# 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__)
Before: '' {} After: 'Euclid' {'_first_name': 'Euclid'}
# 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__)
Before: '' {} After: 'Euler' {'_first_name': 'Euler'}