# Contributing to python-stdnum

This document describes general guidelines for contributing new formats or other enhancement to python-stdnum.

## Adding number formats

Basically any number or code that has some validation mechanism available or some common formatting is eligible for inclusion into this library. If the only specification of the number is “it consists of 6 digits” implementing validation may not be that useful.

Contributions of new formats or requests to implement validation for a format should include the following:

- The format name and short description.

- References to (official) sources that describe the format.

- A one or two paragraph description containing more details of the number (e.g. purpose and issuer and possibly format information that might be useful to end users).

- If available, a link to an (official) validation service for the number, reference implementations or similar sources that allow validating the correctness of the implementation.

- A set of around 20 to 100 “real” valid numbers for testing (more is better during development but only around 100 will be retained for regression testing).

- If the validation depends on some (online) list of formats, structures or parts of the identifier (e.g. a list of region codes that are part of the number) a way to easily update the registry information should be available.

## Code contributions

Improvements to python-stdnum are most welcome. Integrating contributions will be done on a best-effort basis and can be made easier if the following are considered:

- Ideally contributions are made as GitHub pull requests, but contributions by email (privately or through the python-stdnum-users mailing list) can also be considered.

- Submitted contributions will often be reformatted and sometimes restructured for consistency with other parts.

- Contributions will be acknowledged in the release notes.

- Contributions should add or update a copyright statement if you feel the contribution is significant.

- All contribution should be made with compatible applicable copyright.

- It is not needed to modify the NEWS, README.md or files under docs for new formats; these files will be updated on release.

- Marking valid numbers as invalid should be avoided and are much worse than marking invalid numbers as valid. Since the primary use case for python-stdnum is to validate entered data having an implementation that results in “computer says no” should be avoided.

- Number format implementations should include links to sources of information: generally useful links (e.g. more details about the number itself) should be in the module docstring, if it relates more to the implementation (e.g. pointer to reference implementation, online API documentation or similar) a comment in the code is better

- Country-specific numbers and codes go in a country or region package (e.g. stdnum.eu.vat or stdnum.nl.bsn) while global numbers go in the toplevel name space (e.g. stdnum.isbn).

- All code should be well tested and achieve 100% code coverage.

- Existing code structure conventions (e.g. see README for interface) should be followed.

- Git commit messages should follow the usual 7 rules.

- Declarative or functional constructs are preferred over an iterative approach, e.g.:

      s = sum(int(c) for c in number)

  over:

      s = 0
      for c in number:
          s += int(c)

## Testing

Tests can be run with tox. Some basic code style tests can be run with tox -e flake8 and most other targets run the test suite with various supported Python interpreters.

Module implementations have a couple of smaller test cases that also serve as basic documentation of the happy flow.

More extensive tests are available, per module, in the tests directory. These tests (also doctests) cover more corner cases and should include a set of valid numbers that demonstrate that the module works correctly for real numbers.

The normal tests should never require online sources for execution. All functions that deal with online lookups (e.g. the EU VIES service for VAT validation) should only be tested using conditional unittests.

## Finding test numbers

Some company numbers are commonly published on a company’s website contact page (e.g. VAT or other registration numbers, bank account numbers). Doing a web search limited to a country and some key words generally turn up a lot of pages with this information.

Another approach is to search for spreadsheet-type documents with some keywords that match the number. This sometimes turns up lists of companies (also occasionally works for personal identifiers).

For information that is displayed on ID cards or passports it is sometimes useful to do an image search.

For dealing with numbers that point to individuals it is important to:

- Only keep the data that is needed to test the implementation.

- Ensure that no actual other data relation to a person or other personal information is kept or can be inferred from the kept data.

- The presence of a number in the test set should not provide any information about the person (other than that there is a person with the number or information that is present in the number itself).

Sometimes numbers are part of a data leak. If this data is used to pick a few sample numbers from the selection should be random and the leak should not be identifiable from the picked numbers. For example, if the leaked numbers pertain only to people with a certain medical condition, membership of some organisation or other specific property the leaked data should not be used.

## Reverse engineering

Sometimes a number format clearly has a check digit but the algorithm is not publicly documented. It is sometimes possible to reverse engineer the used check digit algorithm from a large set of numbers.

For example, given numbers that, apart from the check digit, only differ in one digit will often expose the weights used. This works reasonably well if the algorithm uses modulo 11 is over a weighted sums over the digits.

See https://github.com/arthurdejong/python-stdnum/pull/203#issuecomment-623188812

## Registries

Some numbers or parts of numbers use validation base on a registry of known good prefixes, ranges or formats. It is only useful to fully base validation on these registries if the update frequency to these registries is very low.

If there is a registry that is used (a list of known values, ranges or otherwise) the downloaded information should be stored in a data file (see the stdnum.numdb module). Only the minimal amount of data should be kept (for validation or identification).

The data files should be able to be created and updated using a script in the update directory.