Selective importing and package dependencies

[David Pratt]

Hi. I am in the midst of preparing a package to convert between
various schemas including orms. The issue is I don't want django,
slqalchemy, storm, rdflib etc. as hard dependencies of the package.
Each module is a schema to schema conversion. As an example, I have
imports for sqlalchemy with classes and methods that use them.

from sqlalchemy.util import OrderedDict
from sqlalchemy import types as rdbtype
import sqlalchemy as sa

I have my own ideas about how I might do this but looking for
recommendations from others how they would handle this so the result
would be:

1. no hard dependencies on any of these other packages
2. load the module without failure.
3. import the dependent package if available to perform the conversion

Many thanks
David