#!/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__}')