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item_50.py
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#!/usr/bin/env PYTHONHASHSEED=1234 python3
# Copyright 2014-2019 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.
# Reproduce book environment
import random
random.seed(1234)
import logging
from pprint import pprint
from sys import stdout as STDOUT
# Write all output to a temporary directory
import atexit
import gc
import io
import os
import tempfile
TEST_DIR = tempfile.TemporaryDirectory()
atexit.register(TEST_DIR.cleanup)
# Make sure Windows processes exit cleanly
OLD_CWD = os.getcwd()
atexit.register(lambda: os.chdir(OLD_CWD))
os.chdir(TEST_DIR.name)
def close_open_files():
everything = gc.get_objects()
for obj in everything:
if isinstance(obj, io.IOBase):
obj.close()
atexit.register(close_open_files)
# Example 1
class Field:
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:
# Class attributes
first_name = Field('first_name')
last_name = Field('last_name')
prefix = Field('prefix')
suffix = Field('suffix')
# Example 3
cust = Customer()
print(f'Before: {cust.first_name!r} {cust.__dict__}')
cust.first_name = 'Euclid'
print(f'After: {cust.first_name!r} {cust.__dict__}')
# Example 4
class Customer:
# Left side is redundant with right side
first_name = Field('first_name')
last_name = Field('last_name')
prefix = Field('prefix')
suffix = Field('suffix')
# Example 5
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 6
class DatabaseRow(metaclass=Meta):
pass
# Example 7
class Field:
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 8
class BetterCustomer(DatabaseRow):
first_name = Field()
last_name = Field()
prefix = Field()
suffix = Field()
# Example 9
cust = BetterCustomer()
print(f'Before: {cust.first_name!r} {cust.__dict__}')
cust.first_name = 'Euler'
print(f'After: {cust.first_name!r} {cust.__dict__}')
# Example 10
try:
class BrokenCustomer:
first_name = Field()
last_name = Field()
prefix = Field()
suffix = Field()
cust = BrokenCustomer()
cust.first_name = 'Mersenne'
except:
logging.exception('Expected')
else:
assert False
# Example 11
class Field:
def __init__(self):
self.name = None
self.internal_name = None
def __set_name__(self, owner, name):
# Called on class creation for each descriptor
self.name = name
self.internal_name = '_' + 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 12
class FixedCustomer:
first_name = Field()
last_name = Field()
prefix = Field()
suffix = Field()
cust = FixedCustomer()
print(f'Before: {cust.first_name!r} {cust.__dict__}')
cust.first_name = 'Mersenne'
print(f'After: {cust.first_name!r} {cust.__dict__}')