Python-3.5 typing.Generic子类从不调用`__init__`

from typing import Generic,TypeVar

T = TypeVar('T')
class Foo(Generic[T]):
    def __init__(self,value: T):
        self.value = value

Bar = Foo[str]

foo = Foo('foo')
bar = Bar('bar')

print(type(foo),end=' ')
print(foo.value)

print(type(bar),end=' ')
print(bar.value) # AttributeError

根据文件：

The Generic base class uses a metaclass that defines getitem() so
that LoggedVar[t] is valid as a type:

这是一个错误,还是我不明白的东西？

编辑

在PDB中逐步执行Foo(‘foo’)情况,注意最后调用__init__：

> /home/kjw53/test.py(1)<module>()
-> from typing import Generic,TypeVar
(Pdb) break /usr/lib/python3.5/typing.py:1078
Breakpoint 1 at /usr/lib/python3.5/typing.py:1078
(Pdb) c
> /usr/lib/python3.5/typing.py(1078)__new__()
-> return next_in_mro.__new__(_gorg(cls))
(Pdb) p cls
__main__.Foo[~T]
(Pdb) p next_in_mro
<class 'object'>
(Pdb) p _gorg(cls)
__main__.Foo[~T]
(Pdb) s
--Call--
> /usr/lib/python3.5/typing.py(858)_gorg()
-> def _gorg(a):
(Pdb) r
--Return--
> /usr/lib/python3.5/typing.py(863)_gorg()->__main__.Foo[~T]
-> return a
(Pdb) n
--Return--
> /usr/lib/python3.5/typing.py(1078)__new__()-><__main__.Foo...x7f57094b1a20>
-> return next_in_mro.__new__(_gorg(cls))
(Pdb) s
--Call--
> /home/kjw53/test.py(5)__init__()
-> def __init__(self,value: T):

继续,现在击中Bar(‘bar’)案.注意__init__未被调用,尽管其他所有相关内容都相同.

(Pdb) c
> /usr/lib/python3.5/typing.py(1078)__new__()
-> return next_in_mro.__new__(_gorg(cls))
(Pdb) p cls
__main__.Foo[str]
(Pdb) p next_in_mro
<class 'object'>
(Pdb) p _gorg(cls)
__main__.Foo[~T]
(Pdb) s
--Call--
> /usr/lib/python3.5/typing.py(858)_gorg()
-> def _gorg(a):
(Pdb) r
--Return--
> /usr/lib/python3.5/typing.py(863)_gorg()->__main__.Foo[~T]
-> return a
(Pdb) n
--Return--
> /usr/lib/python3.5/typing.py(1078)__new__()-><__main__.Foo...x7f57094b1a58>
-> return next_in_mro.__new__(_gorg(cls))
(Pdb) s
> /home/kjw53/test.py(13)<module>()
-> print(type(foo),end=' ')