convert to click for command line parsing and dispatching

[anarcat]

it was informally discussed (on IRC) the idea of converting the current tangled mess of argparse calls into the much cleaner click dispatcher.

this could get rid of a lot of code in archiver.py and make borg easier to extend.

it seems that click works in both 2.7 and 3.3 or higher, so it could be a problem until we drop support for 3.2 (#65).

[anarcat]

i tested click quickly in borgbackup/backupdata#1

it is kind of cool, but it's very idiomatic... for example, you can't have help= on "arguments". they somewhat assume they are sub-commands, which is fine for some times like serve, create and so on, but will be more annoying for the arguments to those commands, which will need to be described in the dispatcher's doc string.

i was also able to have a backwards compatibility layer to avoid depending on click, which may be nice...

[anarcat]

some parts of click are not so elegant:

http://click.pocoo.org/5/options/#boolean-flags

@click.option('--shout/--no-shout', default=False)

Boolean flags are options that can be enabled or disabled. This can be accomplished by defining two flags in one go separated by a slash (/) for enabling or disabling the option. (If a slash is in an option string, Click automatically knows that it’s a boolean flag and will pass is_flag=True implicitly.)

i would have expected this to be magic, ie. that you would not have to specify --no-whatever at all...

[ThomasWaldmann]

that would be less general. just think about --verbose and --quiet influencing the same "verbosity" boolean flag. with that magic, you'ld have to use --no-verbose.

[anarcat]

well, sure, but why can't we specify multiple arguments explicitely? something like:

@click.option('--verbose', toggle=['--quiet', '--no-verbose'])

and --no-verbose could be implicit (ie. not explicitely documented in each boolean option). i just find the string parsing to be clunky...

[ThomasWaldmann]

@anarcat btw, can you say how likely is it that we run into major problems when converting to click (like not being able to realize some arg parsing we have now with argparse or introducing bugs by the conversion or finding blocker bugs in click)?