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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def validate_feature (name):
 """ Checks if all name is a valid feature. Otherwise, raises an exception. """

    assert isinstance(name, basestring)
    if name not in __all_features:
        raise InvalidFeature ("'%s' is not a valid feature name" % name)
    else:
        return __all_features[name]