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item_064.py
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#!/usr/bin/env PYTHONHASHSEED=1234 python3
# Copyright 2014-2024 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.
### 책 예제에 맞는 환경 설정을 시작함
import random
random.seed(1234)
import logging
from pprint import pprint
from sys import stdout as STDOUT
# 모든 출력을 임시 디렉터리로 보냄
import atexit
import gc
import io
import os
import tempfile
TEST_DIR = tempfile.TemporaryDirectory()
atexit.register(TEST_DIR.cleanup)
# 윈도우에서 프로세스가 제대로 종료되도록 함
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)
### 책 예제에 맞는 환경설정 끝
print("아이템 64")
print("Example 1")
class Field:
def __init__(self, column_name):
self.column_name = column_name
self.internal_name = "_" + self.column_name
print("Example 2")
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)
print("Example 3")
class Customer:
# Class attributes
first_name = Field("first_name")
last_name = Field("last_name")
prefix = Field("prefix")
suffix = Field("suffix")
print("Example 4")
cust = Customer()
print(f"이전: {cust.first_name!r} {cust.__dict__}")
cust.first_name = "유클리드"
print(f"이후: {cust.first_name!r} {cust.__dict__}")
print("Example 5")
class Customer:
# =의 좌변과 우변의 정보가 불필요하게 중복된다
first_name = Field("first_name")
last_name = Field("last_name")
prefix = Field("prefix")
suffix = Field("suffix")
print("Example 6")
class Meta(type):
def __new__(meta, name, bases, class_dict):
for key, value in class_dict.items():
if isinstance(value, Field):
value.column_name = key
value.internal_name = "_" + key
cls = type.__new__(meta, name, bases, class_dict)
return cls
print("Example 7")
class DatabaseRow(metaclass=Meta):
pass
print("Example 8")
class Field:
def __init__(self):
# 이 두 정보를 메타클래스가 채워 준다
self.column_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)
print("Example 9")
class BetterCustomer(DatabaseRow):
first_name = Field()
last_name = Field()
prefix = Field()
suffix = Field()
print("Example 10")
cust = BetterCustomer()
print(f"이전: {cust.first_name!r} {cust.__dict__}")
cust.first_name = "오일러"
print(f"이후: {cust.first_name!r} {cust.__dict__}")
print("Example 11")
try:
class BrokenCustomer: # 상속을 잊어버림
first_name = Field()
last_name = Field()
prefix = Field()
suffix = Field()
cust = BrokenCustomer()
cust.first_name = "메르센"
except:
logging.exception('이 예외가 발생해야 함')
else:
assert False
print("Example 12")
class Field:
def __init__(self):
self.column_name = None
self.internal_name = None
def __set_name__(self, owner, column_name):
# Called on class creation for each descriptor
self.column_name = column_name
self.internal_name = "_" + column_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)
print("Example 13")
class FixedCustomer: # No parent class
first_name = Field()
last_name = Field()
prefix = Field()
suffix = Field()
cust = FixedCustomer()
print(f"이전: {cust.first_name!r} {cust.__dict__}")
cust.first_name = "메르센"
print(f"이후: {cust.first_name!r} {cust.__dict__}")