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N-grams based similarity score

Project description


A method for similarity scoring of two strings.

The method, namely nratio, belongs to the class SequenceMatcherExtended, which is an extension of the SequenceMatcher class of the difflib package. In particular, nratio (method of SequenceMatcherExtended) is an augmenation of ratio (method of SequenceMatcher).

ngramratio is to be pronounced as "n gram ratio". The library uses n-grams to find a similarity score via a division (ratio) of the number of matched characters by the total number of characters. See below for more details.


To compute a similarity score based on matching n-grams (with n>=1 chosen by the user) rather than matching single characters (as in the case of the ratio method).


To install the Python library run:

pip install ngramratio

The library will be installed as ngramratio to bin on Linux (e.g. /usr/bin); or as ngramratio.exe to Scripts in your Python installation on Windows (e.g. C:\Python27\Scripts\ngramratio.exe).

You may consider installing the library only for the current user:

pip install ngramratio --user

In this case the library will be installed to ~/.local/bin/ngramratio on Linux and to %APPDATA%\Python\Scripts\ngramratio.exe on Windows.

Library usage

The module provides a method, nratio, which takes an integer number (the user's required minimum n-gram length, i.e. number of consecutive characters, to be matched) and outputs a similarity index (float number in [0,1]).

First step: initialize an object of class SequenceMatcherExtended specifying the two strings to be compared:

    >>> from ngramratio import ngramratio

    >>> SequenceMatcherExtended = ngrmaratio.SequenceMatcherExtended

    >>> a = "ab cde" # string 1
    >>> b = "bcde"   # string 2

    >>> s = SequenceMatcherExtended(a, b)

Alternatively, the last line can be rewritten more generally as

    >>> s = SequenceMatcherExtended(None, a, b, None)

where the first and last arguments are used to specify that no string will be considered junk. For more information on these arguments, see the documentation of the original difflib package.

Second step: apply the ratio and nratio methods and compare similarity scores:

    >>> s.ratio()
    >>> # Matches any character. Matches: "b" (length 1), "cde"(length 3). Score: (3+1)*2/10.
    >>> s.nratio(1)
    >>> # Matches substring of length 1 or more. It replicates `ratio()`'s functionality.
    >>> s.nratio(2)
    >>> # Matches substring of length 2 or more. Matches: "cde"(length 3). Score: 3*2/10.
    >>> s.nratio(3)
    >>> # Matches substring of length 3 or more. Matches: "cde"(length 3). Score: 3*2/10.
    >>> s.nratio(4)
    >>> # Matches substring of length 4 or more. Score 0/10.

The similarity score is computed as the number of characters matched (m) mutiplied by two (2) and divided by the total numer of characters (T) of the two strings, i.e. similarity score = 2m/T. Note that Python always returns a float upon computing a division.

Testing in a virtual environment

This project uses pytest testing framework with tox and docker to automate testing in different python environments. Tests are stored in the test/ folder.

To test a specific python version, for example version 3.6, edit the last few characters of the script to py36 AND change the image to python 3.6 on line 4 of the docker-compose.yaml file.

To run tests, run bash _scripts/ This will start a docker container using the specified python image. After testing, or before testing a different python version, run bash _scripts/ to remove the docker container.

The library has been tested successfully for python >= 3.6.

Testing on your local machine with no v.e.

You can use tox directly in your local machine. Make sure to install tox, pytest before testing.

On Linux tox expects to find executables like python3.6, python3.10 etc. On Windows it looks for C:\Python36\python.exe and C:\Python310\python.exe respectively.

To test a specific Python environment, use the -e option. For example, to test against Python 3.7 run:

tox -e py37

in the root of the project source tree.

To fix code formatting (this will install pre-commit as a dependency), run:

tox -e lint

See the tox.ini file in the repository to learn more about the testing instructions being used.


Contributions should include tests and an explanation for the changes they propose. Documentation (examples, docstrings, should be updated accordingly.

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