nskipgrams 0.4.0

Getting Ngrams from a Sequence

If you simply need ngrams from a sequence, ngrams_from_seq is what you’re looking for:

>>> from nskipgrams import ngrams_from_seq
>>> for ngram in ngrams_from_seq("abcdef", n=2):
...     print(ngram)
('a', 'b')
('b', 'c')
('c', 'd')
('d', 'e')
('e', 'f')

Initializing an Ngram Collection

>>> from nskipgrams import Ngrams
>>> char_ngrams = Ngrams(n=3)  # handles unigrams, bigrams, and trigrams

Adding Ngrams

>>> char_ngrams.add_from_seq("my cats")
>>> char_ngrams.add_from_seq("your cat", count=2)

Here, a sequence is anything that can be iterated over, and the corresponding ngrams are extracted from the individual elements off of the sequence. For example, if the sequence is a text string like "my cats" above, then the ngrams are character-based (hence the chosen variable name char_ngrams).

To add a single ngram:

>>> char_ngrams.add(("y", "o", "u"))

As a best practice, it is recommended that an ngram be represented as a tuple regardless of what the individual elements are, e.g., ("y", "o", "u") for character-based ngrams. As output examples show below, the tuple data type is also what this package uses to represent ngrams.

Accessing Ngrams

>>> for ngram, count in char_ngrams.ngrams_with_counts(n=1):  # unigrams
...     print(ngram, count)
...
('m',), 1
('y',), 3
(' ',), 3
('c',), 3
('a',), 3
('t',), 3
('s',), 1
('o',), 2
('u',), 2
('r',), 2
>>>
>>> for ngram, count in char_ngrams.ngrams_with_counts(n=2):  # bigrams
...     print(ngram, count)
...
('m', 'y'), 1
('y', ' '), 1
('y', 'o'), 2
(' ', 'c'), 3
('c', 'a'), 3
('a', 't'), 3
('t', 's'), 1
('o', 'u'), 2
('u', 'r'), 2
('r', ' '), 2
>>>
>>> for ngram, count in char_ngrams.ngrams_with_counts(n=3):  # trigrams
...     print(ngram, count)
...
('m', 'y', ' '), 1
('y', ' ', 'c'), 1
('y', 'o', 'u'), 3
(' ', 'c', 'a'), 3
('c', 'a', 't'), 3
('a', 't', 's'), 1
('o', 'u', 'r'), 2
('u', 'r', ' '), 2
('r', ' ', 'c'), 2

Accessing Ngrams with a Specific Prefix

>>> for ngram, count in char_ngrams.ngrams_with_counts(n=3, prefix=("y",)):
...     print(ngram, count)
...
('y', ' ', 'c'), 1
('y', 'o', 'u'), 3

Accessing the Count of a Specific Ngram

>>> char_ngrams.count(("c", "a", "t"))
3

Checking Membership

To check if an ngram has an exact match in the collection so far:

>>> ("c", "a", "t") in char_ngrams
True

Combining Collections of Ngrams

To combine collections of ngrams (e.g., when you process data sources in parallel and have multiple Ngrams objects):

>>> char_ngrams1 = Ngrams(n=2)
>>> char_ngrams1.add_from_seq("my cat")
>>> set(char_ngrams1.ngrams_with_counts(n=2))
{((' ', 'c'), 1),
 (('a', 't'), 1),
 (('c', 'a'), 1),
 (('m', 'y'), 1),
 (('y', ' '), 1)}
>>>
>>> char_ngrams2 = Ngrams(n=2)
>>> char_ngrams2.add_from_seq("your cats")
>>> set(char_ngrams2.ngrams_with_counts(n=2))
{((' ', 'c'), 1),
 (('a', 't'), 1),
 (('c', 'a'), 1),
 (('o', 'u'), 1),
 (('r', ' '), 1),
 (('t', 's'), 1),
 (('u', 'r'), 1),
 (('y', 'o'), 1)}
>>>
>>> char_ngrams3 = Ngrams(n=2)
>>> char_ngrams3.add_from_seq("her cats")
>>> set(char_ngrams3.ngrams_with_counts(n=2))
{((' ', 'c'), 1),
 (('a', 't'), 1),
 (('c', 'a'), 1),
 (('e', 'r'), 1),
 (('h', 'e'), 1),
 (('r', ' '), 1),
 (('t', 's'), 1)}
>>>
>>> char_ngrams1.combine(char_ngrams2, char_ngrams3)  # `combine` takes as many Ngrams objects as desired
>>> set(char_ngrams1.ngrams_with_counts(n=2))
{((' ', 'c'), 3),
 (('a', 't'), 3),
 (('c', 'a'), 3),
 (('e', 'r'), 1),
 (('h', 'e'), 1),
 (('m', 'y'), 1),
 (('o', 'u'), 1),
 (('r', ' '), 2),
 (('t', 's'), 2),
 (('u', 'r'), 1),
 (('y', ' '), 1),
 (('y', 'o'), 1)}

If you don’t want to mutate any of the Ngrams instances (the combine method works in-place and mutates these_ngrams when these_ngrams.combine is called), then you can create an empty ngram collection and combine into it all of your ngrams:

>>> collections = [char_ngrams1, char_ngrams2, char_ngrams3]
>>> all_ngrams = Ngrams(n=2)  # A new, empty collection of ngrams
>>> all_ngrams.combine(*collections)

Any “Sequences” and their Corresponding “Ngrams” Work

While the examples above use text strings as sequences and character-based ngrams, another common usage in computational linguistics and NLP is to have segmented phrases/sentences as sequences and word-based ngrams:

>>> from nskipgrams import Ngrams
>>> word_ngrams = Ngrams(n=2)
>>> word_ngrams.add_from_seq(("in", "the", "beginning"))
>>> word_ngrams.add_from_seq(("in", "the", "end"))
>>> for ngram, count in word_ngrams.ngrams_with_counts(n=2):
...     print(ngram, count)
...
('in', 'the'), 2
('the', 'beginning'), 1
('the', 'end'), 1

Skipgrams

Ngrams are a special case of skipgrams, with skip = 0. The class Skipgrams works the same as Ngrams, with the following differences:

• Skipgrams has the method skipgrams_with_counts rather than ngrams_with_counts. skipgrams_with_counts also has the keyword argument skip (in addition to n and prefix).

• For Skipgrams, the methods add and count, as well as collection instantiation (i.e., __init__), also have a meaningful skip keyword argument.

The function skipgrams_from_seq works the same as ngrams_from_seq, but has the skip keyword argument (in addition to seq and n).