render an ini file based on env vars

.env

##############################################################################
# Settings made available in site.ini.in.
#
# After making changes re-run render-config
#
# Override settings by making a .env.local file.
##############################################################################
DEBUG=yes

WEB_CONCURRENCY=10
TRUSTED_PROXY=""
TRUSTED_PROXY_HEADERS=""
TRUSTED_PROXY_COUNT=1
LOG_UNTRUSTED_PROXY_HEADERS=yes
BIND_HOST=127.0.0.1

# heroku defines the PORT variable but we rename it to BIND_PORT
PORT=10001
BIND_PORT=${PORT}

DATABASE_URL=...

RETRY_ATTEMPTS=3

render_config.py

from dotenv import load_dotenv
import jinja2
from jinja2.ext import Extension
from jinja2.lexer import Token
from markupsafe import Markup
import os
from pyramid.settings import asbool
import textwrap


def asbool_filter(value):
    return asbool(value)

def multiline_filter(value):
    # strip only the leading whitespaces from the first line
    value = value.strip(' ').strip('\n')
    if value:
        lines = textwrap.dedent(value).split('\n')
        return '\n    ' + '\n    '.join(lines)
    return ''

def ini_filter(value):
    if isinstance(value, Markup):
        return value
    if not isinstance(value, str):
        return value
    return value.replace('%', '%%')

class IniEverythingExtension(Extension):
    """
    Insert a `|ini` filter at the end of every variable substitution.

    This will ensure that all injected values are converted to INI.
    """
    def filter_stream(self, stream):
        # This is based on
        # https://github.com/indico/indico/blob/master/indico/web/flask/templating.py.
        for token in stream:
            if token.type == 'variable_end':
                yield Token(token.lineno, 'pipe', '|')
                yield Token(token.lineno, 'name', 'ini')
            yield token

def render_template(path, context):
    base, name = os.path.split(path)
    if not base:
        base = os.getcwd()
    env = jinja2.Environment(
        loader=jinja2.FileSystemLoader([base]),
        undefined=jinja2.StrictUndefined,
        extensions=[IniEverythingExtension],
    )
    env.filters['bool'] = asbool_filter
    env.filters['multiline'] = multiline_filter
    env.filters['ini'] = ini_filter

    template = env.get_template(name)
    return template.render(context)
    
def parse_args():
    parser = argparse.ArgumentParser()
    parser.add_argument('--env')
    parser.add_argument('--env-file')
    parser.add_argument('--template', default='site.ini.in')
    parser.add_argument('-o', '--output-file', default='site.ini')

def main(cli, args):
    if args.env_file:
        load_dotenv(args.env_file)

    else:
        override = args.env or os.environ.get('DOTENV')
        for path in (
            f'.env.{override}.local' if override else None,
            f'.env.{override}' if override else None,
            '.env.local',
            '.env',
        ):
            if path and os.path.exists(path):
                load_dotenv(path)

    result = render_template(args.template, {
        'env': os.environ,
    })

    with open(args.output_file, 'w', encoding='utf8') as fp:
        fp.write(result.rstrip() + '\n')

site.ini.in

{%- set DEBUG = env.DEBUG | bool -%}
[alembic]
script_location = myapp.alembic:migrations
file_template = %%(year)d%%(month).2d%%(day).2d_%%(rev)s

sqlalchemy.url = {{ env.DATABASE_URL }}

[app:api]
use = egg:myapp

sqlalchemy.url = {{ env.DATABASE_URL }}

retry.attempts = {{ env.RETRY_ATTEMPTS }}
tm.annotate_user = no

pyramid.reload_assets = {{ 'yes' if DEBUG else 'no' }}
pyramid.reload_templates = {{ 'yes' if DEBUG else 'no' }}
pyramid.debug_authorization = no
pyramid.debug_notfound = no
pyramid.debug_routematch = no
pyramid.default_locale_name = en

{%- if DEBUG %}
pyramid.includes =
    pyramid_debugtoolbar
    
debugtoolbar.hosts = 0.0.0.0/0
debugtoolbar.show_on_exc_only = yes
debugtoolbar.active_panels = performance
{%- endif %}

[filter:tracker]
use = egg:request-id
exclude_prefixes =
    /static/
    /_debug_toolbar/

[pipeline:main]
pipeline =
    tracker
    api

[server:main]
use = egg:waitress#main
host = {{ env.BIND_HOST }}
port = {{ env.BIND_PORT }}
threads = {{ env.WEB_CONCURRENCY }}
{%- if env.TRUSTED_PROXY %}
trusted_proxy = {{ env.TRUSTED_PROXY }}
trusted_proxy_headers = {{ env.TRUSTED_PROXY_HEADERS }}
trusted_proxy_count = {{ env.TRUSTED_PROXY_COUNT }}
{%- endif %}
log_untrusted_proxy_headers = {{ env.LOG_UNTRUSTED_PROXY_HEADERS }}
clear_untrusted_proxy_headers = yes

[loggers]
keys = root, alembic, sqlalchemy, myapp, translogger

[handlers]
keys = console, translogger

[formatters]
keys = generic, minimal

[logger_root]
level = INFO
handlers = console

[logger_alembic]
level = INFO
handlers =
qualname = alembic

[logger_sqlalchemy]
level = WARN
handlers =
qualname = sqlalchemy

[logger_myapp]
level = DEBUG
handlers =
qualname = myapp

[logger_translogger]
level = INFO
handlers = translogger
qualname = request_id
propagate = 0

[handler_console]
class = StreamHandler
args = (sys.stderr,)
level = NOTSET
formatter = generic

[handler_translogger]
class = StreamHandler
args = (sys.stderr,)
level = NOTSET
formatter = minimal

[formatter_generic]
class = Formatter
format = %(asctime)s %(levelname)-5.5s [%(name)s][%(threadName)s] %(message)s

[formatter_minimal]
class = Formatter
format = %(message)s

what is {{ foo }}? Does it count foo as a variable?