Contributing to IOSACal


IOSACal is written in Python 3, and it makes heavy use of the NumPy library for the internal management of calibration curves and calibrated samples. Calibration curves, radiocarbon dates and calibrated curves are handled internally as ndarray objects. ndarray objects are matrices that can be easily manipulated through slicing, flipping, summing and other typical operations.

Generation of plots is done through Matplotlib, another Python library built on top of NumPy. Matplotlib can natively read ndarray objects and plot them in a graphical form. Far from being just a set of plotting functions, Matplotlib allows the drawing of complex plots like those created by IOSACal.

Development happens in a public git repository at Codeberg, the free home for free projects. The default branch name is main.

Codebase structure

The iosacal directory in the root of the Git repository is a Python package that contains the following source code files:

  • is first of all there to declare that this directory is a Python package. It also imports three objects (R, combine and iplot) so that they can be imported directly from the root package with from iosacal import R - that is enough to calibrate a date when using a Python interpreter. And it contains the current version that is propagated to other parts of the program.
  • contains the command line app, based on Click
  • contains the main classes to instantiate calibration curves and radiocarbon determinations, and functions that work directly on them like combine
  • contains functions to compute Highest Posterior Density and helper classes to format the resulting confidence intervals
  • contains two large functions based on Matplotlib that are respectively dedicated to plotting a single date or multiple (“stacked”) dates. The small iplot function is useful for usage with Jupyter notebooks
  • contains functions and classes for working with Sum of Probability Distributions of calibrated dates
  • contains one function to format calibration results for output to a terminal or a Markdown document
  • the data subdirectory contains the calibration curves in the

standard .14c format.

Tests are in a separate directory called tests in the root of the Git repository. Each file corresponds to one of the modules listed above and has the name prefixed with test_, such as

Documentation is in the docs directory and is a collection of files in reStructured Text format, for use with Sphinx. There is also a file called .readthedocs.yml in the root of the Git repository, that contains some settings for Read the Docs, the service that publishes the formatted documentation you are probably reading right now. If you’re contributing changes to the source code, please always check that the documentation is updated and include relevant changes to the documentation as well.


Contributing to IOSACal can be done by suggesting improvements or pointing out bugs and limitations of the program. This kind of contribution works by opening a new issue at <>. Please always open a new issue to let other contributors know that you’re working on a specific problem.

Code contributions are based on pull requests, as is common in modern open source software. You will need Git to clone the repository on your development machine.

However, if you’re not familiar with Git or you don’t want to open an account specifically for contributing, you can send the modified files by email or attach them to the issue.

Coding guidelines

The maintainer prefers to use black with the default settings to format source code.


The IntCal09 and IntCal13 calibration curves have a varying resolution. For example, in IntCal09 data spacing changes from 5 years for the range from 0 to 11.2 to cal kBP, 10 yrs for 11.2–15 cal kBP, 20 yrs for 15–25 cal kBP, 50 yrs for 25–40 cal kBP, and 100 yrs for 40–50 cal kBP [REI2009]. Other curves follow a similar pattern.

This means that the output intervals would follow these limitations. IOSACal uses the interp function of NumPy to perform linear interpolation of the calibration curves and obtain more fine-grained results, particularly concerning probability intervals.

[REI2009]Reimer PJ, Baillie MGL, Bard E, Bayliss A, Beck JW, Blackwell PG, Bronk Ramsey C, Buck CE, Burr GS, Edwards RL, Friedrich M, Grootes PM, Guilderson TP, Hajdas I, Heaton TJ, Hogg AG, Hughen KA, Kaiser KF, Kromer B, McCormac FG, Manning SW, Reimer RW, Richards DA, Southon JR, Talamo S, Turney CSM, van der Plicht J, Weyhenmeyer CE. 2009. IntCal09 and Marine09 radiocarbon age calibration curves, 0–50,000 years cal BP. Radiocarbon 51(4):1111–50.