Dependency Parsing

• this is another method for information extraction
• each words are first converted into tokens
• dependency parse links tokens directly to other tokens
• each link has three parts:
  1. head - the word that governs, each token has exactly 1 head, unless its the ROOT of the sentence (main verb, etc)
  2. dependent/modifier - the word that modifies the head
  3. label - the grammatical relation such as (nsubj, det, amod, ...)

Each link/arc joins exactly 2 tokens. The dependency depends on the head, so the direction is from head to dependency. In displacy, the arrowhead points at the dependent.

A dependent is a general term, and modifier is a subtype of dependent.

nlp = spacy.load("en_core_web_sm")
sentence = "I saw a kitten eating chicken in the kitchen."
 
doc = nlp(sentence)
 
print('{:<15}|{:<8}|{:<15}|{:<20}'.format('Token','Relation','Head','Children'))
print('-'*70)
 
for token in doc:
	#Print the token, dependency nature, head and all dependents of the token
	print("{:<15} | {:<18} | {:<15} | {:<20}"
		.format(str(token.text), str(token.dep_), str(token.head.text), \
		str([child for child in token.children])))

displacy.render(doc, style='dep', jupyter=True, options={'distance':120})

Core arguments

Label	Meaning	Example (head in italics)
ROOT	Head of the whole sentence, depends on nothing	The drone transmits signals
nsubj	Nominal subject (subject performing the action)	The drone transmits
nsubjpass	Passive nominal subject (subject receiving the action)	The signal was jammed
csubj	Clausal subject (clause contains a subject and a verb)	What he said matters
csubjpass	Clausal passive subject	Whether it flew was disputed
dobj	Direct object	transmits signals
dative	Indirect object (spaCy's name for iobj)	gave him the data
pobj	Object of a preposition	in the sky
attr	Attribute: complement of a copula	it is a drone
oprd	Object predicate	considered it useless
agent	The "by" marking the agent in a passive	jammed by the operator

Dependency parsing links words directly to each other, and the other view of sentence structure builds phrases from rules, which is Context-free Grammar (CFG).