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#!/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 LazyDB(object):

def __init__(self):

self.exists = 5

def __getattr__(self, name):

value = 'Value for %s' % name

setattr(self, name, value)

return value

# Example 2

data = LazyDB()

print('Before:', data.__dict__)

print('foo: ', data.foo)

print('After: ', data.__dict__)

# Example 3

class LoggingLazyDB(LazyDB):

def __getattr__(self, name):

print('Called __getattr__(%s)' % name)

return super().__getattr__(name)

data = LoggingLazyDB()

print('exists:', data.exists)

print('foo: ', data.foo)

print('foo: ', data.foo)

# Example 4

class ValidatingDB(object):

def __init__(self):

self.exists = 5

def __getattribute__(self, name):

print('Called __getattribute__(%s)' % name)

try:

return super().__getattribute__(name)

except AttributeError:

value = 'Value for %s' % name

setattr(self, name, value)

return value

data = ValidatingDB()

print('exists:', data.exists)

print('foo: ', data.foo)

print('foo: ', data.foo)

# Example 5

try:

class MissingPropertyDB(object):

def __getattr__(self, name):

if name == 'bad_name':

raise AttributeError('%s is missing' % name)

value = 'Value for %s' % name

setattr(self, name, value)

return value

data = MissingPropertyDB()

data.foo # Test this works

data.bad_name

except:

logging.exception('Expected')

else:

assert False

# Example 6

data = LoggingLazyDB()

print('Before: ', data.__dict__)

print('foo exists: ', hasattr(data, 'foo'))

print('After: ', data.__dict__)

print('foo exists: ', hasattr(data, 'foo'))

# Example 7

data = ValidatingDB()

print('foo exists: ', hasattr(data, 'foo'))

print('foo exists: ', hasattr(data, 'foo'))

# Example 8

class SavingDB(object):

def __setattr__(self, name, value):

# Save some data to the DB log

super().__setattr__(name, value)

# Example 9

class LoggingSavingDB(SavingDB):

def __setattr__(self, name, value):

print('Called __setattr__(%s, %r)' % (name, value))

super().__setattr__(name, value)

data = LoggingSavingDB()

print('Before: ', data.__dict__)

data.foo = 5

print('After: ', data.__dict__)

data.foo = 7

print('Finally:', data.__dict__)

# Example 10

class BrokenDictionaryDB(object):

def __init__(self, data):

self._data = data

def __getattribute__(self, name):

print('Called __getattribute__(%s)' % name)

return self._data[name]

# Example 11

try:

data = BrokenDictionaryDB({'foo': 3})

data.foo

except:

logging.exception('Expected')

else:

assert False

# Example 12

class DictionaryDB(object):

def __init__(self, data):

self._data = data

def __getattribute__(self, name):

data_dict = super().__getattribute__('_data')

return data_dict[name]

data = DictionaryDB({'foo': 3})

print(data.foo)