Tricky issue when converting CountVectorizer or TfidfVectorizer#

This issue is described at scikit-learn/issues/13733. If a CountVectorizer or a TfidfVectorizer produces a token with a space, skl2onnx cannot know if it a bi-grams or a unigram with a space.

A simple example impossible to convert#

import pprint
import numpy
from numpy.testing import assert_almost_equal
from onnxruntime import InferenceSession
from sklearn.feature_extraction.text import TfidfVectorizer
from skl2onnx import to_onnx
from skl2onnx.sklapi import TraceableTfidfVectorizer
import skl2onnx.sklapi.register  # noqa

corpus = numpy.array(
        "This is the first document.",
        "This document is the second document.",
        "Is this the first document?",

pattern = r"\b[a-z ]{1,10}\b"
mod1 = TfidfVectorizer(ngram_range=(1, 2), token_pattern=pattern)
TfidfVectorizer(ngram_range=(1, 2), token_pattern='\\b[a-z ]{1,10}\\b')
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Unigrams and bi-grams are placed into the following container which maps it to its column index.

{'document': 0,
 'document ': 1,
 'document  is the ': 2,
 'is the ': 3,
 'is the  second ': 4,
 'is this ': 5,
 'is this  the first ': 6,
 'second ': 7,
 'second  document': 8,
 'the first ': 9,
 'the first  document': 10,
 'this ': 11,
 'this  document ': 12,
 'this is ': 13,
 'this is  the first ': 14}


    to_onnx(mod1, corpus)
except RuntimeError as e:
There were ambiguities between n-grams and tokens. 2 errors occurred. You can fix it by using class TraceableTfidfVectorizer.
You can learn more at
Unable to split n-grams 'is this  the first ' into tokens ('is', 'this', 'the', 'first ') existing in the vocabulary. Token 'is' does not exist in the vocabulary..
Unable to split n-grams 'this is  the first ' into tokens ('this', 'is', 'the', 'first ') existing in the vocabulary. Token 'this' does not exist in the vocabulary..


Class TraceableTfidfVectorizer is equivalent to sklearn.feature_extraction.text.TfidfVectorizer but stores the unigrams and bi-grams of the vocabulary with tuple instead of concatenating every piece into a string.

mod2 = TraceableTfidfVectorizer(ngram_range=(1, 2), token_pattern=pattern)

{('document',): 0,
 ('document ',): 1,
 ('document ', 'is the '): 2,
 ('is the ',): 3,
 ('is the ', 'second '): 4,
 ('is this ',): 5,
 ('is this ', 'the first '): 6,
 ('second ',): 7,
 ('second ', 'document'): 8,
 ('the first ',): 9,
 ('the first ', 'document'): 10,
 ('this ',): 11,
 ('this ', 'document '): 12,
 ('this is ',): 13,
 ('this is ', 'the first '): 14}

Let’s check it produces the same results.

assert_almost_equal(mod1.transform(corpus).todense(), mod2.transform(corpus).todense())

Conversion. Line import skl2onnx.sklapi.register was added to register the converters associated to these new class. By default, only converters for scikit-learn are declared.

onx = to_onnx(mod2, corpus)
sess = InferenceSession(onx.SerializeToString(), providers=["CPUExecutionProvider"])
got =, {"X": corpus})

Let’s check if there are discrepancies…

assert_almost_equal(mod2.transform(corpus).todense(), got[0])

Total running time of the script: (0 minutes 0.037 seconds)

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