More explicit error when trying to access the child of non-container node

[odelalleau]

When using ${a.b}, if a is not a container node, the previous code was failing with an obscure AssertionError.

This commit was originally the first commit in the stack in #321 but was unrelated to other changes in that PR, so it makes more sense to create a different PR.

I don't think it's worth adding an issue and a news item for this (since it seems pretty minor) but let me know if I should.

Note: the motivation for adding test_assertion_error() is that otherwise coverage would drop below 100%.

[omry]

curious about test_assertion_error.
what line in the code is not covered unless you have that?

[omry]

regardless, this looks fine.
I will merge after you answer my question.

[odelalleau]

curious about test_assertion_error.
what line in the code is not covered unless you have that?

This line:

omegaconf/omegaconf/_utils.py

Line 606 in 800e8a7

raise

[omry]

Okay, let's keep that - it may come in handy next time there is some random assertion.

[omry]

Generally, the contact of OmegaConf.select is that it returns None if the field is not found.
This fix is causing inconsistent behavior:

In [14]: cfg
Out[14]: {'a': 10, 'b': None}

In [15]: print(OmegaConf.select(cfg, "foo.bar"))
None

In [16]: print(OmegaConf.select(cfg, "b.bar"))
...
ConfigKeyError: Error trying to access b.bar: node `b` is not a container and thus cannot contain `bar`
    full_key: b.bar
    object_type=dict

Obviously this is better than assertion error, but it's not the correct fix, at least for the purposes of the public OmegaConf.select().