Does this show what you mean?

>>> sqrt(.1).n(40)
0.3162277660168379411764760789083084091544

>>> sqrt(x).n(40,subs=dict([(x,.1)]))
0.3162277660168379419769730258850242641672
>>> sqrt(Float(.1,40)).n(40)
0.3162277660168379419769730258850242641672>>>


On Fri, Dec 6, 2013 at 8:13 PM, Aaron Meurer <notifications@github.com>wrote:

> In sympy/core/basic.py:
>
> > +        limiting behavior of the expression:
> > +
> > +        >>> (x**3 - 3*x).subs({x: oo})
> > +        nan
> > +
> > +        If the substitution will be followed by numerical
> > +        evaluation, it is better to pass the substitution to
> > +        evalf as
> > +
> > +        >>> sqrt(x).evalf(subs={x: 16}, n=17)
> > +        4.0000000000000000
> > +
> > +        rather than
> > +
> > +        >>> sqrt(x).subs({ x: 1}).evalf()
> > +        4.00000000000000
>
> Aside from accuracy loss, another very important use case is when you
> change the default precision.
>
> —
> Reply to this email directly or view it on GitHub<https://github.com/sympy/sympy/pull/2649/files#r8179786>
> .
>