How to identify the subject of a sentence?

Question

Can Python + NLTK be used to identify the subject of a sentence? From what I have learned till now is that a sentence can be broken into a head and its dependents. For e.g. "I shot an elephant". In this sentence, I and elephant are dependents to shot. But How do I discern that the subject in this sentence is I.

Answer 1

You can use Spacy.

Code

import spacy
nlp = spacy.load('en')
sent = "I shot an elephant"
doc=nlp(sent)

sub_toks = [tok for tok in doc if (tok.dep_ == "nsubj") ]

print(sub_toks) 

Answer 2

As NLTK book (exercise 29) says, "One common way of defining the subject of a sentence S in English is as the noun phrase that is the child of S and the sibling of VP."

Look at tree example: indeed, "I" is the noun phrase that is the child of S that is the sibling of VP, while "elephant" is not.