At times you will want to create two projects that are very similar, but differ just in one or two attributes. For example, you may define a project with one set of samples, and then want an identical project but using a different sample annotation sheet. Or, you may define a project to run on a particular reference genome, and want to define a second project that is identical, but uses a different reference genome.

You could simply define 2 complete PEPs, but this would duplicate information and make it harder to maintain. Instead, you can use subprojects, which allow you to encode additional similar projects all within the original project_config.yaml file. Subprojects are like mini embedded project_config.yaml files that can be activated by software.

For example, consider this project_config.yaml:

  sample_annotation: annotation.csv

	  sample_annotation: annotation2.csv
	  sample_annotation: annotation3.csv

If you load this configuration file, it will by default use the annotation.csv file specified in the primary metadata section, as you would expect. If you don’t directly specify that you want to use a subproject, then they are ignored. But if you choose, you may activate one of the two subprojects, which are called my_project2 and my_project3. If you activate my_project2, then the metadata.sample_annotation attribute would now have a value of annotation2.csv; everything else about the project would be the same as the primary project, because these existing values are not overridden.

Practically what happens under the scenes is that the primary project is first loaded, and then, if a subproject is activated, it overrides any attributes with those specified in the subproject.

How do you activate a subproject?

Activating a subproject depends on what software you’re using to load your PEP. You can tell looper to load a particular subproject by passing --sp subproject-name on the command line. You can activate a subproject in peppy or pepr by passing an argument, subproject=my_project2, when you construct the Project object.

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