I'm using Marko to parse a Markdown document, split it into multiple documents according to some headings, and then render each of those sub-documents separately. While in the process of doing this, I discovered a strange behaviour:

>>> from marko.block import Document
>>> d1 = Document()
>>> d1.children.append(1)
>>> d1
<Document children=[1]>
>>> d1.children
[1]
>>> d2 = Document()
>>> d2
<Document children=[1]>
>>> d2.children
[1]

Looking at Marko's code, it's because you define and assign children = [] as a class variable on BlockElement, and then rely on each subclass to re-initialize it. But, I guess since Document is meant to be virtual, it doesn't do that. So any manually-constructed Document will share the same instance of children, which I'm sure is not intended.

Perhaps what I'm missing is some other way to accomplish the goal of constructing a Document object (which can then be render()-ed) from a sequence of Nodes (filtered out of another Document), rather than from a string. But I don't think this is currently possible, so the only way to do this is to break the virtual-ness of Document and just initialize one. In which case, something like this PR is required.