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Cornélio Sousa's questions

Martin Hope
Cornélio Sousa
Asked: 2024-06-28 06:26:42 +0800 CST
  • 9

我正在尝试使用 Python (CPython 3.10.13) 进行元编程，并注意到一些奇怪的行为object.__new__（至少对我来说很奇怪）。看看以下实验（不是实际代码，只是一个实验）和评论。请注意，object.__new__似乎根据第一个参数改变了它的行为：

# Empty class inherit __new__ and __init__ from object
class Empty:
    pass

# Confirmation of inheritance
assert Empty.__new__ is object.__new__, "Different __new__"
assert Empty.__init__ is object.__init__, "Different __init__"

empty_obj = Empty()
uinit_empty_obj = object.__new__(Empty)

assert type(empty_obj) is type(uinit_empty_obj), "Different types"

try:
    object.__new__(Empty, 10, 'hi', hello='bye')
except TypeError as e:
    # repr(e) mentioned the Empty class
    print(repr(e))

# Overwrite the object __new__ and __init__ methods
# __new__ and __init__ with the same signature
class Person:
    def __new__(cls, name, age):
        """Does nothing bassicaly. Just overwrite `object.__new__`."""
        print(f'Inside {cls.__name__}.__new__')
        return super().__new__(cls)
    
    def __init__(self, name, age):
        print(f'Inside {type(self).__name__}.__init__')
        self.name = name
        self.age = age

a_person = Person('John Doe', 25)
uinit_person = Person.__new__(Person, 'Michael', 40)

try:
    # Seems an obvious error since object() doesn't take any arguments
    another_uinit_person = object.__new__(Person, 'Ryan', 25)
except TypeError as e:
    # Indeed raises TypeError, but now there isn't a mention of the Person class in repr(e)
    print('`another_uinit_person` :', repr(e))

# Now, some weird things happen (well, weird for me).
# Inherit __new__ from object and overwrite __init__.
# __new__ and __init__ with unmatching signatures.
# A basic Python class. Works just fine like suppose to.
class Vehicle:
    def __init__(self, model):
        self.model = model

# Confirmation of __new__ inheritance.
assert Vehicle.__new__ is object.__new__, "Nop, it isn't"

a_vehicle = Vehicle('Honda')

# I would understand if CPython autogenerated a __new__ method matching __init__
# or a __new__ method that accepts all arguments.
# The following try-except-else suggests the last, but the assert statement above 
# indicates that Vehicle.__new__ is actually object.__new__.
try:
    # Doesn't raise any exceptions
    uinit_vehicle = Vehicle.__new__(Vehicle, 'Honda', 10, ('four-wheels',), hello='bye')
except Exception as e:
    print(repr(e))
else:
    print('`uinit_vehicle` : constructed just fine', uinit_vehicle)

# Now the following runs just fine
try:
    # Doesn't raise any exceptions
    another_unit_vehicle = object.__new__(Vehicle, 'Toyota')
    another_unit_vehicle = object.__new__(Vehicle, 'Toyota', 100, four_wheels=True)
except Exception as e:
    print(repr(e))
else:
    print('`another_unit_vehicle` : constructed just fine:', another_unit_vehicle)

我得到了以下输出：

TypeError('Empty() takes no arguments')
Inside Person.__new__
Inside Person.__init__
Inside Person.__new__
`another_uinit_person` : TypeError('object.__new__() takes exactly one argument (the type to instantiate)')
`uinit_vehicle` : constructed just fine <__main__.Vehicle object at 0x00000244D15A7A90>
`another_unit_vehicle` : constructed just fine: <__main__.Vehicle object at 0x00000244D15A7A30>

我的问题：

  1. 为什么第一个TypeError提到了Empty课程而第二个只是提到了object.__new__？
  2. 为什么object.__new__(Person, 'Ryan', 25)提高了TypeError，object.__new__(Vehicle, 'Toyota')而object.__new__(Vehicle, 'Toyota', 100, four_wheels=True)没提高呢？

基本上：object.__new__引擎盖下面有什么？

在我看来，它正在对第一个参数__new__和/或__init__覆盖方法（如果有）执行有点奇怪的检查。

python