How to get value from multiple type possibilities

[sgleske-ias]

I have a YAML file which describes languages and toolchains (e.g. .jervis.yml or Travis CI .travis.yml).

Goal

Get a version of a toolchain from a YAML file (e.g. python) when the toolchain could look like different types (or have nothing defined).

YAML object type possibilities

Please note: I would look to the python key for the version of python.

python not defined.

language: python

or python as a number

language: python
python: 3.6

or python as a string

language: python
python: "3.6"

or python as a seq

language: python
python:
  - 3.6
  - 2.7

Goal

Trying to get the version of python from a list (the first item only), or fall back to getting the value (e.g. 3.6), or fall back to a documented default 2.7.

My hack

In bash, I could put in a series of defaults. For example,

python_version="$(yq eval '.python[0]' .jervis.yml)"
if [ -z "${python_version:-}" -o "${python_version:-}" = null ]; then
    python_version="$(yq eval '.python // "2.7"' .jervis.yml)"
fi

echo "Python version found: ${python_version}"

The above script is the only way I could figure out how to make it work. Is there a yq one liner which would work? I'm at my wit's end on this one.

My failed attempt at a one liner

Please note: I tried

yq eval '.python[0] // .python // "2.7"' .jervis.yml

But it breaks down so I must be misunderstanding it. I also attempted some type checking but it didn't appear to work the way I thought it would.

yq eval '(.python | select((. | tag) == "!!seq") | .[0]) // .python // "2.7"' .jervis.yml

[mikefarah]

Yeah I see - let me take a look I think it's a problem with the // operator

[mikefarah]

Yep found the bug - will fix in the next release

[mikefarah]

FYI - with the new release, the following will work:

yq e '(.python | select(tag == "!!seq") | .[0]) // .python // "2.7"'

[sgleske-ias]

Great; thanks @mikefarah ! I thought it could be a bug but I wasn't trusting my instinct since I'm new to yq.

Edit for credit: 25e0a82 nice

[mikefarah]

Hi @sgleske-ias this has been fixed in the latest release (4.7.0) let me know how it goes :)

[sgleske-ias]

@mikefarah Thanks! I confirmed the fix by downloading 4.7.1.