I need to provide a REST call from Plone to another service. Do I need to integrate something like Flask into Plone or can I do it without any other framework/library?
You can build your own REST API in plone with plone.rest:
https://pypi.python.org/pypi/plone.rest
plone.rest will most likely become part of Plone core at some point.
If you want a ready-to-use out-of-the-box solution have a look at plone.restapi:
https://github.com/plone/plone.restapi
There is no plone.restapi alpha release yet. Though, there will be one soon.
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