I agree with @amueller that it is not great style to have a helper function that is a 4-liner just to avoid an "if" statement in the main code.

The reason that I dislike such style is that when reading the calling code, you don't know what this function is doing, especially given the fact that '_fit' as a name doesn't doesn't tell what this does differently from calling the fit_function. All in all, the code is riddled with those small helper functions, and I find this hard to read.