Best practice for lazy loading Python modules

class Handler(...):
    ...
    def render_with_jinja2(self, values, template_name):
        import jinja2
        env = jinja2.Environment(...)
        ...

There's no need to cache the imported module; Python does that already.

There's no reason for you to keep track of imports manually -- the VM maintains a list of modules that have already been imported, and any subsequent attempts to import that module result in a quick dict lookup in sys.modules and nothing else.

The difference between your code and

def render_with_jinja2(self, values, template_name):
    import jinja2
    env = jinja2.Environment(...)

is zero -- when we hit that code, if jinja2 hasn't been imported, it is imported then. If it already has been, execution continues on.

Nice pattern from sqlalchemy: dependency injection:

@util.dependencies("sqlalchemy.orm.query")
def merge_result(query, *args):
    #...
    query.Query(...)

Instead of declaring all "import" statements at the top of the module, it will only import a module when it's actually needed by a function. This can resolve circular dependency problems.

The other answers have covered the actual details but if you are interested in a lazy loading library, check out apipkg which is part of the py package (py.test fame).