Significant figures calculation

[Harold]

> > I tried to modify the DecimalContext (e.g. getcontext().prec = 2) but
> > that did not lead to the correct behavior.
>
> Really? It works for me.

You are right, I did something wrong when attempting to set the
precision.
And the trick with rounding the decimal with the unary + is neat.
It's the first time for me to play with decimals, so bare with me if I
miss
the obvious.

However, this approach forces you to know the number of significant
figures
beforehand -- which is precisely what the arithmetics should do for
you.
What would indeed be nice, is Jerry's suggestion to obtain the number
of
significant bits and write a small wrapper around the number protocoll
implementation that accounts significance and have __str__/__repr__
set
the context dynamically.

I haven't seen anything obvious in the docs, though it might be
possible
to use log10 of the length of some normalized string representation.