I think that there are very few non-programmer, numerically-naive, TI-89 users working with Python,
For numerical applications there is Numpy.
and I think that shaping Python specifically for them is a mistake. It is completely appropriate to build an interactive calculator using Decimals aimed at the TI-89-using audience, and I applaud that. But that's an application written in Python, not Python itself.
The decimal module really should only be used for financial applications; there should be a separate high-precision float type.
Another note on the 'd' notation - this might be confused with the FORTRAN 'd' for double precision number. Would you write, 1.2e3d or 1.2d3?