a problem with subclassing a dict

[Fernando Pérez]

Rajarshi Guha wrote:

> Hi,
>   I was playing with some code from a tutorial by GvR
> (http://www.python.org/2.2/descrintro.html) - the relevant code is:
> 
> class mydict(dict):
> 
>     def __init__(self, default=None):
>         dict.__init__(self)
>         self.default = default
> 
>     def __getitem__(self, key):
>         try:
>             return dict.__getitem__(self, key)
>         except KeyError:
>             return self.default
> 
> md = mydict("Sorry  - no such key!!")
> md = {1:2}
> 
> print md[1]
> print md[2]
> ---------------------------------
> 
> I get:
> 
> 2
> Traceback (most recent call last):
>   File "p2.py", line 22, in ?
>     print md[2]
> KeyError: 2
> 
> 
> Should'nt I be seeing a message instead of the Traceback?

The problem is simply that your second statement (md={1:2}) wiped out the 
assignment of md and now md is a regular dict:

In [17]: md = mydict("Sorry  - no such key!!")

In [18]: md[1]
Out[18]: 'Sorry  - no such key!!'

In [19]: type md
-------> type (md)
Out[19]: <class '__main__.mydict'>

In [20]: md = {1:2}

In [21]: type md
-------> type (md)
Out[21]: <type 'dict'>

In [22]: md[1]
Out[22]: 2

In [23]: md[2]
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)

?

KeyError: 2

So, it's not a problem with the new classes. You simply assigned md to a 
normal dict, you can't expect it then to behave like a mydict, can you?

Cheers,

f.