Can't deepcopy bytes-derived class

[Terry Reedy]

On 12/8/2010 7:11 PM, Ned Deily wrote:
> In article<idosir$45i$1 at dough.gmane.org>, Terry
> Reedy<tjreedy at udel.edu>  wrote:
>> On 12/8/2010 2:42 PM, Dan wrote:
>>> I have a simple type derived from bytes...
>>>
>>> class atom(bytes): pass
>>>
>>> ... that I cannot deepcopy().  The session below demonstrates
>>> how deepcopy() of "bytes" works fine, but deepcopy() of "atom"
>>> does not.
> [...]
>>> Python 3.1.2 (r312:79149, Mar 21 2010, 00:41:52) [MSC v.1500 32
>>> bit (Intel)] Type "help", "copyright", "credits" or "license" for
>>> more information.
>>>>>> import copy class atom(bytes):
>>> ... 	pass ...
>>>>>> copy.deepcopy(b'abc')
>>> b'abc'
>>>>>> copy.deepcopy(atom(b'abc'))
>>> Traceback (most recent call last): File "C:\Program Files
>>> (x86)\Wing IDE 3.2\src\debug\tserver \_sandbox.py", line 1,
>>> in<module> # Used internally for debug sandbox under external
>>> interpreter File "C:\Python31\Lib\copy.py", line 173, in
>>> deepcopy y = _reconstruct(x, rv, 1, memo) File
>>> "C:\Python31\Lib\copy.py", line 280, in _reconstruct y =
>>> callable(*args) File "C:\Python31\Lib\copyreg.py", line 88, in
>>> __newobj__ return cls.__new__(cls, *args) builtins.TypeError:
>>> string argument without an encoding
>>
>> You could trace through copy.py and copyreg.py to see where bytes
>> and atom get treated differently.
>>
>> This might be a bug that should be reported on the tracker, but I
>> do not know. Let see what anyone else says.
>
> FWIW, the example still fails with Python 3.1.3 but seems to work OK
> with a recent alpha build of Python 3.2.  What specific change might
> have fixed it doesn't come immediately to mind.

According to difflib.unified_diff, the only difference between
Lib/copy.py from 3.1 to 3.2 is 4 lines:
+
+def _deepcopy_method(x, memo): # Copy instance methods
+    return type(x)(x.__func__, deepcopy(x.__self__, memo))
+_deepcopy_dispatch[types.MethodType] = _deepcopy_method

(and none in copyreg.py)

These were added in rev76572 as part of
http://bugs.python.org/issue1515

Guido decreed this to be a new feature rather than bugfix, so the change 
went into future 2.7 and 3.2 and not existing 2.6 and 3.1.

Tto verify that these cause the change, comment out in 3.2 or add to 
3.1. I have not read through the issue to understand it or why it would 
affect the OP's code case.

-- 
Terry Jan Reedy