How can I use __setitem__ method of dict object?

[Steve Holden]

jeremito wrote:
> Please excuse me if this is obvious to others, but I can't figure it
> out.  I am subclassing dict, but want to prevent direct changing of
> some key/value pairs.  For this I thought I should override the
> __setitem__ method as such:
> 
> 
> class xs(dict):
>     """
>     XS is a container object to hold information about cross sections.
>     """
> 
>     def __new__(cls, xS=1.0, xF=1.0, xG=1.0, nu=1.0, debug=0):
>         """
>         """
>         x = {}
>         x['xS'] = xS
>         x['xF'] = xF
>         x['nu'] = nu
>         x['xG'] = xG
>         x['xA'] = x['xG'] + x['xF']
>         x['xT'] = x['xA'] + x['xS']
> 
>         return x
> 
>     def __setitem__(self, key, value):
>         """
>         I have overridden this method to prevent setting xT or xA
> outside the
>         class.
>         """
>         print "I am in __setitem__"
>         if key == 'xT':
>             raise AttributeError("""Can't change xT.  Please change,
> xF, xS, or xG""")
> 
> 
> But I can't even get __setitem__ to run.  Example:
> Python 2.5 (r25:51918, Sep 19 2006, 08:49:13)
> [GCC 4.0.1 (Apple Computer, Inc. build 5341)] on darwin
> Type "help", "copyright", "credits" or "license" for more information.
>>>> import xs
>>>> cs = xs.xs()
>>>> cs
> {'xA': 2.0, 'xF': 1.0, 'xG': 1.0, 'xS': 1.0, 'nu': 1.0, 'xT': 3.0}
>>>> cs['xT'] = 3.1415
>>>> cs
> {'xA': 2.0, 'xF': 1.0, 'xG': 1.0, 'xS': 1.0, 'nu': 1.0, 'xT':
> 3.1415000000000002}
> 
> 
> Is this what the __setitem__ method is for?  If not, how can I do what
> I want to do?

  >>> class d(dict):
  ...   def __setitem__(self, k, v):
  ...     print "Setting", k
  ...     dict.__setitem__(self, k, v)
  ...
  >>> dd = d()
  >>> dd['steve'] = 'holden'
Setting steve
  >>> dd['steve']
'holden'
  >>>

I believe the problem is that your __new__ method does not return an 
object of type xs but a dict, so it does not inherit the __getitem__ 
method from xs but instead from dict.

regards
  Steve
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