Filtering on how words are being used

Yesterday I wrote about how you could use the spaCy Python library to find proper nouns in a document. Now suppose you want to refine this and find proper nouns that are the subjects of sentences or proper nouns that are direct objects.

This post was motivated by a project in which I needed to pull out company names from a large amount of text, and it was important to know how the company name was being used.

Dependency labels

Tokens in spaCy have a dependency label attribute dep (or dep_ for its string representation). Dependency labels tell you how a word is being used. For example, dobj tells you the word is being used as a direct object, and nsubj tells you its being used as a nominal subject.

In yesterday’s post the line

    if tok.pos_ == "PROPN":

filtered tokens to look for proper nouns. We could modify the script to also tell us how the proper nouns are being used by printing tok.dep_.

There are three proper nouns in the opening paragraph of Moby Dick: Ishmael, November, and Cato.

Call me Ishmael. … whenever it is a damp, drizzly November in my soul … With a philosophical flourish Cato throws himself upon his sword …

If we run

    if tok.pos_ == "PROPN":
        print(tok, tok.dep_)

on the first paragraph we get

    Ishmael oprd
    November attr
    Cato nsubj

but it’s not obvious what the output means. If we wrap tok.dep_ with spacy.explain we get a more verbose explanation.

    Ishmael object predicate
    November attribute
    Cato nominal subject

Pulling out subjects

Now suppose we wanted to pull out words that are subjects. We could filter on tok.dep_ == "nsubj" but there are more kinds of subjects than just nominal subjects. There are six kinds of subjects:

  1. nsubj: nominal subject
  2. nsubjpass: nominal passive subject
  3. csubj: clausal subject
  4. csubjpass: clausal passive subject
  5. agent: agent
  6. expl: expletive

Finding the range of possible values for dependency labels takes some digging. I don’t believe it’s in the spaCy documentation per se, but if you’re persistent you’ll find a link this list or the paper it came from.