__init__.py

import imp
import sys

from .cs50 import *

class CustomImporter(object):
    """
    Import cs50.SQL lazily so that rest of library can be used without SQLAlchemy installed.

    https://docs.python.org/3/library/imp.html
    http://xion.org.pl/2012/05/06/hacking-python-imports/
    http://dangerontheranger.blogspot.com/2012/07/how-to-use-sysmetapath-with-python.html
    """
    def find_module(self, fullname, path=None):
        if fullname == "cs50.SQL":
            return self
        return None
    def load_module(self, name):
        if name in sys.modules:
            return sys.modules[name]
        from .sql import SQL
        sys.modules[name] = SQL
        return SQL
sys.meta_path.append(CustomImporter())