow import DAG
 from airflow.configuration import conf
@@ -45,7 +46,7 @@
     with DAG(
         dag_id='example_kubernetes_executor',
         schedule_interval=None,
-        start_date=datetime(2021, 1, 1),
+        start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
         catchup=False,
         tags=['example3'],
     ) as dag:

airflow/example_dags/example_latest_only_with_trigger.py

@@ -20,7 +20,9 @@
 """
 
 # [START example]
-import datetime as dt
+import datetime
+
+import pendulum
 
 from airflow import DAG
 from airflow.operators.dummy import DummyOperator
@@ -29,8 +31,8 @@
 
 with DAG(
     dag_id='latest_only_with_trigger',
-    schedule_interval=dt.timedelta(hours=4),
-    start_date=dt.datetime(2021, 1, 1),
+    schedule_interval=datetime.timedelta(hours=4),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
     tags=['example3'],
 ) as dag:

airflow/example_dags/example_nested_branch_dag.py

@@ -21,7 +21,7 @@
 ``none_failed_min_one_success`` trigger rule such that they are skipped whenever their corresponding
 ``BranchPythonOperator`` are skipped.
 """
-from datetime import datetime
+import pendulum
 
 from airflow.models import DAG
 from airflow.operators.dummy import DummyOperator
@@ -30,7 +30,7 @@
 
 with DAG(
     dag_id="example_nested_branch_dag",
-    start_date=datetime(2021, 1, 1),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
     schedule_interval="@daily",
     tags=["example"],

airflow/example_dags/example_passing_params_via_test_command.py

@@ -18,10 +18,12 @@
 
 """Example DAG demonstrating the usage of the params arguments in templated arguments."""
 
+import datetime
 import os
-from datetime import datetime, timedelta
 from textwrap import dedent
 
+import pendulum
+
 from airflow import DAG
 from airflow.decorators import task
 from airflow.operators.bash import BashOperator
@@ -58,9 +60,9 @@ def print_env_vars(test_mode=None):
 with DAG(
     "example_passing_params_via_test_command",
     schedule_interval='*/1 * * * *',
-    start_date=datetime(2021, 1, 1),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
-    dagrun_timeout=timedelta(minutes=4),
+    dagrun_timeout=datetime.timedelta(minutes=4),
     tags=['example'],
 ) as dag:
     run_this = my_py_command(params={"miff": "agg"})

airflow/example_dags/example_python_operator.py

@@ -23,9 +23,10 @@
 import logging
 import shutil
 import time
-from datetime import datetime
 from pprint import pprint
 
+import pendulum
+
 from airflow import DAG
 from airflow.decorators import task
 
@@ -34,7 +35,7 @@
 with DAG(
     dag_id='example_python_operator',
     schedule_interval=None,
-    start_date=datetime(2021, 1, 1),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
     tags=['example'],
 ) as dag:

airflow/example_dags/example_short_circuit_operator.py

@@ -17,7 +17,7 @@
 # under the License.
 
 """Example DAG demonstrating the usage of the ShortCircuitOperator."""
-from datetime import datetime
+import pendulum
 
 from airflow import DAG
 from airflow.models.baseoperator import chain
@@ -27,7 +27,7 @@
 
 with DAG(
     dag_id='example_short_circuit_operator',
-    start_date=datetime(2021, 1, 1),
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
     catchup=False,
     tags=['example'],
 ) as dag:

airflow/example_dags/example_skip_dag.py

@@ -18,7 +18,7 @@
 
 """Example DAG demonstrating the DummyOperator and a custom DummySkipOperator which skips by default."""
 
-from datetime import datetime
+import pendulum
 
 from airflow import DAG
 from airflow.exceptions import AirflowSkipException
@@ -55,6 +55,11 @@ def create_test_pipeline(suffix, trigger_rule):
     join >> final
 
 
-with DAG(dag_id='example_skip_dag', start_date=datetime(2021, 1, 1), catchup=False, tags=['example']) as dag:
+with DAG(
+    dag_id='example_skip_dag',
+    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
+    catchup=False,
+    tags=['example'],
+) as dag:
     create_test_pipeline('1', TriggerRule.ALL_SUCCESS)
     create_test_pipeline('2', TriggerRule.ONE_SUCCESS)