Specify NLTK feature grammar within Python function in code

Question

I have parsed input string by loading grammar specified within a .fcfg file as given in the NLTK book. Is there anyway to specify this grammar within the Python function itself?

Grammar:

% start S
S[SEM=(?np + WHERE + ?vp)] -> NP[SEM=?np] VP[SEM=?vp]
VP[SEM=(?v + ?pp)] -> IV[SEM=?v] PP[SEM=?pp]
VP[SEM=(?v + ?ap)] -> IV[SEM=?v] AP[SEM=?ap]
NP[SEM=(?det + ?n)] -> Det[SEM=?det] N[SEM=?n]
PP[SEM=(?p + ?np)] -> P[SEM=?p] NP[SEM=?np]
AP[SEM=?pp] -> A[SEM=?a] PP[SEM=?pp]
NP[SEM='Country="greece"'] -> 'Greece'
NP[SEM='Country="china"'] -> 'China'
Det[SEM='SELECT'] -> 'Which' | 'What'
N[SEM='City FROM city_table'] -> 'cities'
IV[SEM=''] -> 'are'
A[SEM=''] -> 'located'
P[SEM=''] -> 'in'

I need this because I need to create the grammar dynamically wrt to input string before parsing.

Answer 1

Yes, use the nltk.grammar.FeatureGrammar.fromstring() function, eg

from nltk import grammar, parse
from nltk.parse.generate import generate

# If person is always 3rd, we can skip the PERSON feature.
g = """
S[SEM=(?np + WHERE + ?vp)] -> NP[SEM=?np] VP[SEM=?vp]
VP[SEM=(?v + ?pp)] -> IV[SEM=?v] PP[SEM=?pp]
VP[SEM=(?v + ?ap)] -> IV[SEM=?v] AP[SEM=?ap]
NP[SEM=(?det + ?n)] -> Det[SEM=?det] N[SEM=?n]
PP[SEM=(?p + ?np)] -> P[SEM=?p] NP[SEM=?np]
AP[SEM=?pp] -> A[SEM=?a] PP[SEM=?pp]
NP[SEM='Country="greece"'] -> 'Greece'
NP[SEM='Country="china"'] -> 'China'
Det[SEM='SELECT'] -> 'Which' | 'What'
N[SEM='City FROM city_table'] -> 'cities'
IV[SEM=''] -> 'are'
A[SEM=''] -> 'located'
P[SEM=''] -> 'in'
"""

grammar =  grammar.FeatureGrammar.fromstring(g)

for sent in generate(grammar, n=30):
    print(sent)

[out]:

['Which', 'cities', 'are', 'in', 'Which', 'cities']
['Which', 'cities', 'are', 'in', 'What', 'cities']
['Which', 'cities', 'are', 'in', 'Greece']
['Which', 'cities', 'are', 'in', 'China']
['Which', 'cities', 'are', 'located', 'in', 'Which', 'cities']
['Which', 'cities', 'are', 'located', 'in', 'What', 'cities']
['Which', 'cities', 'are', 'located', 'in', 'Greece']
['Which', 'cities', 'are', 'located', 'in', 'China']
['What', 'cities', 'are', 'in', 'Which', 'cities']
['What', 'cities', 'are', 'in', 'What', 'cities']
['What', 'cities', 'are', 'in', 'Greece']
['What', 'cities', 'are', 'in', 'China']
['What', 'cities', 'are', 'located', 'in', 'Which', 'cities']
['What', 'cities', 'are', 'located', 'in', 'What', 'cities']
['What', 'cities', 'are', 'located', 'in', 'Greece']
['What', 'cities', 'are', 'located', 'in', 'China']
['Greece', 'are', 'in', 'Which', 'cities']
['Greece', 'are', 'in', 'What', 'cities']
['Greece', 'are', 'in', 'Greece']
['Greece', 'are', 'in', 'China']
['Greece', 'are', 'located', 'in', 'Which', 'cities']
['Greece', 'are', 'located', 'in', 'What', 'cities']
['Greece', 'are', 'located', 'in', 'Greece']
['Greece', 'are', 'located', 'in', 'China']
['China', 'are', 'in', 'Which', 'cities']
['China', 'are', 'in', 'What', 'cities']
['China', 'are', 'in', 'Greece']
['China', 'are', 'in', 'China']
['China', 'are', 'located', 'in', 'Which', 'cities']
['China', 'are', 'located', 'in', 'What', 'cities']