[AIRFLOW-6010] Remove cyclic imports and pylint disables

.pre-commit-config.yaml

@@ -197,6 +197,33 @@ repos:
         entry: "pydevd.*settrace\\("
         pass_filenames: true
         files: \.py$
+      - id: base-operator
+        language: pygrep
+        name: Make sure BaseOperator is imported from airflow.models.baseoperator in core
+        entry: "from airflow.models import.* BaseOperator"
+        files: \.py$
+        pass_filenames: true
+        exclude: >
+          (?x)
+          ^airflow/gcp/.*$|
+          ^airflow/hooks/.*$|
+          ^airflow/operators/.*$|
+          ^airflow/sensors/.*$|
+          ^airflow/providers/.*$|
+          ^airflow/contrib/.*$
+      - id: base-operator
+        language: pygrep
+        name: Make sure BaseOperator is imported from airflow.models outside of core
+        entry: "from airflow.models.baseoperator import.* BaseOperator"
+        pass_filenames: true
+        files: >
+          (?x)
+          ^airflow/gcp/.*$|
+          ^airflow/hooks/.*$|
+          ^airflow/operators/.*$|
+          ^airflow/sensors/.*$|
+          ^airflow/providers/.*\.py$|
+          ^airflow/contrib/.*\.py$
       - id: build
         name: Check if image build is needed
         entry: ./scripts/ci/pre_commit_ci_build.sh

CONTRIBUTING.rst

@@ -414,7 +414,7 @@ image built locally):
 =================================== ================================================================ ============
 **Hooks**                             **Description**                                                 **Breeze**
 =================================== ================================================================ ============
-``airflow-settings``                  Check if airflow import settings are used well.
+``base-operator``                     Checks that BaseOperator is imported properly
 ----------------------------------- ---------------------------------------------------------------- ------------
 ``build``                             Builds image for check-apache-licence, mypy, pylint, flake8.         *
 ----------------------------------- ---------------------------------------------------------------- ------------
@@ -513,6 +513,35 @@ You can always skip running the tests by providing ``--no-verify`` flag to the
 
 To check other usage types of the pre-commit framework, see `Pre-commit website <https://pre-commit.com/>`__.
 
+Importing Airflow core objects
+==============================
+
+When you implement core features or DAGs you might need to import some of the core objects or modules.
+Since Apache Airflow can be used both as application (by internal classes) and as library (by DAGs), there are
+different ways those core objects and packages are imported.
+
+Airflow imports some of the core objects directly to 'airflow' package so that they can be used from there.
+
+Those criteria were assumed for choosing what import path to use:
+
+* If you work on a core feature inside Apache Airflow, you should import the objects directly from the
+  package where the object is defined - this minimises the risk of cyclic imports.
+* If you import the objects from any of 'providers' classes, you should import the objects from
+  'airflow' or 'airflow.models', It is very important for back-porting operators/hooks/sensors
+  to Airflow 1.10.* (AIP-21)
+* If you import objects from within a DAG you write, you should import them from 'airflow' or
+  'airflow.models' package where stable location of such import is important.
+
+Those checks enforced for the most important and repeated objects via pre-commit hooks as described below.
+
+BaseOperator
+------------
+
+The BaseOperator should be imported:
+* as ``from airflow.models import BaseOperator`` in external DAG/operator
+* as ``from airflow.models.baseoperator import BaseOperator`` in Airflow core to avoid cyclic imports
+
+
 Travis CI Testing Framework
 ===========================
 

UPDATING.md

@@ -41,6 +41,14 @@ assists users migrating to a new version.
 
 ## Airflow Master
 
+### Changes to settings
+
+CONTEXT_MANAGER_DAG was removed from settings. It's role has been taken by `DagContext` in
+'airflow.models.dag'. One of the reasons was that settings should be rather static than store
+dynamic context from the DAG, but the main one is that moving the context out of settings allowed to
+untangle cyclic imports between DAG, BaseOperator, SerializedDAG, SerializedBaseOperator which was
+part of AIRFLOW-6010.
+
 #### Change default aws_conn_id in EMR operators
 
 The default value for the [aws_conn_id](https://airflow.apache.org/howto/manage-connections.html#amazon-web-services) was accidently set to 's3_default' instead of 'aws_default' in some of the emr operators in previous 

airflow/__init__.py

@@ -31,6 +31,8 @@
 # pylint:disable=wrong-import-position
 from typing import Callable, Optional
 
+# noinspection PyUnresolvedReferences
+from airflow import utils
 from airflow import settings
 from airflow import version
 from airflow.utils.log.logging_mixin import LoggingMixin

airflow/api/common/experimental/delete_dag.py

@@ -22,7 +22,8 @@
 
 from airflow import models
 from airflow.exceptions import DagNotFound
-from airflow.models import DagModel, SerializedDagModel, TaskFail
+from airflow.models import DagModel, TaskFail
+from airflow.models.serialized_dag import SerializedDagModel
 from airflow.settings import STORE_SERIALIZED_DAGS
 from airflow.utils.db import provide_session
 from airflow.utils.log.logging_mixin import LoggingMixin

airflow/api/common/experimental/mark_tasks.py

@@ -24,7 +24,8 @@
 from sqlalchemy import or_
 
 from airflow.jobs import BackfillJob
-from airflow.models import BaseOperator, DagRun, TaskInstance
+from airflow.models import DagRun, TaskInstance
+from airflow.models.baseoperator import BaseOperator
 from airflow.operators.subdag_operator import SubDagOperator
 from airflow.utils import timezone
 from airflow.utils.db import provide_session

airflow/configuration.py

@@ -574,7 +574,7 @@ def get_airflow_test_config(airflow_home):
 has_option = conf.has_option
 remove_option = conf.remove_option
 as_dict = conf.as_dict
-set = conf.set # noqa
+set = conf.set  # noqa
 
 for func in [load_test_config, get, getboolean, getfloat, getint, has_option,
              remove_option, as_dict, set]:

airflow/contrib/operators/dynamodb_to_s3.py

@@ -32,7 +32,7 @@
 from boto.compat import json  # type: ignore
 
 from airflow.contrib.hooks.aws_dynamodb_hook import AwsDynamoDBHook
-from airflow.models.baseoperator import BaseOperator
+from airflow.models import BaseOperator
 from airflow.providers.amazon.aws.hooks.s3 import S3Hook
 
 

airflow/contrib/operators/qubole_operator.py

@@ -24,7 +24,7 @@
     COMMAND_ARGS, HYPHEN_ARGS, POSITIONAL_ARGS, QuboleHook, flatten_list,
 )
 from airflow.hooks.base_hook import BaseHook
-from airflow.models.baseoperator import BaseOperator, BaseOperatorLink
+from airflow.models import BaseOperator, BaseOperatorLink
 from airflow.models.taskinstance import TaskInstance
 from airflow.utils.decorators import apply_defaults
 

airflow/gcp/operators/bigquery.py

@@ -31,7 +31,7 @@
 from airflow.exceptions import AirflowException
 from airflow.gcp.hooks.bigquery import BigQueryHook
 from airflow.gcp.hooks.gcs import GoogleCloudStorageHook, _parse_gcs_url
-from airflow.models.baseoperator import BaseOperator, BaseOperatorLink
+from airflow.models import BaseOperator, BaseOperatorLink
 from airflow.models.taskinstance import TaskInstance
 from airflow.operators.check_operator import CheckOperator, IntervalCheckOperator, ValueCheckOperator
 from airflow.utils.decorators import apply_defaults

airflow/models/__init__.py

@@ -18,7 +18,7 @@
 # under the License.
 """Airflow models"""
 from airflow.models.base import ID_LEN, Base  # noqa: F401
-from airflow.models.baseoperator import BaseOperator  # noqa: F401
+from airflow.models.baseoperator import BaseOperator, BaseOperatorLink  # noqa: F401
 from airflow.models.connection import Connection  # noqa: F401
 from airflow.models.dag import DAG, DagModel  # noqa: F401
 from airflow.models.dagbag import DagBag  # noqa: F401
@@ -28,7 +28,6 @@
 from airflow.models.kubernetes import KubeResourceVersion, KubeWorkerIdentifier  # noqa: F401
 from airflow.models.log import Log  # noqa: F401
 from airflow.models.pool import Pool  # noqa: F401
-from airflow.models.serialized_dag import SerializedDagModel  # noqa: F401
 from airflow.models.skipmixin import SkipMixin  # noqa: F401
 from airflow.models.slamiss import SlaMiss  # noqa: F401
 from airflow.models.taskfail import TaskFail  # noqa: F401
@@ -39,4 +38,5 @@
 
 # Load SQLAlchemy models during package initialization
 # Must be loaded after loading DAG model.
+# noinspection PyUnresolvedReferences
 import airflow.jobs  # noqa: F401 isort # isort:skip