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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 Homework:
def __init__(self):
self._grade = 0
@property
def grade(self):
return self._grade
@grade.setter
def grade(self, value):
if not (0 <= value <= 100):
raise ValueError(
'Grade must be between 0 and 100')
self._grade = value
# Example 2
galileo = Homework()
galileo.grade = 95
assert galileo.grade == 95
# Example 3
class Exam:
def __init__(self):
self._writing_grade = 0
self._math_grade = 0
@staticmethod
def _check_grade(value):
if not (0 <= value <= 100):
raise ValueError(
'Grade must be between 0 and 100')
# Example 4
@property
def writing_grade(self):
return self._writing_grade
@writing_grade.setter
def writing_grade(self, value):
self._check_grade(value)
self._writing_grade = value
@property
def math_grade(self):
return self._math_grade
@math_grade.setter
def math_grade(self, value):
self._check_grade(value)
self._math_grade = value
galileo = Exam()
galileo.writing_grade = 85
galileo.math_grade = 99
assert galileo.writing_grade == 85
assert galileo.math_grade == 99
# Example 5
class Grade:
def __get__(self, instance, instance_type):
pass
def __set__(self, instance, value):
pass
class Exam:
# Class attributes
math_grade = Grade()
writing_grade = Grade()
science_grade = Grade()
# Example 6
exam = Exam()
exam.writing_grade = 40
# Example 7
Exam.__dict__['writing_grade'].__set__(exam, 40)
# Example 8
exam.writing_grade
# Example 9
Exam.__dict__['writing_grade'].__get__(exam, Exam)
# Example 10
class Grade:
def __init__(self):
self._value = 0
def __get__(self, instance, instance_type):
return self._value
def __set__(self, instance, value):
if not (0 <= value <= 100):
raise ValueError(
'Grade must be between 0 and 100')
self._value = value
# Example 11
class Exam:
math_grade = Grade()
writing_grade = Grade()
science_grade = Grade()
first_exam = Exam()
first_exam.writing_grade = 82
first_exam.science_grade = 99
print('Writing', first_exam.writing_grade)
print('Science', first_exam.science_grade)
# Example 12
second_exam = Exam()
second_exam.writing_grade = 75
print(f'Second {second_exam.writing_grade} is right')
print(f'First {first_exam.writing_grade} is wrong; '
f'should be 82')
# Example 13
class Grade:
def __init__(self):
self._values = {}
def __get__(self, instance, instance_type):
if instance is None:
return self
return self._values.get(instance, 0)
def __set__(self, instance, value):
if not (0 <= value <= 100):
raise ValueError(
'Grade must be between 0 and 100')
self._values[instance] = value
# Example 14
from weakref import WeakKeyDictionary
class Grade:
def __init__(self):
self._values = WeakKeyDictionary()
def __get__(self, instance, instance_type):
if instance is None:
return self
return self._values.get(instance, 0)
def __set__(self, instance, value):
if not (0 <= value <= 100):
raise ValueError(
'Grade must be between 0 and 100')
self._values[instance] = value
# Example 15
class Exam:
math_grade = Grade()
writing_grade = Grade()
science_grade = Grade()
first_exam = Exam()
first_exam.writing_grade = 82
second_exam = Exam()
second_exam.writing_grade = 75
print(f'First {first_exam.writing_grade} is right')
print(f'Second {second_exam.writing_grade} is right')