Determining tense of a sentence Python

Question

Following several other posts, [e.g. Detect English verb tenses using NLTK , Identifying verb tenses in python, Python NLTK figure out tense ] I wrote the following code to determine tense of a sentence in Python using POS tagging:

from nltk import word_tokenize, pos_tag

def determine_tense_input(sentence):
    text = word_tokenize(sentence)
    tagged = pos_tag(text)

    tense = {}
    tense["future"] = len([word for word in tagged if word[1] == "MD"])
    tense["present"] = len([word for word in tagged if word[1] in ["VBP", "VBZ","VBG"]])
    tense["past"] = len([word for word in tagged if word[1] in ["VBD", "VBN"]]) 
    return(tense)

This returns a value for the usage of past/present/future verbs, which I typically then take the max value of as the tense of the sentence. The accuracy is moderately decent, but I am wondering if there is a better way of doing this.

For example, is there now by-chance a package written which is more dedicated to extracting the tense of a sentence? [note - 2 of the 3 stack-overflow posts are 4-years old, so things may have now changed]. Or alternatively, should I be using a different parser from within nltk to increase accuracy? If not, hope the above code may help someone else!

Answer 1

You can strengthen your approach in various ways. You could think more about the grammar of English and add some more rules based on whatever you observe; or you could push the statistical approach, extract some more (relevant) features and throw the whole lot at a classifier. The NLTK gives you plenty of classifiers to play with, and they're well documented in the NLTK book.

You can have the best of both worlds: Hand-written rules can be in the form of features that are fed to the classifier, which will decide when it can rely on them.

Answer 2

You could use the Stanford Parser to get a dependency parse of the sentence. The root of the dependency parse will be the 'primary' verb that defines the sentence (I'm not too sure what the specific linguistic term is). You can then use the POS tag on this verb to find its tense, and use that.

Answer 3

As of http://dev.lexalytics.com/wiki/pmwiki.php?n=Main.POSTags, the tags mean

MD  Modal verb (can, could, may, must)
VB  Base verb (take)
VBC Future tense, conditional
VBD Past tense (took)
VBF Future tense
VBG Gerund, present participle (taking)
VBN Past participle (taken)
VBP Present tense (take)
VBZ Present 3rd person singular (takes)

so that your code would be

tense["future"] = len([word for word in tagged if word[1] in ["VBC", "VBF"])