effectivepython/example_code/item_33.py at master · AISTLAB/effectivepython · GitHub

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#!/usr/bin/env python3

# 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.

# Preamble to mimick book environment

import logging

from pprint import pprint

from sys import stdout as STDOUT

# Example 1

class Meta(type):

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

orig_print = __builtins__.print

print = pprint

print((meta, name, bases, class_dict))

print = orig_print

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

class MyClass(object, metaclass=Meta):

stuff = 123

def foo(self):

pass

# Example 3

class ValidatePolygon(type):

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

# Don't validate the abstract Polygon class

if bases != (object,):

if class_dict['sides'] < 3:

raise ValueError('Polygons need 3+ sides')

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

class Polygon(object, metaclass=ValidatePolygon):

sides = None # Specified by subclasses

@classmethod

def interior_angles(cls):

return (cls.sides - 2) * 180

class Triangle(Polygon):

sides = 3

print(Triangle.interior_angles())

# Example 4

try:

print('Before class')

class Line(Polygon):

print('Before sides')

sides = 1

print('After sides')

print('After class')

except:

logging.exception('Expected')

else:

assert False

effectivepython/example_code/item_33.py at master · AISTLAB/effectivepython · GitHub

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