fixes airflow provider init sequence (#569)

dlt/common/configuration/providers/airflow.py

@@ -1,5 +1,3 @@
-from airflow.models import Variable
-
 from .toml import VaultTomlProvider
 
 
@@ -13,6 +11,7 @@ def name(self) -> str:
 
     def _look_vault(self, full_key: str, hint: type) -> str:
         """Get Airflow Variable with given `full_key`, return None if not found"""
+        from airflow.models import Variable
         return Variable.get(full_key, default_var=None)  # type: ignore
 
     @property

dlt/common/configuration/specs/config_providers_context.py

@@ -105,28 +105,36 @@ def _airflow_providers() -> List[ConfigProvider]:
     task context will not be available. Still we want the provider to function so
     we just test if Airflow can be imported.
     """
-    if not is_airflow_installed():
-        return []
+
+    providers: List[ConfigProvider] = []
 
     try:
         # hide stdio. airflow typically dumps tons of warnings and deprecations to stdout and stderr
         with contextlib.redirect_stdout(io.StringIO()), contextlib.redirect_stderr(io.StringIO()):
-            from airflow.operators.python import get_current_context  # noqa
-
+            # try to get dlt secrets variable. many broken Airflow installations break here. in that case do not create
             from airflow.models import Variable # noqa
             from dlt.common.configuration.providers.airflow import AirflowSecretsTomlProvider
             # probe if Airflow variable containing all secrets is present
             from dlt.common.configuration.providers.toml import SECRETS_TOML_KEY
             secrets_toml_var = Variable.get(SECRETS_TOML_KEY, default_var=None)
 
+            # providers can be returned - mind that AirflowSecretsTomlProvider() requests the variable above immediately
+            providers = [AirflowSecretsTomlProvider()]
+
+            # check if we are in task context and provide more info
+            from airflow.operators.python import get_current_context  # noqa
+            ti = get_current_context()["ti"]
+
+        # log outside of stderr/out redirect
         if secrets_toml_var is None:
             message = f"Airflow variable '{SECRETS_TOML_KEY}' was not found. " + \
                 "This Airflow variable is a recommended place to hold the content of secrets.toml." + \
                 "If you do not use Airflow variables to hold dlt configuration or use variables with other names you can ignore this warning."
-            ti = get_current_context()["ti"]
             ti.log.warning(message)
+
     except Exception:
         # do not probe variables when not in task context
         pass
 
-    return [AirflowSecretsTomlProvider()]
+    # airflow not detected
+    return providers

tests/helpers/airflow_tests/test_airflow_provider.py

@@ -27,6 +27,9 @@ def test_dag():
         from dlt.common.configuration.providers.airflow import AirflowSecretsTomlProvider
 
         Variable.set(SECRETS_TOML_KEY, SECRETS_TOML_CONTENT)
+        # make sure provider works while creating DAG
+        provider = AirflowSecretsTomlProvider()
+        assert provider.get_value("api_key", str, None, "sources")[0] == "test_value"
 
         @task()
         def test_task():
@@ -78,6 +81,7 @@ def test_dag():
 
         # this will initialize provider context
         api_key = secrets["sources.api_key"]
+        assert api_key == "test_value"
 
         @task()
         def test_task():