Using Both DictLoaders and extra_template_basedirs

I have a custom template, extending from HTMLExporter. I pass a path to the directory with this template through `extra_template_basedirs` in the `HTMLExporter` Constructor. I then want to extend my custom template programmatically (i.e., the full inheritance is HTMLTemplate > Custom Template in filesystem > Custom template defined through DictLoader). The DictLoader template changes between invocations, so it must be programmatic. However, because the templates passed through extra_loaders are loaded before the ones passed through extra_template_basedirs, the dynamic DictLoader template cannot find the custom template in the filesystem. Is it possible to do this with nbconvert?

In the case of an XY problem, my overall use case is that I want to define a variable in my custom template, that can be set programmatically when we're exporting the notebook.

Start at the HTMLExporter constructor and trace how extra_loaders and extra_template_basedirs are ordered. Check whether a DictLoader template can inherit from the filesystem template while changing between exports; done means the dynamic template resolves that inheritance, or the supported limitation and alternative are documented.

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