The translation of the article was prepared in anticipation of the start of the " Python Developer. Basic" course .
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- . , , . Python .
Python «, ».
. :
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;
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, , Python, , «». , , , , . , metaclass ( ), type() (.. _call_ ). , metaclass , type.
. , _new_ type._new_:
class LittleMeta(type):
def __new__(cls, clsname, superclasses, attributedict):
print("clsname: ", clsname)
print("superclasses: ", superclasses)
print("attributedict: ", attributedict)
return type.__new__(cls, clsname, superclasses, attributedict)
LittleMeta :
class S:
pass
class A(S, metaclass=LittleMeta):
pass
a = A()clsname: A
superclasses: (<class '__main__.S'>,)
attributedict: {'__module__': '__main__', '__qualname__': 'A'} , LittleMeta._new_, type._new_.
EssentialAnswers, augment_answer:
x = input("Do you need the answer? (y/n): ")
if x.lower() == "y":
required = True
else:
required = False
def the_answer(self, *args):
return 42
class EssentialAnswers(type):
def __init__(cls, clsname, superclasses, attributedict):
if required:
cls.the_answer = the_answer
class Philosopher1(metaclass=EssentialAnswers):
pass
class Philosopher2(metaclass=EssentialAnswers):
pass
class Philosopher3(metaclass=EssentialAnswers):
pass
plato = Philosopher1()
print(plato.the_answer())
kant = Philosopher2()
# let's see what Kant has to say :-)
print(kant.the_answer())Do you need the answer? (y/n): y
42
42«Type and Class Relationship» , Python :
type(classname, superclasses, attributes_dict)
, . . Philosopher1, Philosopher2 Philosopher3 «» EssentialAnswers. EssentialAnswers type:
EssentialAnswer(classname, superclasses, attributes_dict)
, :
EssentialAnswer('Philopsopher1',
(),
{'__module__': '__main__', '__qualname__': 'Philosopher1'})Philosopher .
- , . , . , . , , , .
class Singleton(type):
_instances = {}
def __call__(cls, *args, **kwargs):
if cls not in cls._instances:
cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
return cls._instances[cls]
class SingletonClass(metaclass=Singleton):
pass
class RegularClass():
pass
x = SingletonClass()
y = SingletonClass()
print(x == y)
x = RegularClass()
y = RegularClass()
print(x == y)True
False
Singleton-, Singleton, :
class Singleton(object):
_instance = None
def __new__(cls, *args, **kwargs):
if not cls._instance:
cls._instance = object.__new__(cls, *args, **kwargs)
return cls._instance
class SingletonClass(Singleton):
pass
class RegularClass():
pass
x = SingletonClass()
y = SingletonClass()
print(x == y)
x = RegularClass()
y = RegularClass()
print(x == y)True
False