//- 💫 DOCS > USAGE > PROCESSING PIPELINES > ATTRIBUTE HOOKS

p
    |  Hooks let you customize some of the behaviours of the #[code Doc],
    |  #[code Span] or #[code Token] objects by adding a component to the
    |  pipeline. For instance, to customize the
    |  #[+api("doc#similarity") #[code Doc.similarity]] method, you can add a
    |  component that sets a custom function to
    |  #[code doc.user_hooks['similarity']]. The built-in #[code Doc.similarity]
    |  method will check the #[code user_hooks] dict, and delegate to your
    |  function if you've set one. Similar results can be achieved by setting
    |  functions to #[code Doc.user_span_hooks] and #[code Doc.user_token_hooks].

+code("Polymorphic similarity example").
    span.similarity(doc)
    token.similarity(span)
    doc1.similarity(doc2)

p
    |  By default, this just averages the vectors for each document, and
    |  computes their cosine. Obviously, spaCy should make it easy for you to
    |  install your own similarity model. This introduces a tricky design
    |  challenge. The current solution is to add three more dicts to the
    |  #[code Doc] object:

+aside("Implementation note")
    |  The hooks live on the #[code Doc] object because the #[code Span] and
    |  #[code Token] objects are created lazily, and don't own any data. They
    |  just proxy to their parent #[code Doc]. This turns out to be convenient
    |  here — we only have to worry about installing hooks in one place.

+table(["Name", "Description"])
    +row
        +cell #[code user_hooks]
        +cell Customise behaviour of #[code doc.vector], #[code doc.has_vector], #[code doc.vector_norm] or #[code doc.sents]

    +row
        +cell #[code user_token_hooks]
        +cell Customise behaviour of #[code token.similarity], #[code token.vector], #[code token.has_vector], #[code token.vector_norm] or #[code token.conjuncts]

    +row
        +cell #[code user_span_hooks]
        +cell Customise behaviour of #[code span.similarity], #[code span.vector], #[code span.has_vector], #[code span.vector_norm] or #[code span.root]

p
    |  To sum up, here's an example of hooking in custom #[code .similarity()]
    |  methods:

+code("Add custom similarity hooks").
    class SimilarityModel(object):
        def __init__(self, model):
            self._model = model

        def __call__(self, doc):
            doc.user_hooks['similarity'] = self.similarity
            doc.user_span_hooks['similarity'] = self.similarity
            doc.user_token_hooks['similarity'] = self.similarity

        def similarity(self, obj1, obj2):
            y = self._model([obj1.vector, obj2.vector])
            return float(y[0])