I've found some differences between decimal and cdecimal. 

cdecimal 2.3 does not support the __ceil__ and __floor__ methods that exist in decimal. math.ceil converts a cdecimal.Decimal instance into a float before finding the ceiling. This can generate incorrect results.

>>> import decimal
>>> import math
>>> math.ceil(decimal.Decimal("12345678901234567890.1"))
12345678901234567168

The decimal module in previous versions returns the correct answer 12345678901234567891

cdecimal.Decimal instances do not emulate the various single-underscore methods of a decimal.Decimal instance. In gmpy2, I use _int, _exp, _sign, and _is_special to convert a decimal.Decimal into an exact fraction. I realize the issue is with gmpy2 and I will fix gmpy2, but there may be other code that uses those methods.