#!/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 Meta(type):

def __new__(meta, name, bases, class_dict):

global print

orig_print = print

print(f'* Running {meta}.__new__ for {name}')

print('Bases:', bases)

print = pprint

print(class_dict)

print = orig_print

return type.__new__(meta, name, bases, class_dict)

class MyClass(metaclass=Meta):

stuff = 123

def foo(self):

pass

class MySubclass(MyClass):

other = 567

def bar(self):

pass

# Example 2

class ValidatePolygon(type):

def __new__(meta, name, bases, class_dict):

# Only validate subclasses of the Polygon class

if bases:

if class_dict['sides'] < 3:

raise ValueError('Polygons need 3+ sides')

return type.__new__(meta, name, bases, class_dict)

class Polygon(metaclass=ValidatePolygon):

sides = None # Must be specified by subclasses

@classmethod

def interior_angles(cls):

return (cls.sides - 2) * 180

class Triangle(Polygon):

sides = 3

class Rectangle(Polygon):

sides = 4

class Nonagon(Polygon):

sides = 9

assert Triangle.interior_angles() == 180

assert Rectangle.interior_angles() == 360

assert Nonagon.interior_angles() == 1260

# Example 3

try:

print('Before class')

class Line(Polygon):

print('Before sides')

sides = 2

print('After sides')

print('After class')

except:

logging.exception('Expected')

else:

assert False

# Example 4

class BetterPolygon:

sides = None # Must be specified by subclasses

def __init_subclass__(cls):

super().__init_subclass__()

if cls.sides < 3:

raise ValueError('Polygons need 3+ sides')

@classmethod

def interior_angles(cls):

return (cls.sides - 2) * 180

class Hexagon(BetterPolygon):

sides = 6

assert Hexagon.interior_angles() == 720

# Example 5

try:

print('Before class')

class Point(BetterPolygon):

sides = 1

print('After class')

except:

logging.exception('Expected')

else:

assert False

# Example 6

class ValidateFilled(type):

def __new__(meta, name, bases, class_dict):

# Only validate subclasses of the Filled class

if bases:

if class_dict['color'] not in ('red', 'green'):

raise ValueError('Fill color must be supported')

return type.__new__(meta, name, bases, class_dict)

class Filled(metaclass=ValidateFilled):

color = None # Must be specified by subclasses

# Example 7

try:

class RedPentagon(Filled, Polygon):

color = 'blue'

sides = 5

except:

logging.exception('Expected')

else:

assert False

# Example 8

class ValidatePolygon(type):

def __new__(meta, name, bases, class_dict):

# Only validate non-root classes

if not class_dict.get('is_root'):

if class_dict['sides'] < 3:

raise ValueError('Polygons need 3+ sides')

return type.__new__(meta, name, bases, class_dict)

class Polygon(metaclass=ValidatePolygon):

is_root = True

sides = None # Must be specified by subclasses

class ValidateFilledPolygon(ValidatePolygon):

def __new__(meta, name, bases, class_dict):

# Only validate non-root classes

if not class_dict.get('is_root'):

if class_dict['color'] not in ('red', 'green'):

raise ValueError('Fill color must be supported')

return super().__new__(meta, name, bases, class_dict)

class FilledPolygon(Polygon, metaclass=ValidateFilledPolygon):

is_root = True

color = None # Must be specified by subclasses

# Example 9

class GreenPentagon(FilledPolygon):

color = 'green'

sides = 5

greenie = GreenPentagon()

assert isinstance(greenie, Polygon)

# Example 10

try:

class OrangePentagon(FilledPolygon):

color = 'orange'

sides = 5

except:

logging.exception('Expected')

else:

assert False

# Example 11

try:

class RedLine(FilledPolygon):

color = 'red'

sides = 2

except:

logging.exception('Expected')

else:

assert False

# Example 12

class Filled:

color = None # Must be specified by subclasses

def __init_subclass__(cls):

super().__init_subclass__()

if cls.color not in ('red', 'green', 'blue'):

raise ValueError('Fills need a valid color')

# Example 13

class RedTriangle(Filled, BetterPolygon):

color = 'red'

sides = 3

ruddy = RedTriangle()

assert isinstance(ruddy, Filled)

assert isinstance(ruddy, BetterPolygon)

# Example 14

try:

print('Before class')

class BlueLine(Filled, BetterPolygon):

color = 'blue'

sides = 2

print('After class')

except:

logging.exception('Expected')

else:

assert False

# Example 15

try:

print('Before class')

class BeigeSquare(Filled, BetterPolygon):

color = 'beige'

sides = 4

print('After class')

except:

logging.exception('Expected')

else:

assert False

# Example 16

class Top:

def __init_subclass__(cls):

super().__init_subclass__()

print(f'Top for {cls}')

class Left(Top):

def __init_subclass__(cls):

super().__init_subclass__()

print(f'Left for {cls}')

class Right(Top):

def __init_subclass__(cls):

super().__init_subclass__()

print(f'Right for {cls}')

class Bottom(Left, Right):

def __init_subclass__(cls):

super().__init_subclass__()

print(f'Bottom for {cls}')