"dunders" (i.e. names with two leading and two trailing underscores) https://www.python.org/dev/peps/pep-0008/#module-level-dunder-names __init__ __dict__
class ModelBase(type): "Metaclass for all models" def __new__(cls, name, bases, attrs): # If this isn't a subclass of Model, don't do anything special. … https://github.com/django/django/blob/e27211a0deae2f1d402537f0ebb64ad4ccf6a4da/django/core/meta/__init__.py#L394 githubܦ༝Ͱ༰қʹͨͲΕͨi18nϒϥϯνͬͯݺΕͯͨόʔδϣϯʢ2005/12ʣͷͷ ←typeΛܧঝ͍ͯͯ͠ɺ __new__ ϝιου͕ఆٛ͞Ε͍ͯΔ ϝλΫϥε
class Spam: def __init__(self): self.value = 0 @property def value(self): return self._value @value.setter def value(self, value): if type(value) is not int: raise ValueError(f"value should be int, but {value}") if value < 0: raise ValueError(f"value souldn't be negative") self._value = value >>> spam = Spam() >>> spam.value = 1 >>> spam.__dict__ {'_value': 1} https://github.com/tsuyukimakoto/pyconjp2020/blob/master/notebook/03.property_01.ipynb >>> spam.value = "10" --------------------------------------------------------------------------- ValueError Traceback (most recent call last) in ----> 1 spam.value = "10" in value(self, value) 10 def value(self, value): 11 if type(value) is not int: ---> 12 raise ValueError(f"value should be int, but {value}") 13 if value < 0: 14 raise ValueError(f"value souldn't be negative") ValueError: value should be int, but 10
class Spam: def __init__(self): self.value = 0 @property def value(self): return self._value @value.setter def value(self, value): if type(value) is not int: raise ValueError(f"value should be int, but {value}") if value < 0: raise ValueError(f"value souldn't be negative") self._value = value >>> spam = Spam() >>> spam.value = 1 >>> spam.__dict__ {'_value': 1} https://github.com/tsuyukimakoto/pyconjp2020/blob/master/notebook/03.property_01.ipynb >>> spam.value = -1 --------------------------------------------------------------------------- ValueError Traceback (most recent call last) in ----> 1 spam.value = -1 in value(self, value) 12 raise ValueError(f"value should be int, but {value}") 13 if value < 0: ---> 14 raise ValueError(f"value souldn't be negative") 15 self._value = value ValueError: value souldn't be negative >>> spam.value = 1 >>> spam.__dict__ {'_value': 1}
https://github.com/tsuyukimakoto/pyconjp2020/blob/master/notebook/03.property_02.ipynb def check_positive_integer(value): if type(value) is not int: raise ValueError(f"value should be int, but {value}") if value < 0: raise ValueError(f"value souldn't be negative") class Spam: def __init__(self): self.value1 = 0 self.value2 = 0 @property def value1(self): return self._value1 @value1.setter def value1(self, value): check_positive_integer(value) self._value1 = value @property def value2(self): return self._value2 @value2.setter def value2(self, value): check_positive_integer(value) self._value2 = value
https://github.com/tsuyukimakoto/pyconjp2020/blob/master/notebook/03.property_03.ipynb class PositiveInteger: def __init__(self, value): if type(value) is not int: raise ValueError(f"value should be int, but {value}") if value < 0: raise ValueError(f"value souldn't be negative") self.value = value class Spam: def __init__(self): self.value1 = None self.value2 = None >>> spam = Spam() >>> spam.value1 = PositiveInteger(10) >>> spam.value2 = PositiveInteger(20) >>> print(spam.value1.value, spam.value2.value) 10 20
from abc import ABC, abstractmethod registry = {} def register(cls): registry[cls.__name__] = cls class AutoRegisterMeta(type): def __new__(cls, name, bases, class_dict): newclass = type.__new__(cls, name, bases, class_dict) register(newclass) return newclass class AbstractPlugin(ABC): @abstractmethod def run(self): pass class MyClassA(AbstractPlugin, metaclass=AutoRegisterMeta): def run(self): return f"return from {self.__class__.__name__}.run" --------------------------------------------------------------------------- TypeError Traceback (most recent call last) in 18 19 ---> 20 class MyClassA(AbstractPlugin, metaclass=AutoRegisterMeta): 21 def run(self): 22 return f"return from {self.__class__.__name__}.run" TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases https://github.com/tsuyukimakoto/pyconjp2020/blob/master/notebook/06_metaclass_01.ipynb
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