Tests & documentation

The scientific community broadly discusses the capabilities and shortcomings of hydrological models from a theoretical point of view. Many sensitivity studies address the negative impacts of low data quality. By contrast, we are not aware of any studies estimating the adverse effects of bugs and misleading documentation of hydrological computer models. With little attention paid to these issues during evaluation processes (e.g. during peer-review), there is a risk of publishing impaired model results, possibly compromising the drawn conclusions. See, for example, the commentary of Hutton et al., addressing this topic from the scientific perspective.

This section describes strategies on how to keep the danger of severe bugs and outdated documentation to a reasonable degree. We try to keep the code and the documentation in sync by connecting them as strong as possible, using the “docstring” and “doctest” features of Python.

The first “connection” is writing each documentation section as close as possible next to the related source code. For very general topics, like the one you are reading now, it does not make sense, but you have to write all explanations addressing specific HydPy features as docstrings. Docstrings are documentation strings which are attached to the Python objects they explain. When extending HydPy, it is a strict requirement that each newly implemented public member (including the sub-members, e.g. the public methods of a class) comes with its own docstring. Our Travis CI based continuous integration workflow recognises any missing docstrings by using Pylint and reports them as errors.

The second “connection” is to use and extend the functionalities of Sphinx, which collects the source code, the docstrings, and the usual documentation files to generate the online documentation. Sphinx relies on the reStructuredText format, hence follow this format when writing docstrings and regular documentation files. However, instead of using its regular referencing style, make use of “substitutions” as defined by class Substituter of module autodoctools. Write, for example, the class name “Substituter” within vertical bars to reference the corresponding class properly. This short syntax allows making frequent use of substitutions. A helpful side effect is that, during the generation of the HTML pages, wrong substitutions result in warnings, interpreted as errors by our Travis CI based continuous integration workflow (see section Continuous Integration). This mechanism increases chances that, when documentation adjustments do not accompany future code changes, the Travis CI based workflow breaks, enforcing the responsible programmer to adapt the documentation.

The third “connection” is to define a sufficient number of doctests. Doctests are documentation sections containing valid Python code followed by the expected results. For developers, doctest is not always as convenient as other unit testing frameworks (e.g. unittest), but it offers the great advantage to define tests that are understandable for non-programmers as well. In HydPy, at best each (sub)member should define its own doctests, telling in conjunction with some “normal” explanations about its purpose and usage. Non-programmers should be enabled to learn using HydPy by repeating the doctests. Besides their intuitiveness, doctests (like substitutions) offer the significant advantage of keeping source code and documentation in sync. As long as the following three-line doctest remains in the documentation, one can be sure that the current core package contains a module named “objecttools”:

>>> from hydpy.core import objecttools
>>> objecttools.classname(objecttools)
'module'

The Python script test_everything.py collects the doctests of all modules and executes them.

We measure the “code coverage” (the number of executed code lines divided by the total number of code lines), determined by the coverage library. The code coverage of HydPy is 100 %. To make sure future changes do not undo this favourable situation, Travis CI reports all future changes introducing uncovered lines as failures.