from decimal import Decimal
names = dict(a=Decimal('3.625'), b=Decimal(2))
expr = '(a + 2.625) / b' # expression entered by end-user
eval(expr, names)

Ideally I'd like to have a way to tell the interpreter to use Decimal
by default instead of float (but only in the eval() calls). I
understand the performance implications and they are of no concern. I'm
also willing to define a single global Decimal context for the
expressions (not sure if that matters or not). Is there a way to do
what I want without rolling my own parser and/or interpreter? Is there
some other alternative that would solve my problem?

What about :

c = compile(thestring, thestring, '<eval>')

cc = new.code( ...all args from c's attributes, except the 5th
one, constants, which should instead be:
decimalize(c.co_consts)...)

Alex Martelli wrote:

What about :

c = compile(thestring, thestring, '<eval>')

cc = new.code( ...all args from c's attributes, except the 5th
one, constants, which should instead be:
decimalize(c.co_consts)...)
Wow, what an elegant solution! I had no hope that it would be this

Heh, funny, I was originally parsing your response as ironic, because
_I_ don't think of this as elegant -- too boilerplatey (as the expansion
I showed right after dispays!)... took me a sec to see you really mean
it!-)
simple. I always wondered what compile() was useful for and now I know
at least one thing. I'll try it out tomorrow. Thanks a lot Alex!