Output specified letter groups in --list

[Badboy-16]

Trying to fix #19

Usage:

# List all letter groups
$ timezone-converter --list
# List timezones beginning with 'm'
$ timezone-converter --list m
# List timezones beginning with 'b', 'd', or 't'
$ timezone-converter --list bdt

[ibLeDy]

I left some comments, I hope you don't mind doing some changes.

(clicked approve by mistake, sorry for the confusion)

[ibLeDy]

I left some comments, I hope you don't mind doing some changes.

[ibLeDy]

I updated the comments, as the suggestion to use [None] as the const value was not really optimal. My improved suggestion is to just use True.

[Badboy-16]

Thanks for the suggestions from both of you! I pushed some new commits according to the requested changes.

However, for the comparison part in list_view.ListView._sort_and_group, I used if self.letters rather than if self.letters is True, as I found the latter doesn't seem to work when testing the --list output.

I also tested on some pseudo-code in Python shell.

>>> a = ['x', 'y', 'z']
>>> if a is True:
...     print('foo')
... 
>>> if a:
...     print('bar')
... 
bar

[ibLeDy]

Yeah, the suggestion was that we could use True as a value for const, but it was not a really good option. However, using ascii_lowercase solves all issues and aven simplifies the code a bit.

[ibLeDy]

I left a comment regarding an unnecesary if statement, but this looks good!

[ibLeDy]

This if statement is not necessary, as we are using list(string.ascii_lowercase) as a value for const in main.py.

[ibLeDy]

I seems that I always forget that the request changes button exists, sorry about that.

Please @Badboy-16 , take a look at the comment about the unnecessary if statement. When that is fixed this will be ready to be merged!

[Badboy-16]

Have just removed the if statement and the test output looks ok 🎉

[ibLeDy]

Thanks again for your time and for this contribution! I'll create a new release in a bit