Apache Airflow version:
1.10.10

Kubernetes version (if you are using kubernetes) (use kubectl version):

Environment:

• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others:

What happened:
I set store_serialized_dags as an environment variable: export AIRFLOW__CORE__STORE_SERIALIZED_DAGS=True.

The default value of store_dag_code is %(store_serialized_dags)s, so I expect it to now also be True. However, it's not (it's False).

What you expected to happen:
I expect store_dag_code to have the same value as store_serialized_dags.

It's because the interpolation is applied to the value set in .cfg, but not when it's set via an env var.

How to reproduce it:

Was able to reproduce it in a test (add this to test_configuration.py):

    def test_interpolation_from_env_var(self):
        """Test if interpolation works when the substituted value is set by an environment variable."""
        test_config = '''[test]
key1 = testme
key2 = %(key1)s
'''
        with mock.patch.dict('os.environ', AIRFLOW__TEST__KEY1="something_else"):
            test_conf = AirflowConfigParser(default_config=test_config)
            key2 = test_conf.get("test", "key2")
            self.assertEqual(key2, "something_else")

So far I haven't been able to make any sense of airflow/configuration.py, to fix it.

Anything else we need to know: