As mentioned, this is similar to #1240 and #944 .

@ssbarnea @insspb
I'm biased, but I'd gravitate toward my more simple solution that involves less code. For the same reason I'd also vote for using the _extensions keyword in the json for both local and installed extensions.

If anything, make the folder name in #1240 just extensions instead of local_extensions to go along with ./hooks/.

@con-f-use As I mention earlier some tests required.
At least these behavior cases should be covered:
User requested an custom extension, extension exist and work.
User requested an custom extension, but extension not exist in extension folder files.
User requested an custom extension, file/class exist in folder, but is nor an extension.

This feature can be useful for some part of users, who want to deliver own extensions without pip packages. But it is not just path extension.

User requested an custom extension, but extension not exist in extension folder files.
User requested an custom extension, file/class exist in folder, but is nor an extension

I wouldn't worry about these two cases. The python import system will catch the first one.

For the last, IMHO it is not necessary to check if it is an extension. If Jinja cannot find the extension, it will complain and I'd leave it to Jinja. Besides we have no way of knowing, whether the user wants to include a pip installed extension with "_extensions": "SomeName", or if SomeName is supposed to be in the ./extension directory but isn't.

I'd not explicitly forbid that something other than Jinja extension - it's actually a nice feature if it can be a normal python package or module in there and the directory is not permanently added to the path. Also, we already have arbitrary code execution by virtue of having hooks. Elaborate verification would just over-complicate things with no benefit.

That being said, I will write a test, that verifies an actual Jinja2 extension can be imported and used. Just not today ;-)

@con-f-use I just found #1240 and seems it is more correct implementation of same thing and almost done.
I will close this one and #944 as duplicates of #1240