Help using forward slashes inside f-strings

[ali-ramadhan]

I'm playing around with path formats and noticed that if I put a slash inside an f-string then Moe fails to parse it as it thinks it's an unterminated f-string (see track_path and stack trace below).

I tried some combinations but couldn't get this to work (probably my Python is rusty). I don't think the nested f-string is the issue but rather it seems like the track_path was split with / as a separator?

track_path = "{f'Disc {track.disc:1d}/{track.disc:1d}-' if album.disc_total > 1 else ''}{track.track_num:02d}. {track.title}{track.path.suffix}"
                                     ^

Stacktrace:

Traceback (most recent call last):
  File "/home/alir/build/Moe/.venv/bin/moe", line 6, in <module>
    sys.exit(main())
  File "/home/alir/build/Moe/moe/cli.py", line 168, in main
    _parse_args(args)
  File "/home/alir/build/Moe/moe/cli.py", line 114, in _parse_args
    parsed_args.func(args=parsed_args)
  File "/home/alir/build/Moe/moe/plugins/add/add_cli.py", line 87, in _parse_args
    _add_path(path, album)
  File "/home/alir/build/Moe/moe/plugins/add/add_cli.py", line 122, in _add_path
    moe_add.add_item(Album.from_dir(path))
  File "/home/alir/build/Moe/moe/plugins/add/add_core.py", line 78, in add_item
    session.flush()
  File "/home/alir/build/Moe/.venv/lib/python3.9/site-packages/sqlalchemy/orm/session.py", line 3429, in flush
    self._flush(objects)
  File "/home/alir/build/Moe/.venv/lib/python3.9/site-packages/sqlalchemy/orm/session.py", line 3458, in _flush
    self.dispatch.before_flush(self, flush_context, objects)
  File "/home/alir/build/Moe/.venv/lib/python3.9/site-packages/sqlalchemy/event/attr.py", line 334, in __call__
    fn(*args, **kw)
  File "/home/alir/build/Moe/moe/library/lib_item.py", line 160, in _edit_before_flush
    config.CONFIG.pm.hook.edit_new_items(items=new_items)
  File "/home/alir/build/Moe/.venv/lib/python3.9/site-packages/pluggy/hooks.py", line 286, in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
  File "/home/alir/build/Moe/.venv/lib/python3.9/site-packages/pluggy/manager.py", line 93, in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
  File "/home/alir/build/Moe/.venv/lib/python3.9/site-packages/pluggy/manager.py", line 84, in <lambda>
    self._inner_hookexec = lambda hook, methods, kwargs: hook.multicall(
  File "/home/alir/build/Moe/.venv/lib/python3.9/site-packages/pluggy/callers.py", line 208, in _multicall
    return outcome.get_result()
  File "/home/alir/build/Moe/.venv/lib/python3.9/site-packages/pluggy/callers.py", line 80, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/home/alir/build/Moe/.venv/lib/python3.9/site-packages/pluggy/callers.py", line 187, in _multicall
    res = hook_impl.function(*args)
  File "/home/alir/build/Moe/moe/plugins/move/move_core.py", line 71, in edit_new_items
    copy_item(item)
  File "/home/alir/build/Moe/moe/plugins/move/move_core.py", line 238, in copy_item
    _copy_album(item)
  File "/home/alir/build/Moe/moe/plugins/move/move_core.py", line 253, in _copy_album
    _copy_file_item(track)
  File "/home/alir/build/Moe/moe/plugins/move/move_core.py", line 263, in _copy_file_item
    dest = fmt_item_path(item)
  File "/home/alir/build/Moe/moe/plugins/move/move_core.py", line 102, in fmt_item_path
    new_path = _fmt_track_path(item)
  File "/home/alir/build/Moe/moe/plugins/move/move_core.py", line 132, in _fmt_track_path
    track_path = _eval_path_template(config.CONFIG.settings.move.track_path, track)
  File "/home/alir/build/Moe/moe/plugins/move/move_core.py", line 151, in _eval_path_template
    path_part = _lazy_fstr_item(template_part, lib_item)
  File "/home/alir/build/Moe/moe/plugins/move/move_core.py", line 199, in _lazy_fstr_item
    return eval(f'f"""{template}"""')
  File "<string>", line 1
    f"""{f'Disc {track.disc:1d}"""
                                 ^
SyntaxError: f-string: unterminated string

[ali-ramadhan]

Indeed looks like track_path is split with / as a separator so you cannot have / inside an f-string in track_path otherwise the splitting produces unterminated f-strings.

Moe/moe/plugins/move/move_core.py

Line 148 in de6ebd2

template_parts = template.split("/")

This might not be an issue for me since I'm planning on writing a plugin to handle disc and track numbering for me, but I'm wondering whether this is an intentional limitation or a bug.

This can also show up in cases where you want to divide a number, e.g. {f'track.sample_rate/1000'}kHz.

[jtpavlock]

That's correct, forward slashes are explicitly used to separate paths, so you can't have them inside an f-string variable. I don't necessarily consider this a bug, but I do think a note should be added in the documentation noting the limitation.

I don't think this limitation is too difficult to work around, and I think it would be a huge pain to try to allow the behavior so I don't really consider it worthwhile to implement.

The main issue is discerning forward slashes that separate paths from forward slashes that appear from variables such as in an artist name or something. That's why we split first and then sanitize any forward slashes that appear afterwards so they don't appear in the final path.

Edit: docs added in e392114

[ali-ramadhan]

Thanks for clarifying and for updating the documentation. Indeed it was easy to work around for the {f'track.sample_rate/1000'}kHz case, but I'll open a new discussion the case where new directories are created under certain conditions.