how-to: avoid putting artifacts in your repo for testing purposes (and why)

[gregcaporaso]

• use transform_format (q2-sapienns has a good example)
• to know the to and from values to provide as input
  • export the artifact that you would have used in your test using qiime tools export. the format that is reported on the command line should be used as to (alternatively you can find this using qiime tools peek- it's the Data format that is reported)
  • the view type that your function takes as input (or which you would have passed as view_type to Artifact.load("...").view(view_type)) should be provided as from
• generally this is a good way to go as:
  • it makes for easier test developing and debugging since you can see the test data more easily, and
  • generally speaking (though not in all cases - e.g., if the underlying format is binary) building your tests on data files (or objects) makes for more informative diffs if/when the test data changes across commits.

[gregcaporaso]

Pending qiime2/qiime2#757, these instructions would have to use qiime2.plugin.util.transform instead of TestPluginBase.transform_format. (The q2-sapienns example works b/c the transformer being used is defined in q2-sapienns.)