Project Status: Active – The project has reached a stable, usable state and is being actively developed.

Frog for Python

This is a Python binding to the Natural Language Processing suite Frog. Frog is intended for Dutch and performs part-of-speech tagging, lemmatisation, morphological analysis, named entity recognition, shallow parsing, and dependency parsing. The tool itseelf is implemented in C++ (



For easy installation, please use our LaMachine distribution


  • Make sure to first install Frog and all its dependencies
  • Install Cython if not yet available on your system: $ sudo apt-get cython cython3 (Debian/Ubuntu, may differ for others)
  • Run: $ sudo python install



from __future__ import print_function, unicode_literals #to make this work on Python 2 as well as Python 3

import frog

frog = frog.Frog(frog.FrogOptions(parser=False))
output = frog.process_raw("Dit is een test")
print("RAW OUTPUT=",output)
output = frog.process("Dit is nog een test.")
print("PARSED OUTPUT=",output)


RAW OUTPUT= 1   Dit     dit     [dit]   VNW(aanw,pron,stan,vol,3o,ev)
0.777085        O       B-NP
2       is      zijn    [zijn]  WW(pv,tgw,ev)   0.999891        O
3       een     een     [een]   LID(onbep,stan,agr)     0.999113        O
4       test    test    [test]  N(soort,ev,basis,zijd,stan)     0.789112
O       I-NP

PARSED OUTPUT= [{'chunker': 'B-NP', 'index': '1', 'lemma': 'dit', 'ner':
'O', 'pos': 'VNW(aanw,pron,stan,vol,3o,ev)', 'posprob': 0.777085, 'text':
'Dit', 'morph': '[dit]'}, {'chunker': 'B-VP', 'index': '2', 'lemma':
'zijn', 'ner': 'O', 'pos': 'WW(pv,tgw,ev)', 'posprob': 0.999966, 'text':
'is', 'morph': '[zijn]'}, {'chunker': 'B-NP', 'index': '3', 'lemma': 'nog',
'ner': 'O', 'pos': 'BW()', 'posprob': 0.99982, 'text': 'nog', 'morph':
'[nog]'}, {'chunker': 'I-NP', 'index': '4', 'lemma': 'een', 'ner': 'O',
'pos': 'LID(onbep,stan,agr)', 'posprob': 0.995781, 'text': 'een', 'morph':
'[een]'}, {'chunker': 'I-NP', 'index': '5', 'lemma': 'test', 'ner': 'O',
'pos': 'N(soort,ev,basis,zijd,stan)', 'posprob': 0.903055, 'text': 'test',
'morph': '[test]'}, {'chunker': 'O', 'index': '6', 'eos': True, 'lemma':
'.', 'ner': 'O', 'pos': 'LET()', 'posprob': 1.0, 'text': '.', 'morph':

Available keyword arguments for FrogOptions:

  • tok – True/False – Do tokenisation? (default: True)
  • lemma – True/False – Do lemmatisation? (default: True)
  • morph – True/False – Do morpholigical analysis? (default: True)
  • daringmorph – True/False – Do morphological analysis in new experimental style? (default: False)
  • mwu – True/False – Do Multi Word Unit detection? (default: True)
  • chunking – True/False – Do Chunking/Shallow parsing? (default: True)
  • ner – True/False – Do Named Entity Recognition? (default: True)
  • parser – True/False – Do Dependency Parsing? (default: False).
  • xmlin – True/False – Input is FoLiA XML (default: False)
  • xmlout – True/False – Output is FoLiA XML (default: False)
  • docid – str – Document ID (for FoLiA)
  • numThreads – int – Number of threads to use (default: unset, unlimited)

You can specify a Frog configuration file explicitly as second argument upon instantiation, otherwise the default one is used:

frog = frog.Frog(frog.FrogOptions(parser=False), "/path/to/your/frog.cfg")

A third parameter, a dictionary, can be used to override specific configuration values (same syntax as Frog’s --override option), you may want to leave the second parameter empty if you want to load the default configuration:

frog = frog.Frog(frog.FrogOptions(parser=False), "", { "tokenizer.rulesFile": "tokconfig-nld-twitter" })

FoLiA support

Frog supports output in the FoLiA XML format (set FrogOptions(xmlout=True)), as well as FoLiA input (set FrogOptions(xmlin=True)). The FoLiA format exposes more details about the linguistic annotation in a more structured and more formal way.

Whenever FoLiA output is requested, the process() method will return an instance of folia.Document, which is provided by the FoLiApy library. This loads the entire FoLiA document in memory and allows you to inspect it in any way you see fit. Extensive documentation for this library can be found here:

An example can be found below:

from frog import Frog, FrogOptions

frog = Frog(FrogOptions(parser=True,xmlout=True))
output = frog.process("Dit is een FoLiA test.")
#output is now no longer a string but an instance of folia.Document, provided by the FoLiA library in PyNLPl (pynlpl.formats.folia)

print("Inspecting FoLiA output (just a small example):")
for word in output.words():
    print(word.text() + " " + word.pos() + " " + word.lemma())