ERROR Failed with KeyError: 'slnt'

Observed Behavior

After making jost[slnt,wght].ttf in with fontmake as below

fontmake -g source/jost.glyphs -o variable --round-instances -a --output-path source/vf/jost[slnt,wght].ttf

When I put output jost[slnt,wght].ttf through fontbakery I get the following error

💔 ERROR: Check variable font instances have correct coordinate values
com.google.fonts/check/varfontinstancecoordinates

💔 ERROR Failed with KeyError: 'slnt'

Expected Behavior

Should not have an issue with 'slnt"

Others have experienced this as well

@thundernixon had a similar error with his ‘mono’ axis

 >> com.google.fonts/check/varfont_instance_coordinates
   Check variable font instances have correct coordinate values
   with font_betas/recursive-MONO_CASL_wght_slnt_ital--full_gsub--2019_11_19-19_44.ttf

   * ERROR: Failed with KeyError: 'MONO'
          File "/Users/stephennixon/type-repos/recursive/venv/lib/python3.7/site-packages/fontbakery/checkrunner.py", line 344, in
_exec_check
            for sub_result in result:  # Might raise.
          File "/Users/stephennixon/type-repos/recursive/venv/lib/python3.7/site-packages/fontbakery/profiles/googlefonts.py", lin
e 3974, in com_google_fonts_check_varfont_instance_coordinates
            if instance.coordinates[axis] != expected_instance.coordinates[axis]:

[m4rc1e]

I’m partway through refactoring these checks. I’ve used @arialcrime’s family as a reference.

The updated VF instance checks will do the following:

• Submit a warn if instance names are not fully parsable. It will also output the unparsable tokens.
• Submit a fail if instance coordinates are incorrect for known axes
• Submit a fail if the fvar contains known axes and they’re not mentioned in instance names.
• Submit a fail if instance names is incorrectly ordered
• If any fail or warn occurs, output a hint to study the GF spec (wip)

    * WARN: Instance "144 G100 Thin": contains the following unparsable tokens "['144', 'G100']"
    * FAIL: Instance "144 G100 Thin": does not contain "opsz" token.
    * WARN: Instance "144 G100 Light": contains the following unparsable tokens "['144', 'G100']"
    * FAIL: Instance "144 G100 Light": does not contain "opsz" token.
    * WARN: Instance "144 G100 Regular": contains the following unparsable tokens "['144', 'G100']"
    * FAIL: Instance "144 G100 Regular": does not contain "opsz" token.
    ...
    * WARN: Instances may be incorrect. See our spec for further info ...

[thundernixon]

As far as I can tell, it seems that the problem is with fontbakery’s instance_parse() function, which appears to set up expected instance axis values based on the instance name … BUT only includes values for opsz, wght, and wdth.

https://github.com/googlefonts/fontbakery/blob/d78810411152f449b1d302f7401d4e3e2c2bd775/Lib/fontbakery/parse.py#L183-L194

We (probably) can’t really predict the values of other axes, like slnt or MONO. So basically, I think we just need something that only checks the name based on the axes that are in the instance parse, but doesn’t try to check for axes that aren’t.