======================
 Allow Unknown Extras
======================

Sometimes we need to allow unknown extras.

The ``allow-unknown-extras`` option lets us do that in a buildout
configuration, just as we can directly calling ``easy_install``
works exactly like the one provided in ``easy_install``.

Let's create a develop egg that requires a bogus extra.

    >>> mkdir(sample_buildout, 'allowdemo')
    >>> write(sample_buildout, 'allowdemo', 'dependencydemo.py',
    ...       'import eggrecipekss.core')
    >>> write(sample_buildout, 'allowdemo', 'setup.py',
    ... '''from setuptools import setup; setup(
    ...     name='allowdemo', py_modules=['dependencydemo'],
    ...     zip_safe=True, version='1')
    ... ''')

Now let's configure the buildout to use the develop egg with a bogus extra.

    >>> write(sample_buildout, 'buildout.cfg',
    ... '''
    ... [buildout]
    ... develop = allowdemo
    ... parts = eggs
    ...
    ... [eggs]
    ... recipe = zc.recipe.egg:eggs
    ... eggs = allowdemo[bad_extra]
    ... ''')

Now we can run the buildout and see that it fails:

    >>> print_(system(buildout), end='') # doctest: +ELLIPSIS
    Develop: '/sample-buildout/allowdemo'
    Installing eggs...
    ...
    While:
      Installing eggs.
    Error: Couldn't find the required extra...

If we flip the option on, the buildout succeeds

    >>> write(sample_buildout, 'buildout.cfg',
    ... '''
    ... [buildout]
    ... develop = allowdemo
    ... parts = eggs
    ... allow-unknown-extras = true
    ...
    ... [eggs]
    ... recipe = zc.recipe.egg:eggs
    ... eggs = allowdemo[bad_extra]
    ... ''')


Now we can run the buildout and only get a warning::

    >>> print_(system(buildout), end='') # doctest: +ELLIPSIS
    Develop: '/sample-buildout/allowdemo'
    Installing eggs...
    allowdemo 1 does not provide the extra 'bad_extra'