item_33.py

#!/usr/bin/env python3

# Copyright 2014 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.

# 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