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removed 1st edition code

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Luciano Ramalho
2020-03-11 18:03:55 -03:00
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4
24-class-metaprog/README.rst
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Sample code for Chapter 21 - "Class metaprogramming"
From the book "Fluent Python" by Luciano Ramalho (O'Reilly, 2015)
http://shop.oreilly.com/product/0636920032519.do

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24-class-metaprog/bulkfood/bulkfood_v6.py
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"""
A line item for a bulk food order has description, weight and price fields::
>>> raisins = LineItem('Golden raisins', 10, 6.95)
>>> raisins.weight, raisins.description, raisins.price
(10, 'Golden raisins', 6.95)
A ``subtotal`` method gives the total price for that line item::
>>> raisins.subtotal()
69.5
The weight of a ``LineItem`` must be greater than 0::
>>> raisins.weight = -20
Traceback (most recent call last):
...
ValueError: value must be > 0
No change was made::
>>> raisins.weight
10
The value of the attributes managed by the descriptors are stored in
alternate attributes, created by the descriptors in each ``LineItem``
instance::
# BEGIN LINEITEM_V6_DEMO
>>> raisins = LineItem('Golden raisins', 10, 6.95)
>>> dir(raisins)[:3]
['_NonBlank#description', '_Quantity#price', '_Quantity#weight']
>>> LineItem.description.storage_name
'_NonBlank#description'
>>> raisins.description
'Golden raisins'
>>> getattr(raisins, '_NonBlank#description')
'Golden raisins'
# END LINEITEM_V6_DEMO
If the descriptor is accessed in the class, the descriptor object is
returned:
>>> LineItem.weight # doctest: +ELLIPSIS
<model_v6.Quantity object at 0x...>
>>> LineItem.weight.storage_name
'_Quantity#weight'
The `NonBlank` descriptor prevents empty or blank strings to be used
for the description:
>>> br_nuts = LineItem('Brazil Nuts', 10, 34.95)
>>> br_nuts.description = ' '
Traceback (most recent call last):
...
ValueError: value cannot be empty or blank
>>> void = LineItem('', 1, 1)
Traceback (most recent call last):
...
ValueError: value cannot be empty or blank
"""
# BEGIN LINEITEM_V6
import model_v6 as model
@model.entity # <1>
class LineItem:
description = model.NonBlank()
weight = model.Quantity()
price = model.Quantity()
def __init__(self, description, weight, price):
self.description = description
self.weight = weight
self.price = price
def subtotal(self):
return self.weight * self.price
# END LINEITEM_V6

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24-class-metaprog/bulkfood/bulkfood_v7.py
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"""
A line item for a bulk food order has description, weight and price fields::
>>> raisins = LineItem('Golden raisins', 10, 6.95)
>>> raisins.weight, raisins.description, raisins.price
(10, 'Golden raisins', 6.95)
A ``subtotal`` method gives the total price for that line item::
>>> raisins.subtotal()
69.5
The weight of a ``LineItem`` must be greater than 0::
>>> raisins.weight = -20
Traceback (most recent call last):
...
ValueError: value must be > 0
No change was made::
>>> raisins.weight
10
The value of the attributes managed by the descriptors are stored in
alternate attributes, created by the descriptors in each ``LineItem``
instance::
>>> raisins = LineItem('Golden raisins', 10, 6.95)
>>> dir(raisins)[:3]
['_NonBlank#description', '_Quantity#price', '_Quantity#weight']
>>> LineItem.description.storage_name
'_NonBlank#description'
>>> raisins.description
'Golden raisins'
>>> getattr(raisins, '_NonBlank#description')
'Golden raisins'
If the descriptor is accessed in the class, the descriptor object is
returned:
>>> LineItem.weight # doctest: +ELLIPSIS
<model_v7.Quantity object at 0x...>
>>> LineItem.weight.storage_name
'_Quantity#weight'
The `NonBlank` descriptor prevents empty or blank strings to be used
for the description:
>>> br_nuts = LineItem('Brazil Nuts', 10, 34.95)
>>> br_nuts.description = ' '
Traceback (most recent call last):
...
ValueError: value cannot be empty or blank
>>> void = LineItem('', 1, 1)
Traceback (most recent call last):
...
ValueError: value cannot be empty or blank
"""
# BEGIN LINEITEM_V7
import model_v7 as model
class LineItem(model.Entity): # <1>
description = model.NonBlank()
weight = model.Quantity()
price = model.Quantity()
def __init__(self, description, weight, price):
self.description = description
self.weight = weight
self.price = price
def subtotal(self):
return self.weight * self.price
# END LINEITEM_V7

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24-class-metaprog/bulkfood/bulkfood_v8.py
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"""
A line item for a bulk food order has description, weight and price fields::
>>> raisins = LineItem('Golden raisins', 10, 6.95)
>>> raisins.weight, raisins.description, raisins.price
(10, 'Golden raisins', 6.95)
A ``subtotal`` method gives the total price for that line item::
>>> raisins.subtotal()
69.5
The weight of a ``LineItem`` must be greater than 0::
>>> raisins.weight = -20
Traceback (most recent call last):
...
ValueError: value must be > 0
No change was made::
>>> raisins.weight
10
>>> raisins = LineItem('Golden raisins', 10, 6.95)
>>> dir(raisins)[:3]
['_NonBlank#description', '_Quantity#price', '_Quantity#weight']
>>> LineItem.description.storage_name
'_NonBlank#description'
>>> raisins.description
'Golden raisins'
>>> getattr(raisins, '_NonBlank#description')
'Golden raisins'
If the descriptor is accessed in the class, the descriptor object is
returned:
>>> LineItem.weight # doctest: +ELLIPSIS
<model_v8.Quantity object at 0x...>
>>> LineItem.weight.storage_name
'_Quantity#weight'
The `NonBlank` descriptor prevents empty or blank strings to be used
for the description:
>>> br_nuts = LineItem('Brazil Nuts', 10, 34.95)
>>> br_nuts.description = ' '
Traceback (most recent call last):
...
ValueError: value cannot be empty or blank
>>> void = LineItem('', 1, 1)
Traceback (most recent call last):
...
ValueError: value cannot be empty or blank
Fields can be retrieved in the order they were declared:
# BEGIN LINEITEM_V8_DEMO
>>> for name in LineItem.field_names():
... print(name)
...
description
weight
price
# END LINEITEM_V8_DEMO
"""
import model_v8 as model
class LineItem(model.Entity):
description = model.NonBlank()
weight = model.Quantity()
price = model.Quantity()
def __init__(self, description, weight, price):
self.description = description
self.weight = weight
self.price = price
def subtotal(self):
return self.weight * self.price

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24-class-metaprog/bulkfood/model_v6.py
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import abc
class AutoStorage:
__counter = 0
def __init__(self):
cls = self.__class__
prefix = cls.__name__
index = cls.__counter
self.storage_name = '_{}#{}'.format(prefix, index)
cls.__counter += 1
def __get__(self, instance, owner):
if instance is None:
return self
else:
return getattr(instance, self.storage_name)
def __set__(self, instance, value):
setattr(instance, self.storage_name, value)
class Validated(abc.ABC, AutoStorage):
def __set__(self, instance, value):
value = self.validate(instance, value)
super().__set__(instance, value)
@abc.abstractmethod
def validate(self, instance, value):
"""return validated value or raise ValueError"""
class Quantity(Validated):
"""a number greater than zero"""
def validate(self, instance, value):
if value <= 0:
raise ValueError('value must be > 0')
return value
class NonBlank(Validated):
"""a string with at least one non-space character"""
def validate(self, instance, value):
value = value.strip()
if len(value) == 0:
raise ValueError('value cannot be empty or blank')
return value
# BEGIN MODEL_V6
def entity(cls): # <1>
for key, attr in cls.__dict__.items(): # <2>
if isinstance(attr, Validated): # <3>
type_name = type(attr).__name__
attr.storage_name = '_{}#{}'.format(type_name, key) # <4>
return cls # <5>
# END MODEL_V6

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24-class-metaprog/bulkfood/model_v7.py
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import abc
class AutoStorage:
__counter = 0
def __init__(self):
cls = self.__class__
prefix = cls.__name__
index = cls.__counter
self.storage_name = '_{}#{}'.format(prefix, index)
cls.__counter += 1
def __get__(self, instance, owner):
if instance is None:
return self
else:
return getattr(instance, self.storage_name)
def __set__(self, instance, value):
setattr(instance, self.storage_name, value)
class Validated(abc.ABC, AutoStorage):
def __set__(self, instance, value):
value = self.validate(instance, value)
super().__set__(instance, value)
@abc.abstractmethod
def validate(self, instance, value):
"""return validated value or raise ValueError"""
class Quantity(Validated):
"""a number greater than zero"""
def validate(self, instance, value):
if value <= 0:
raise ValueError('value must be > 0')
return value
class NonBlank(Validated):
"""a string with at least one non-space character"""
def validate(self, instance, value):
value = value.strip()
if len(value) == 0:
raise ValueError('value cannot be empty or blank')
return value
# BEGIN MODEL_V7
class EntityMeta(type):
"""Metaclass for business entities with validated fields"""
def __init__(cls, name, bases, attr_dict):
super().__init__(name, bases, attr_dict) # <1>
for key, attr in attr_dict.items(): # <2>
if isinstance(attr, Validated):
type_name = type(attr).__name__
attr.storage_name = '_{}#{}'.format(type_name, key)
class Entity(metaclass=EntityMeta): # <3>
"""Business entity with validated fields"""
# END MODEL_V7

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24-class-metaprog/bulkfood/model_v8.py
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import abc
import collections
class AutoStorage:
__counter = 0
def __init__(self):
cls = self.__class__
prefix = cls.__name__
index = cls.__counter
self.storage_name = '_{}#{}'.format(prefix, index)
cls.__counter += 1
def __get__(self, instance, owner):
if instance is None:
return self
else:
return getattr(instance, self.storage_name)
def __set__(self, instance, value):
setattr(instance, self.storage_name, value)
class Validated(abc.ABC, AutoStorage):
def __set__(self, instance, value):
value = self.validate(instance, value)
super().__set__(instance, value)
@abc.abstractmethod
def validate(self, instance, value):
"""return validated value or raise ValueError"""
class Quantity(Validated):
"""a number greater than zero"""
def validate(self, instance, value):
if value <= 0:
raise ValueError('value must be > 0')
return value
class NonBlank(Validated):
"""a string with at least one non-space character"""
def validate(self, instance, value):
value = value.strip()
if len(value) == 0:
raise ValueError('value cannot be empty or blank')
return value
# BEGIN MODEL_V8
class EntityMeta(type):
"""Metaclass for business entities with validated fields"""
@classmethod
def __prepare__(cls, name, bases):
return collections.OrderedDict() # <1>
def __init__(cls, name, bases, attr_dict):
super().__init__(name, bases, attr_dict)
cls._field_names = [] # <2>
for key, attr in attr_dict.items(): # <3>
if isinstance(attr, Validated):
type_name = type(attr).__name__
attr.storage_name = '_{}#{}'.format(type_name, key)
cls._field_names.append(key) # <4>
class Entity(metaclass=EntityMeta):
"""Business entity with validated fields"""
@classmethod
def field_names(cls): # <5>
for name in cls._field_names:
yield name
# END MODEL_V8

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24-class-metaprog/evalsupport.py
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print('<[100]> evalsupport module start')
def deco_alpha(cls):
print('<[200]> deco_alpha')
def inner_1(self):
print('<[300]> deco_alpha:inner_1')
cls.method_y = inner_1
return cls
class MetaAleph(type):
print('<[400]> MetaAleph body')
def __init__(cls, name, bases, dic):
print('<[500]> MetaAleph.__init__')
def inner_2(self):
print('<[600]> MetaAleph.__init__:inner_2')
cls.method_z = inner_2
print('<[700]> evalsupport module end')

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24-class-metaprog/evaltime.py
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from evalsupport import deco_alpha
print('<[1]> evaltime module start')
class ClassOne():
print('<[2]> ClassOne body')
def __init__(self):
print('<[3]> ClassOne.__init__')
def __del__(self):
print('<[4]> ClassOne.__del__')
def method_x(self):
print('<[5]> ClassOne.method_x')
class ClassTwo(object):
print('<[6]> ClassTwo body')
@deco_alpha
class ClassThree():
print('<[7]> ClassThree body')
def method_y(self):
print('<[8]> ClassThree.method_y')
class ClassFour(ClassThree):
print('<[9]> ClassFour body')
def method_y(self):
print('<[10]> ClassFour.method_y')
if __name__ == '__main__':
print('<[11]> ClassOne tests', 30 * '.')
one = ClassOne()
one.method_x()
print('<[12]> ClassThree tests', 30 * '.')
three = ClassThree()
three.method_y()
print('<[13]> ClassFour tests', 30 * '.')
four = ClassFour()
four.method_y()
print('<[14]> evaltime module end')

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24-class-metaprog/evaltime_meta.py
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from evalsupport import deco_alpha
from evalsupport import MetaAleph
print('<[1]> evaltime_meta module start')
@deco_alpha
class ClassThree():
print('<[2]> ClassThree body')
def method_y(self):
print('<[3]> ClassThree.method_y')
class ClassFour(ClassThree):
print('<[4]> ClassFour body')
def method_y(self):
print('<[5]> ClassFour.method_y')
class ClassFive(metaclass=MetaAleph):
print('<[6]> ClassFive body')
def __init__(self):
print('<[7]> ClassFive.__init__')
def method_z(self):
print('<[8]> ClassFive.method_y')
class ClassSix(ClassFive):
print('<[9]> ClassSix body')
def method_z(self):
print('<[10]> ClassSix.method_y')
if __name__ == '__main__':
print('<[11]> ClassThree tests', 30 * '.')
three = ClassThree()
three.method_y()
print('<[12]> ClassFour tests', 30 * '.')
four = ClassFour()
four.method_y()
print('<[13]> ClassFive tests', 30 * '.')
five = ClassFive()
five.method_z()
print('<[14]> ClassSix tests', 30 * '.')
six = ClassSix()
six.method_z()
print('<[15]> evaltime_meta module end')

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24-class-metaprog/factories.py
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"""
record_factory: create simple classes just for holding data fields
# BEGIN RECORD_FACTORY_DEMO
>>> Dog = record_factory('Dog', 'name weight owner') # <1>
>>> rex = Dog('Rex', 30, 'Bob')
>>> rex # <2>
Dog(name='Rex', weight=30, owner='Bob')
>>> name, weight, _ = rex # <3>
>>> name, weight
('Rex', 30)
>>> "{2}'s dog weighs {1}kg".format(*rex) # <4>
"Bob's dog weighs 30kg"
>>> rex.weight = 32 # <5>
>>> rex
Dog(name='Rex', weight=32, owner='Bob')
>>> Dog.__mro__ # <6>
(<class 'factories.Dog'>, <class 'object'>)
# END RECORD_FACTORY_DEMO
The factory also accepts a list or tuple of identifiers:
>>> Dog = record_factory('Dog', ['name', 'weight', 'owner'])
>>> Dog.__slots__
('name', 'weight', 'owner')
"""
# BEGIN RECORD_FACTORY
def record_factory(cls_name, field_names):
try:
field_names = field_names.replace(',', ' ').split() # <1>
except AttributeError: # no .replace or .split
pass # assume it's already a sequence of identifiers
field_names = tuple(field_names) # <2>
def __init__(self, *args, **kwargs): # <3>
attrs = dict(zip(self.__slots__, args))
attrs.update(kwargs)
for name, value in attrs.items():
setattr(self, name, value)
def __iter__(self): # <4>
for name in self.__slots__:
yield getattr(self, name)
def __repr__(self): # <5>
values = ', '.join('{}={!r}'.format(*i) for i
in zip(self.__slots__, self))
return '{}({})'.format(self.__class__.__name__, values)
cls_attrs = dict(__slots__ = field_names, # <6>
__init__ = __init__,
__iter__ = __iter__,
__repr__ = __repr__)
return type(cls_name, (object,), cls_attrs) # <7>
# END RECORD_FACTORY