BagOfWords

Counts tokens in sentences.

This transformer can be used to counts tokens in a given piece of text. It takes care of normalizing the text before tokenizing it. In mini-batch settings, this transformers allows to convert a series of pandas of text into sparse dataframe.

Note that the parameters are identical to those of feature_extraction.TFIDF.

Parameters

• on ('str') – defaults to None

  The name of the feature that contains the text to vectorize. If None, then each learn_one and transform_one will assume that each x that is provided is a str, andnot a dict.

• strip_accents – defaults to True

  Whether or not to strip accent characters.

• lowercase – defaults to True

  Whether or not to convert all characters to lowercase.

• preprocessor ('typing.Callable') – defaults to None

  An optional preprocessing function which overrides the strip_accents and lowercase steps, while preserving the tokenizing and n-grams generation steps.

• stop_words ('set[str]') – defaults to None

  An optional set of tokens to remove.

• tokenizer_pattern – defaults to (?u)\b\w[\w\-]+\b

  The tokenization pattern which is used when no tokenizer function is passed. A single capture group may optionally be specified.

• tokenizer ('typing.Callable') – defaults to None

  A function used to convert preprocessed text into a dict of tokens. By default, a regex formula that works well in most cases is used.

• ngram_range – defaults to (1, 1)

  The lower and upper boundary of the range n-grams to be extracted. All values of n such that min_n <= n <= max_n will be used. For example an ngram_range of (1, 1) means only unigrams, (1, 2) means unigrams and bigrams, and (2, 2) means only bigrams.

Examples

By default, BagOfWords will take as input a sentence, preprocess it, tokenize the preprocessed text, and then return a collections.Counter containing the number of occurrences of each token.

>>> from river import feature_extraction as fx

>>> corpus = [
...     'This is the first document.',
...     'This document is the second document.',
...     'And this is the third one.',
...     'Is this the first document?',
... ]

>>> bow = fx.BagOfWords()

>>> for sentence in corpus:
...     print(bow.transform_one(sentence))
{'this': 1, 'is': 1, 'the': 1, 'first': 1, 'document': 1}
{'this': 1, 'document': 2, 'is': 1, 'the': 1, 'second': 1}
{'and': 1, 'this': 1, 'is': 1, 'the': 1, 'third': 1, 'one': 1}
{'is': 1, 'this': 1, 'the': 1, 'first': 1, 'document': 1}

Note that learn_one does not have to be called because BagOfWords is stateless. You can call it but it won't do anything.

In the above example, a string is passed to transform_one. You can also indicate which field to access if the string is stored in a dictionary:

>>> bow = fx.BagOfWords(on='sentence')

>>> for sentence in corpus:
...     x = {'sentence': sentence}
...     print(bow.transform_one(x))
{'this': 1, 'is': 1, 'the': 1, 'first': 1, 'document': 1}
{'this': 1, 'document': 2, 'is': 1, 'the': 1, 'second': 1}
{'and': 1, 'this': 1, 'is': 1, 'the': 1, 'third': 1, 'one': 1}
{'is': 1, 'this': 1, 'the': 1, 'first': 1, 'document': 1}

The ngram_range parameter can be used to extract n-grams (including unigrams):

>>> ngrammer = fx.BagOfWords(ngram_range=(1, 2))

>>> ngrams = ngrammer.transform_one('I love the smell of napalm in the morning')
>>> for ngram, count in ngrams.items():
...     print(ngram, count)
love 1
the 2
smell 1
of 1
napalm 1
in 1
morning 1
('love', 'the') 1
('the', 'smell') 1
('smell', 'of') 1
('of', 'napalm') 1
('napalm', 'in') 1
('in', 'the') 1
('the', 'morning') 1

BagOfWord allows to build a term-frequency pandas sparse dataframe with the transform_many method.

>>> import pandas as pd
>>> X = pd.Series(['Hello world', 'Hello River'], index = ['river', 'rocks'])
>>> bow = fx.BagOfWords()
>>> bow.transform_many(X=X)
       hello  world  river
river      1      1      0
rocks      1      0      1

Methods

learn_many

learn_one

Update with a set of features x.

A lot of transformers don't actually have to do anything during the learn_one step because they are stateless. For this reason the default behavior of this function is to do nothing. Transformers that however do something during the learn_one can override this method.

Parameters

• x (dict)

Returns

Transformer: self

process_text

transform_many

Transform pandas series of string into term-frequency pandas sparse dataframe.

Parameters

• X ('pd.Series')

transform_one

Transform a set of features x.

Parameters

• x (dict)

Returns

dict: The transformed values.