StrConfigItem returns None instead of 'none' if os.environ is modified

[braingram]

This issue arose while attempting to get stdatamodels tests to run locally (fix and semi-related discussion in: spacetelescope/stdatamodels#117).

A set of tests including a local import of crds always failed after the first call. This was traced back to a use of CRDS_REFERENCE_URI that returned None instead of 'none'. Here's a reproducer:

import os
import pytest

@pytest.fixture
def jail_environ():
    """Lock changes to the environment"""
    original = os.environ.copy()
    try:
        yield
    finally:
        os.environ = original

@pytest.mark.parametrize('value', ['1', '2'])
def test_crds(jail_environ, value):
    import crds
    assert crds.core.config.CRDS_REFERENCE_URI.get() == 'none'

If the import crds is moved outside the test (and no longer 'jailed' by the fixture) the tests pass without issue.

I'm not certain if this is a crds bug as crds might assume that the environment variables are unchanged between calls. A minimal reproducer (without pytest is):

import os
import crds

assert crds.core.config.CRDS_REFERENCE_URI.get() == 'none'
del os.environ['CRDS_REFERENCE_URI']
assert crds.core.config.CRDS_REFERENCE_URI.get() is None

The 'issue' isn't seen with an unmodified environment because the first call to StrConfigItem.get occurs within StrConfigItem.__init__ and the return value is thrown away. This call also sets the environment variable so future calls to StrConfigItem.get return the expected 'none'.

crds/crds/core/config.py

Line 139 in 4b19990

self.get()

I think the issue could be addressed by changing these None values to 'none' (but am not familiar with the inner workings of CRDS to predict any knock-on effects).

crds/crds/core/config.py

Lines 735 to 749 in 4b19990

	CRDS_CONFIG_URI = StrConfigItem("CRDS_CONFIG_URI", None,
	"Defines the URL/URI from which CRDS config files are downloaded.",
	lower=True)
	CRDS_MAPPING_URI = StrConfigItem("CRDS_MAPPING_URI", None,
	"Defines the URL/URI from which CRDS mapping files are downloaded.",
	lower=True)
	CRDS_REFERENCE_URI = StrConfigItem("CRDS_REFERENCE_URI", None,
	"Defines the URL/URI from which CRDS reference files are downloaded.",
	lower=True)
	CRDS_PICKLE_URI = StrConfigItem("CRDS_PICKLE_URI", None,
	"Defines the URL/URI from which CRDS pickle files are downloaded.",
	lower=True)