item_35.py

#!/usr/bin/env python3

# Copyright 2014 Brett Slatkin, Pearson Education Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# Preamble to mimick book environment
import logging
from pprint import pprint
from sys import stdout as STDOUT


# Example 1
class Field(object):
    def __init__(self, name):
        self.name = name
        self.internal_name = '_' + self.name

    def __get__(self, instance, instance_type):
        if instance is None: return self
        return getattr(instance, self.internal_name, '')

    def __set__(self, instance, value):
        setattr(instance, self.internal_name, value)


# Example 2
class Customer(object):
    # Class attributes
    first_name = Field('first_name')
    last_name = Field('last_name')
    prefix = Field('prefix')
    suffix = Field('suffix')


# Example 3
foo = Customer()
print('Before:', repr(foo.first_name), foo.__dict__)
foo.first_name = 'Euclid'
print('After: ', repr(foo.first_name), foo.__dict__)


# Example 4
class Meta(type):
    def __new__(meta, name, bases, class_dict):
        for key, value in class_dict.items():
            if isinstance(value, Field):
                value.name = key
                value.internal_name = '_' + key
        cls = type.__new__(meta, name, bases, class_dict)
        return cls


# Example 5
class DatabaseRow(object, metaclass=Meta):
    pass


# Example 6
class Field(object):
    def __init__(self):
        # These will be assigned by the metaclass.
        self.name = None
        self.internal_name = None
    def __get__(self, instance, instance_type):
        if instance is None: return self
        return getattr(instance, self.internal_name, '')

    def __set__(self, instance, value):
        setattr(instance, self.internal_name, value)


# Example 7
class BetterCustomer(DatabaseRow):
    first_name = Field()
    last_name = Field()
    prefix = Field()
    suffix = Field()


# Example 8
foo = BetterCustomer()
print('Before:', repr(foo.first_name), foo.__dict__)
foo.first_name = 'Euler'
print('After: ', repr(foo.first_name), foo.__dict__)