Allow Jinja extensions to be defined/bundled in the template repository

[ramnes]

This commit permits users to define custom Jinja extensions to be used in the cookiecutter.json file.

For example, with something like this in template/helpers.py:

from jinja2.ext import Extension


def get_app_slug(app_name):
    return app_name.lower()


class HelpersExtension(Extension):

    def __init__(self, environment):
        super(Extension, self).__init__()
        environment.filters["get_app_slug"] = get_app_slug

__all__ = ["HelpersExtension"]

It allows to use that Jinja filter directly:

{
    "app_name": "ExampleProject",
    "app_slug": "{{cookiecutter.app_name|get_app_slug}}",
    "_extensions": ["helpers.HelpersExtension"]
}

This is really just a dumb example, but I'm pretty confident that it could be a powerful feature.

I'm kinda new to Cookiecutter, so please point me to the right direction if there's already another way to achieve something like this that I'm not aware of.

[codecov-io]

Codecov Report

Merging #944 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #944   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          17     17           
  Lines         684    686    +2     
=====================================
+ Hits          684    686    +2

Impacted Files	Coverage Δ
cookiecutter/main.py	100% <100%> (ø)	⬆️

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[ramnes]

@hackebrot any feedback?

[theodesp]

@ramnes Looks good. Have you got any real cases when you've used Jinja Extensions?

[ramnes]

Yeah, at numberly we use our fork and we leverage this feature a lot.

We use filters almost everywhere, because it's so convenient to output different things based on a single input.

The example in my first comment is very basic but shows pretty well how we're using it.

[theodesp]

Could you PM the rest of the Team on Gitter about your progress? 👍
https://gitter.im/audreyr/cookiecutter

[ramnes]

@theodesp I've already done that in July, without much success. 😞

[michaeljoseph]

I would support a more explicit solution to this issue:

• validate the presence of a well known directory in a template repo (called extensions or similar)
• add this directory to the import path when we initialise the Jinja environment
• tests and documentation

[ramnes]

This implementation is how I expected cookiecutter to work in the first place, and it's good to have instinctive behaviors working out of the box.

I'd be open to implement it the way you say, but it would take more time, wouldn't be as obvious since you'd have to look at the docs, whereas here it's super simple and just works…

Don't want to keep it simple? What are the downsides?

[tarqd]

I hacked together something to make this available without a fork.

cookiecutter.json:

{
  "repo_name": "test",
  "foo": "bar",
  "_extensions": [ "cookiecutter_repo_extensions:Extension", "ccext:TemplateUtils" ]
}

# template/extensions/ccext.py
# -*- coding: utf-8 -*-

from jinja2.ext import Extension


def split(value, delim=','):
    return [ x.strip() for x in value.split(delim) ]

class TemplateUtils(Extension):
    def __init__(self, environment):
        super(TemplateUtils, self).__init__(environment)
        environment.filters['split'] = split

Only requires you to have cookiecutter_repo_extensions alongside cookiecutter and then all repo local extensions should load fine (as long as this extension is loaded first).

Not pretty and I'm not great at python so forgive the quality. Just a work-around for people who need want this feature but don't want to maintain a fork.

The code here is if you're interested: https://gist.github.com/ilsken/79f3077a72a9113f3e83ae44c8289f77

[ramnes]

@pydanny gentle ping

As said on gitter two months ago, done is better than perfect, and this is a two lines diff that's sitting here for more than a year now. Can we do something about it?

[ramnes]

Note to myself: just implement that extension directory idea.

[codecov-io]

Codecov Report

Merging #944 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #944   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          18     18           
  Lines         783    785    +2     
=====================================
+ Hits          783    785    +2

Impacted Files	Coverage Δ
cookiecutter/main.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3bc7b98...6b002cd. Read the comment docs.

[insspb]

@ramnes
Thank you for contribution. While this was in review, more correct approach in PR #1240 was introduced.
This one closed as duplicate.

[ramnes]

Great!