lektor_checkfile.py

import os
from lektor.pluginsystem import Plugin


class CheckfilePlugin(Plugin):
    name = 'CheckFile'
    description = u'Plugin to check if a file is present'

    def on_setup_env(self, **extra):

        def check_file(filename):
            fn = os.path.join(self.env.root_path, filename)
            assert os.path.isfile(fn), 'file {} does not exist'.format(fn)
        self.env.jinja_env.globals.update(
            check_file=check_file
        )