[Python-ideas] func attribute & default arg

[Nick Coghlan]

spir wrote:
> Le Sat, 16 May 2009 17:53:09 +1000, Nick Coghlan <ncoghlan at gmail.com>
> s'exprima ainsi:
> 
>> Since option 1 will work for any arbitrary object, while option 2
>> is limited to objects which support copy.copy (or copy.deepcopy,
>> depending on the copying semantics chosen), the "late evaluation of
>> default arguments" (option 1) approach is the more promising angle
>> of attack by far.
> 
> Which objects do not support copy.deepcopy? Really surprised. I used
> and still use it in personal projects for rather complicated custom
> objects* and it works as expected.

Bound methods for one:

>>> class C(object):
...   def m(): pass
...
>>> copy.copy(C().m)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/copy.py", line 95, in copy
    return _reconstruct(x, rv, 0)
  File "/usr/lib/python2.5/copy.py", line 322, in _reconstruct
    y = callable(*args)
  File "/usr/lib/python2.5/copy_reg.py", line 92, in __newobj__
    return cls.__new__(cls, *args)
TypeError: instancemethod expected at least 2 arguments, got 0
>>> copy.deepcopy(C().m)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/copy.py", line 189, in deepcopy
    y = _reconstruct(x, rv, 1, memo)
  File "/usr/lib/python2.5/copy.py", line 322, in _reconstruct
    y = callable(*args)
  File "/usr/lib/python2.5/copy_reg.py", line 92, in __newobj__
    return cls.__new__(cls, *args)
TypeError: instancemethod expected at least 2 arguments, got 0

Copyability really can't be assumed for Python objects - the copy module
does an excellent job of making it "just work" for most objects, but it
really isn't any more fundamental a property than being hashable.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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