Sometimes you have what appears to be a tuple with a leading underscore as in
return _('my_' + bar)
In this case, what's going on is that _() is an alias for a localization function that operates on text to put it into the proper language, etc. based on the locale. For example, Sphinx does this, and you'll find among the imports
from sphinx.locale import l_, _
and in sphinx.locale, _() is assigned as an alias of some localization function.
You can use the following video tutorials to clear all your doubts:-