Lightweight helpers to read settings from environement variables

12factors.py

import os
import re
import sys
from functools import wraps
import logging

logger = logging.getLogger(__name__)

WARN_ON_MISSING = True

NO_ARGUMENT = object()

TRUE_CHOICES = {'on', 'enabled', 'yes', 'true', '1'}


class MissingValueError(ValueError):
    pass


def handle_errors(f):
    @wraps(f)
    def _wrapper(name, *args, **kwargs):
        try:
            return f(name, *args, **kwargs)
        except MissingValueError as e:
            if WARN_ON_MISSING:
                logger.warning("Variable {key} was expected and is missing".format(key=name))
                return None
            else:
                logger.error("Variable {key} is missing".format(key=name))
                sys.exit(1)

        except ValueError as e:
            logger.error("Configuration error for key {key}: {message}".format(key=name, message=str(e)))
            sys.exit(1)

    return _wrapper


@handle_errors
def read(name, default=NO_ARGUMENT):
    if name not in os.environ:
        if default is NO_ARGUMENT:
            raise MissingValueError
        else:
            return default
    return os.environ[name]


@handle_errors
def read_boolean(name, default=NO_ARGUMENT):
    if name not in os.environ:
        if default is NO_ARGUMENT:
            raise MissingValueError
        else:
            return default
    return os.environ[name].strip().lower() in TRUE_CHOICES


@handle_errors
def read_int(name, default=NO_ARGUMENT):
    value = os.environ.get(name)
    if value is None:
        if default is NO_ARGUMENT:
            raise MissingValueError
        else:
            return default
    else:
        if not re.match('^\d+$', value):
            raise ValueError('Only digits are allowed for this value'.format(name=name))
        return int(value)


@handle_errors
def read_list(name, default=NO_ARGUMENT, separator=','):
    """Anythhing like VAR=1, 2, 3 will be converted to ['1', '2', '3']"""
    value = os.environ.get(name)
    if value is None:
        if default is NO_ARGUMENT:
            return []
        else:
            return default
    return [v.strip() for v in value.split(separator) if v.strip()]