Updated set command to support tab completion of values

[kmvanbrunt]

Closes #870 #871

[codecov]

Codecov Report

Merging #873 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #873      +/-   ##
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+ Coverage   97.37%   97.39%   +0.01%     
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  Files          14       14              
  Lines        3580     3602      +22     
==========================================
+ Hits         3486     3508      +22     
  Misses         94       94              

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[tleonhardt]

Looks good

[kotfu]

This all looks pretty good to me. I'll add one suggestion.

If the list of valid values is not known at initialization time (which is when settables would typically get defined), you have to do really awkward stuff in order to validate the new value of a setting when a user changes it. The onchange_cb doesn't have access to self (i.e. the cmd2 subclass), so my callback doesn't have access to all the data in my app.

If I somehow manage to say the input isn't valid, there is no way for my callback to to say "hey this isn't a valid value, I don't want to allow the user to change it" back to cmd2. I think throwing a ValueError exception would be ideal here, and cmd2 could then catch it and not allow the change in the value.

[kmvanbrunt]

@kotfu The onchange_cb can have access to self if you set it to a bound method of your app.

onchange_cb = self.val_changed

As far as validation, that's the job of Settable.val_type. It corresponds to the type field in the argparse.add_argument() function. Any exception it raises is handled in do_set() by printing it with perror() and not changing the value.

[kotfu]

Go it. I missed that val_type was a callable. You already have implemented my suggestion. This looks good to me.

[kmvanbrunt]

@kotfu I update the docstring for Settable to be more clear.

[kotfu]

hold up before we merge this, I need to fix the settings.rst documentation and update examples/environment.py

[tleonhardt]

This looks good so I'm going to merge it