Re: deepcopy chokes with TypeError on dynamically assigned instance method

[Nick Coghlan]

Kanenas wrote:
On Thu, 10 Feb 2005 00:54:04 -0800, Kanenas <kanenas @t comcast d.t
net> wrote:

When an instance has a dynamically assigned instance method, deepcopy
throws a TypeError with the message "TypeError: instancemethod
expected at least 2 arguments, got 0".  

I forgot to mention that the TypeError is thrown only when
constructing an instance from another instance in the same class, e.g.
Foo('bar') is fine but Foo(Foo('bar')) will fail.
Interesting. The problem appears to be that bound methods are not copyable:
>>> x = Foo('bar')
>>> x
Foo([b,a,r])
>>> x.bar
<bound method Foo.bar of Foo([b,a,r])>
>>> copy(x.bar)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "C:\Python24\lib\copy.py", line 95, in copy
    return _reconstruct(x, rv, 0)
  File "C:\Python24\lib\copy.py", line 320, in _reconstruct
    y = callable(*args)
  File "C:\Python24\lib\copy_reg.py", line 92, in __newobj__
    return cls.__new__(cls, *args)
TypeError: instancemethod expected at least 2 arguments, got 0
>>> x.__repr__
<bound method Foo.__repr__ of Foo([b,a,r])>
>>> copy(x.__repr__)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "C:\Python24\lib\copy.py", line 95, in copy
    return _reconstruct(x, rv, 0)
  File "C:\Python24\lib\copy.py", line 320, in _reconstruct
    y = callable(*args)
  File "C:\Python24\lib\copy_reg.py", line 92, in __newobj__
    return cls.__new__(cls, *args)
TypeError: instancemethod expected at least 2 arguments, got 0
Normally, the methods live in the class dictionary, so they don't cause a 
problem with copying the instance.

It turns out this exception actually makes sense, since you *don't* want to copy 
these atributes to the new instance. If you actually copied them, they'd be 
bound to the *old* instance, rather than the new one.

So I expect you'll need to provide a __deepcopy__ in order to correctly generate 
the instancemethods bound to the new instance.

I also realised that the reason the use of a mutable default is OK here is 
because you're deepcopying it.

Cheers,
Nick.
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Nick Coghlan   |   [EMAIL PROTECTED]   |   Brisbane, Australia
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