humanize granularity doesn't behave as expected

[chimaerase]

Issue Description

I'd like to use humanize() to print out human (engineer)-readable durations for a potentially long-running data processing pipeline I'm working on.

For my pipeline, the useful precision for duration output depends on how long a given run takes to execute, and can span seconds to hours. I'd settled on seconds as a useful metric for logging approximate runtime, but humanize() doesn't seem to be giving me the expected output when I add the granularity argument.

>> import arrow
>> now = arrow.utcnow()
>> mins = now.shift(seconds=-200)
>>> mins.humanize(now, only_distance=True)
'3 minutes'
>>> mins.humanize(now, only_distance=True, granularity="second")
'seconds'

I'd expected the final statement to ideally evaluate to "3 minutes 20 seconds", or at least to "200 seconds". If "granularity" is intended to convey a seconds remainder, e.g. in this case of "3 minutes 20 seconds" the output should still be "20 seconds". It's also worth noting that I thought the documentation on the granularity argument could use some fleshing out to disambiguate cases like i've mentioned here.

System Info

• 🖥 OS name and version: alpine3.9
• 🐍 Python version: Python 3.7.3 -->
• 🏹 Arrow version: 0.15.4

[systemcatch]

Hey @chimaerase we have a PR open right now (#722) which should solve the "3 minutes 20 seconds" part of your question.

As to the first part of your post it's definitely a possible bug in humanize, or at the very least surprising behavior. I think it wasn't designed for precision (anything from 10 to 44 seconds returns seconds ago) when granularity is auto. From the looks of it granularity = "second" needs to be altered to allow greater precision.