I guess you meant for me to check whether the following has any problem:
import decimal

class BadFloat(float):
    def as_integer_ratio(self):
        return 1 << 1000

decimal.Decimal.from_float(BadFloat())

so it doesn't crash.
if IIUC, this is because the C implementation of Decimal.from_float()
is PyDecType_FromFloat(), while the relevant part of it is a call to
PyDecType_FromFloatExact().
But PyDecType_FromFloatExact() uses float.as_integer_ratio(), and thus
ISTM that the issue doesn't exist there.

(also, the following doesn't raise an exception, as expected:
import decimal

class BadFloat(float):
    def as_integer_ratio(self):
        raise RuntimeError

decimal.Decimal.from_float(BadFloat())
)