There's nothing "out of idiom" of custom containers in Python (or are numpy arrays or pandas dataframes "out of idiom" now? I could swear that wasn't the case yesterday), and pretty much anything available to builtin containers are also available to custom ones (the exceptions being literal syntax, and true immutability unless the container is written in C).

In fact there are a few domains where they are rather common (e.g. multidict implementations in web framework, pretty much every web framework has one for query and form data; optimized or analysis-focused structures in scientific python)