TransformData in construct/construct

What the code was meant to do, written into the code itself as a docstring

Parses a field on transfored data (or transforms data after building). Parsing reads a specified amount, processes data using a bytes-to-bytes decoding function, then parses subcon using those data. Building does build subcon into separate bytes, then processes using encoding encoding, then writes those data into main stream. Size is reported as `encodeamount`. Used internally to implement :class:`~construct.core.Bitwise` :class:`~construct.core.Bytewise` :class:`~construct.core.ByteSwapped` :class:`~construct.core.BitsSwapped` . Possible use-cases include encryption, obfuscation, byte-level encoding. .. warning:: Remember that subcon must consume (or produce) an amount of bytes that is same as `decodeamount` (or `encodeamount`). .. warning:: Do NOT use seeking/telling classes inside TransformData context. :param subcon: Construct instance :param decodefunc: bytes-to-bytes function, applied before parsing subcon :param decodeamount: integer, amount of bytes to read :param encodefunc: bytes-to-bytes function, applied after building subcon :param encodeamount: integer, amount of bytes to write :raises StreamError: requested reading negative amount, could not read enough bytes, requested writing different amount than actual data, or could not write all bytes :raises StreamError: subcon parsed less than `decodeamount` bytes :raises StreamError: subcon build (and encoder transformed) more or less than `encodeamount` bytes :raises StringError: building from non-bytes value, perhaps unicode Can propagate any exception from the lambdas, possibly non-ConstructError. Example::

The change

The check that tells the two apart

Check file tests/test_core.py, taken without changes from the fix and copied onto the older code, so the exact same check runs against both versions.