One purpose of _source is to make the namedtuple self-documenting -- that has been a considerable benefit over the years and has resulted in many fewer questions and issues than we usually get about our other tools.

Another purpose of _source is so that you can write it out to a module. This lets you pre-compute the named tuple for speed and it makes it possible to easily Cythonize it or to customize it (in particular, people like to take out the error checking).

Since the namedtuple() factory function was already creating the source string as part of its operation, just storing it and exposing the attribute had nearly zero additional cost.

That said, don't worry about it, next week I plan to make this a lazily generated attribute so you all can sleep soundly at night :-)