add pkg version method (and unit tests)

[kwmonroe]

Add helper to return a version string given a package name (currently, only dpkg is supported). This is useful for hookenv.application_version_set when we only have the package manager to rely on.

[petevg]

I grumped about the acceptance and return of null values, but if you have a justification for them that I'm missing, I have no problem merging this code ...

[petevg]

What is the reason for accepting None here? Wouldn't it be simpler to just force the user to pass in a package?

[kwmonroe]

Good point -- no one should call this method without a pkg name.

However, i do still want to make sure the value is truthy. If someone accidentally called this with an empty string, dpkg would happily return the version for every package on the system.

[petevg]

I that I'd rather see us pass the exception up to the caller, rather than returning a null value, which might cause exceptions higher up.

If you have a good reason to need the None, feel free to ignore.

[kwmonroe]

I opted for returning None in case something prevented dpkg from running (eg, something else has the dpkg lock). Since the main use is just for status output, I didn't want to put the charm in a failed state just because we weren't able to determine the pkg version.

I figured the caller would check for a valid return, and if it was None, they could decide whether to set something (like "unkown"), retry later, or do nothing.

[petevg]

@kwmonroe I think that it might make more sense to pass "unknown" or an empty string in the case where we can't get the package. That way, if someone naively joins the string with some other output, they won't necessarily get an error.

[kwmonroe]

ack, i'll do empty string so callers can still do ver = get_package_version(foo) or 'unknown'

[petevg]

This LGTM/+1 (have a slight preference for always returning a string, but I can see the argument for passing back None if we can't get the package version.)

[kwmonroe]

i've split the difference with ya @petevg. we don't accept pkg=None anymore, but we don't pass dpkg exceptions up to the caller.. if something goes afoul, we'll return an empty string.

[petevg]

+1 :-)