__slots__ and class attributes

[Ewald R. de Wit]

I'm running into a something unexpected  for a new-style class
that has both a class attribute and __slots__ defined. If the
name of the class attribute also exists in __slots__, Python
throws an AttributeError. Is this by design (if so, why)?

class A( object ):
__slots__ = ( 'value', )
value = 1

def __init__( self, value = None ):
self.value = value or A.value

a = A()
print a.value


Traceback (most recent call last):
  File "t1.py", line 8, in ?
a = A()
  File "t1.py", line 6, in __init__
self.value = value or A.value
AttributeError: 'A' object attribute 'value' is read-only


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Re: __slots__ and class attributes

[Ewald R. de Wit]

Steven Bethard wrote:
> But why do you want a class level attribute with the same name as an 
> instance level attribute?  I would have written your class as:
>
> class A(object):
>  __slots__ = ['value']
>  def __init__(self, value=1):
>  self.value = value
>
> where the default value you put in the class is simply expressed as a 
> default value to the __init__ parameter.

Thanks for your explanation. The reason why I was doing it was
to have class-level defaults, so that one can easily adjust how
new instances will be made. I'm doing it now with capitilized
class attribute names to avoid the name clash.

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  --  Ewald
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Re: directory bug on linux; workaround?

[Ewald R. de Wit]

Russell E. Owen wrote:
> It seems that the path was to a "fat" file partition and included a 
> directory name that was all uppercase. The directory was created, but 
> using lowercase. I'm not yet sure the version of python.
>
> The workaround for now is to not use fat file partitions. But I was 
> wondering if anyone had a better option?

You can mount the FAT partition with the 'shortname' argument, i.e.
put something like 

/dev/sda1 /usbkey auto 
umask=0,user,iocharset=iso8859-1,sync,kudzu,codepage=850,noauto,exec,users,shortname=winnt,check=s
 0 0

in your /etc/fstab

Btw, this is a general Linux problem and is not really related to
Python (it also happens when working with shells or filemanagers).

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  --  Ewald

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Re: cyclic data structures

[Ewald R. de Wit]

John Salerno wrote:
> I'm having some slight trouble understanding exactly why this creates an 
> infinite loop:
>
> L = [1, 2]
> L.append(L)

I tried this with Python 2.3.5 and it handles this tailbiter in a
very pleasantly surprising way:

>>> l = [ 0, 1 ]
>>> l.append( l )
>>> l
[0, 1, [...]]
>>> l[2]
[0, 1, [...]]
>>> l[2][2][2][2][2][2][2][0]
1

The [...] I have never seen before anywhere, is it documented?

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  --  Ewald

 
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