Tkinter menu item underline syntax [RESOLVED]

[Chris Angelico]

On Thu, Jan 7, 2021 at 9:22 AM Grant Edwards <grant.b.edwards at gmail.com> wrote:
>
> On 2021-01-06, Rich Shepard <rshepard at appl-ecosys.com> wrote:
> > On Wed, 6 Jan 2021, 2QdxY4RzWzUUiLuE at potatochowder.com wrote:
> >
> >> For the sake of future generations who may run into this issue, can you
> >> post the complete, correct call to file_menu.add_command?
> >
> > This is the working version of the stanza I initially posted:
> >
> >       file_menu.add_command(
> >              label = 'New',
> >              command = self.callbacks['file->new', underline: 0],
> >              accelerator = 'Ctrl+N'
> >          )
>
> I'm completely baffled by that. Can somebody explain how this
> expression is evaluated?
>
>    self.callbacks['file->new', underline: 0]
>
> It appears that the dict callbacks is being accessed with the key of
> a tuple comprising a string and a slice.
>

Heh, you had to try it to make sure it was valid? Me too. Here's how
it's parsed:

self.callbacks[ # this bit's non-controversial
    'file->new' # string literal
    , # make tuple
    underline: 0 # slice(underline, 0, None)
]

So it takes the name "underline", whatever that is, and it assumes you
want a slice from underline to zero. That's all the second dimension
of the indexing, where the first is a simple string literal.

ChrisA