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Python implementation of Decimal.from_float() calls __new__ and __init__ methods of subclass.
>>> from _pydecimal import Decimal
>>> class D(Decimal):
... def __new__(cls, *args, **kwargs):
... print('__new__')
... return Decimal.__new__(cls, *args, **kwargs)
... def __init__(self, *args, **kwargs):
... print('__init__')
... 
>>> print(type(D.from_float(42)))
__new__
__init__
<class '__main__.D'>
>>> print(type(D.from_float(42.0)))
__new__
__init__
<class '__main__.D'>
But C implementation doesn't.
>>> from decimal import Decimal
>>> class D(Decimal):
... def __new__(cls, *args, **kwargs):
... print('__new__')
... return Decimal.__new__(cls, *args, **kwargs)
... def __init__(self, *args, **kwargs):
... print('__init__')
... 
>>> print(type(D.from_float(42)))
<class '__main__.D'>
>>> print(type(D.from_float(42.0)))
<class '__main__.D'>
This means that resulting instance of Decimal subclass can be in not valid state.
Example is Decimal enums (see also issue23640).
>>> from decimal import Decimal
>>> from enum import Enum
>>> class D(Decimal, Enum):
... A = Decimal('3.25')
... 
>>> D(Decimal(3.25))
<D.A: Decimal('3.25')>
>>> D(3.25)
<D.A: Decimal('3.25')>
>>> D.from_float(3.25)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/serhiy/py/cpython/Lib/enum.py", line 486, in __repr__
 self.__class__.__name__, self._name_, self._value_)
AttributeError: 'D' object has no attribute '_name_'
A solution is to reproduce Python implementation in C code:
 result = ... # create exact Decimal
 if cls is not Decimal:
 result = cls(result)
 return result

New changeset dd3f48f1df86 by Stefan Krah in branch '3.5':
Issue #27006: from_float(): call the subclass' __new__() and __init__().
https://hg.python.org/cpython/rev/dd3f48f1df86 

Thank you for the detailed summary!

PyDec_CheckExact(type) always return 0. Should be PyDec_CheckExact(result).
And what about other calls of PyDecType_FromFloatExact()? Can they produce broken instance of Decimal subtype?

New changeset 51a7a97c3ed4 by Stefan Krah in branch '3.5':
Issue #27006: Do not use PyDec_CheckExact() on a type.
https://hg.python.org/cpython/rev/51a7a97c3ed4 

> PyDec_CheckExact(type) always return 0. Should be PyDec_CheckExact(result).
'result' is always an exact decimal, because your enum example won't work otherwise.
> And what about other calls of PyDecType_FromFloatExact()? Can they produce broken instance of Decimal subtype?
The PyDecType* functions handle calling the correct allocator, they don't handle calling __new__(). Why should they? The regular non-class-method enum examples worked.