Summary:
Writing a document fragment doesn't work because the 'writexml' method is not implemented.

Problem:
I would like to be able to write out document fragments as XML text, but this functionality is not implemented. Here are the reasons why I think this functionality should be implemented.

1. DOM Level 1 describes a document fragment as a lightweight document. I can write a document as XML text so why not a document fragment?

2. This would be very easy to implement, basically just the child processing loop from Element.writexml:
for node in self.childNodes:
    node.writexml(writer,indent+addindent,addindent,newl)

3. Document fragments are often returned as intermediate results in XML building. The best way I can think of to unit test these results is by writing the document fragment as XML text and comparing to an existing string. (Comparing two document fragments is not guaranteed to work.)

4. Implementing an XML editor on top of minidom would require this functionality, e.g. display a piece of cut XML.

I realize that writing document fragments as XML text is not a common or "core" operation, but it makes sense in the above four cases and perhaps other cases as well. Therefore, to me, implementing this functionality is less a question of "Why?" and more a question of "Why not?"

I will be glad to add any other information as requested.

Can someone please confirm that my assessment of this as an enhancement is correct, thanks.

I agree that writexml should be available for document fragments.

I doubt the additional level of indentation should be added, as you've included in point 2.

Updated target to Python 3.8, since this has aged a bit.

Seems like a reasonable request to me, even if it hasn't been touched or re-requested for years.

Funny enough, DocumentFragment is currently documented as "not implemented" (as for Entity, Notation, CDATASection, CharacterData, DOMImplementation, DocumentType). I'll add a PR to clean those up.

https://docs.python.org/3/library/xml.dom.minidom.html#minidom-and-the-dom-standard

New changeset 2ea8099523581cf2ecc060831a53debb57ff98ee by Serhiy Storchaka (Stefan Behnel) in branch 'master':
bpo-9883: Update list of unimplemented interfaces in minidom. (GH-12677)
https://github.com/python/cpython/commit/2ea8099523581cf2ecc060831a53debb57ff98ee

New changeset a9a065addd175ed37a959118c90377ba60f90036 by Miss Islington (bot) in branch '3.7':
bpo-9883: Update list of unimplemented interfaces in minidom. (GH-12677)
https://github.com/python/cpython/commit/a9a065addd175ed37a959118c90377ba60f90036

I've closed PR1745 as it appeared to be abandoned.