config_descriptors.py

# -*- coding: utf-8 -*-
from __future__ import absolute_import, division, print_function, unicode_literals

import string
from weakref import WeakKeyDictionary


class SafeDict(dict):
    def __missing__(self, key):
        return '{' + key + '}'


class GBQResource(str):
    def __init__(self, value):
        self.all_values = WeakKeyDictionary()
        super(GBQResource, self).__init__(value)
        self.default_value = value
        self.project = self._dataset = self._table = None

        values = value.split('.')
        if len(values) not in (1, 2, 3):
            raise ValueError('Resource needs to be {project}[.{dataset}[.{table}]]')
        if len(values) == 1:
            self.project, = values
        elif len(values) == 2:
            self.project, self._dataset = values
        elif len(values) == 3:
            self.project, self._dataset, self._table = values

    def __get__(self, instance, owner):
        if not self.all_values.get(instance):
            obj = self.all_values[instance] = type(self)(self.default_value)
            return obj
        return self.all_values[instance]

    def __set__(self, instance, value):
        self.all_values[instance] = type(self)(value)

    @property
    def dataset(self):
        if self._dataset:
            return self._dataset
        raise ValueError('Resource is only project, can not provide dataset')

    @property
    def table(self):
        if self._table:
            return self._table
        raise ValueError('Resource is either project or dataset, can not provide table')


class CLASS(object):
    def __new__(cls, *args, **kwargs):
        fmt = string.Formatter()
        obj = super(CLASS, cls).__new__(cls)
        for key, value in kwargs.items():
            if hasattr(obj, key):
                setattr(obj, key, value)
        obj_attrs = {a: getattr(obj, a) for a in dir(obj) if a == a.upper()}
        for i in range(5):  # Four indirection levels
            for attribute, value in obj_attrs.items():
                if not isinstance(value, (str, unicode)):
                    continue
                obj_attrs[attribute] = fmt.vformat(value, (), SafeDict(obj_attrs))

        for attribute, value in obj_attrs.items():
            if not isinstance(value, (str, unicode)):
                continue
            assert '{' not in value, 'Attribute ' + attribute + ' failed with ' + value
            assert '}' not in value, 'Attribute ' + attribute + ' failed with ' + value
            setattr(obj, attribute, value)
        return obj

    FIELD = GBQResource('m.n.o')


a = CLASS(FIELD='z.x.y')
b = CLASS(FIELD='d.e.f')
c = CLASS()

print(a, a.FIELD, a.FIELD.table)
print(b, b.FIELD, b.FIELD.table)
print(c, c.FIELD, c.FIELD.table)