I suggest using the [Extras](http://peak.telecommunity.com/DevCenter/setuptools#declaring-extras-optional-features-with-their-own-dependencies) section of the `setup.py` file to install optional stuff. For example, if you have something like this in your setup file:

```python
setup(
    # Other really important stuff...
    extras_require = {
        'mongo':  ['pymongo==1.9', 'otherlibrary'],
    }
)
```

Then, the user will be able to install the mongo version of diesel by issuing the following `pip` call:

```bash
$ pip install diesel[mongo]
```

I don't know how it would play with the requirements file though