Minimal problem case, if the content of routers.jinja2 is:

from anymodule import {{tag}}

generation will fail with a "list index out of bounds error".

Instead, this:

from anymodule import anything, {{tag}}

will work as expected.

From the trace of the failure, the problem seems to be that imports are checked in order to correctly insert commas, and in the first case a list ends up being empty probably due to the only import being a template variable.

In the following invoked isort code (from the trace), the "just_imports") list ends up being empty, while import string is from anymodule import ending abruptly.

site-packages\isort\parse.py:522

 if "," in import_string.split(just_imports[-1])[-1]:
   trailing_commas.add(import_from)

I'm not sure if this is fixable since it depends on some other libraries' behaviour, but since adding any other name before {{tag}} makes it work, I thought it might have been worth the effort.