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Effective Python in Python 3.6
Hayao Suzuki
Python 3.6 Release Party at Yahoo Japan
January 31, 2017
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Who Are You?
About Me
Name Hayao Suzuki (@CardinalXaro)
Blog http://xaro.hatenablog.jp/
Major Mathematics (Combinatorics, Number Theory)
Work Python Programmer (Not Mathematics!)
Reviewed Books
Effective Python (O’Reilly Japan)
Algorithms in A Nutshell 2nd ed. (O’Reilly Japan)
And more...?
2 / 9
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Today’s Theme
Metaprogramming without Metaclasses
Metaclass can create extremely bizarre behaviors that are
unapproachable to newcomers. (Effective Python, P.87)
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Metaprogramming without Metaclasses
Items about Metaclass in Effective Python
Item 33 Validate Subclasses with Metaclasses
Item 34 Register Class Existence with Metaclasses
Item 35 Annotate Class Attributes with Metaclasses
Using Python 3.6, we can write metaprogramming without
using metaclass.
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Item 33 With Metaclasses
class ValidatePolygon(type):
def __new__(meta, name, bases, class_dict):
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
@classmethod
def interior_angles(cls):
return (cls.sides - 2) * 180
class Triangle(Polygon):
sides = 3 5 / 9
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Item 33 Without Metaclasses
class Polygon(object):
def __init_subclass__(cls, sides, **kwargs):
cls.sides = sides
if cls.sides < 3:
raise ValueError("polygons need 3+ sides.")
@classmethod
def interior_angles(cls):
return (cls.sides - 2) * 180
class Triangle(Polygon, sides=3):
pass
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Item 35 with Metaclasses
class Meta(type):
def __new__(meta, name, bases, class_dict):
for key, value in class_dict.items():
if isinstance(value, Field):
value.name = key
value.internal_name = '_' + key
return type.__new__(meta, name, bases, class_dict)
class Field(object):
def __init__(self):
self.name = None
self.internal_name = None
def __get__(self, instance, instance_type):
if instance is None: return self
return getattr(instance, self.internal_name, '')
def __set__(self, instance, value):
setattr(instance, self.internal_name, value)
class DatabaseRow(object, metaclass=Meta):
pass
class Customer(DatabaseRow):
first_name = Field()
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Item 35 Without Metaclasses
class Field(object):
def __init__(self):
self.name = None
self.internal_name = None
def __get__(self, instance, instance_type):
if instance is None: return self
return getattr(instance, self.internal_name, '')
def __set__(self, instance, value):
setattr(instance, self.internal_name, value)
def __set_name__(self, owner, name):
self.name = name
self.internal_name = '_' + name
class Customer(object):
first_name = Field()
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Summary
Summary
Using Python 3.6, we can write metaprogramming
without using metaclass.
Item 33 to Item 35 in Effective Python no longer have
to use metaclass.
Python 3.6 is good I think.
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