Full-commented-out yaml file contents return ugly error

[jtcohen6]

Describe the bug

A user reports seeing this error when trying to parse their project after upgrading to v0.20.0:

Encountered an error:
argument of type 'NoneType' is not iterable

This is the stacktrace:

File "/usr/local/Cellar/dbt/0.20.0_1/libexec/lib/python3.8/site-packages/dbt/parser/schemas.py", line 560, in check_format_version
    if 'version' not in yaml_dct:
TypeError: argument of type 'NoneType' is not iterable

That's very surprising! The error cropping up in this line:
https://github.com/dbt-labs/dbt/blob/f460d275baa4df8cf8713f27e198a3577d6ea71c/core/dbt/parser/schemas.py#L560

Means that yaml_dct must be set to None, somehow. That method is called here:
https://github.com/dbt-labs/dbt/blob/f460d275baa4df8cf8713f27e198a3577d6ea71c/core/dbt/parser/read_files.py#L21-L23

It's pretty hard to imagine yaml_from_file returning None. I've tried a couple different things to trip this error (an empty .yml file, malformed contents, simply the word 'None'), but none (hah) of my attemps has worked. The only way I've managed to reproduce it is by stepping directly into the validate_yaml or yaml_from_file method calls and setting the yaml dictionary to None.

One important hint: The user reports that this project runs fine on earlier versions of dbt. It's only with the upgrade to v0.20.0 that they started seeing this error.

Steps To Reproduce

???

Expected behavior

Definitely not this error!

The output of dbt --version:

/usr/local/Cellar/dbt/0.20.0_1/

(i.e. dbt v0.20.0 installed via Homebrew)

[nathaniel-may]

I wanted to take a quick peek at this, and drilling all the way down to our call to the yaml loader, we aren't passing along any None values in dbt. Looking at the PyYaml documentation, this line stands out: "If there are no documents in the stream, it returns None." This, of course, is still very odd.

[jtcohen6]

reproduction case

Create a yaml file that's all comments:

# models/schema.yml
# only comments here

Try parsing the project:

$ dbt --no-partial-parse parse
Running with dbt=0.20.0
14:19:08 | Start parsing.
14:19:08 | Dependencies loaded
14:19:08 | ManifestLoader created
Encountered an error:
argument of type 'NoneType' is not iterable

I think the catch here is, source_file.contents is truthy, but yaml.load() of those contents returns None.

[leahwicz]

@nathaniel-may I'm assigning this to you to look into some more