Re: [Tutor] when I create an instance of a class that inherits from Python dictionary, my data disappears

[Kent Johnson]

Bob Gailer wrote:
> [EMAIL PROTECTED] wrote:
>> hi all, I would like to create a class that specializes Python 
>> dictionary.  I would like an instance of this class to store objects 
>> representing html form data, and I would like to have an instance of 
>> this Data_Set class be able to use the Python dictionary method pop to 
>> remove objects as I see fit.  I defined the following:
>>
>> class Data_Set(dict):
>> def __init__(self, dict_of_datum_objects, required=None):
>> self.keys = dict_of_datum_objects.keys
>> self.values = dict_of_datum_objects.values
>> self.required = required
>>
>> For some reason, whenever I create an instance of this class with 
>> data, all I get is an empty dictionary. 

>> Am I doing something wrong ?
> Err, yes. You are assuming that assigning to self.keys and self.values 
> creates dictionary entries. All that actually does is assign the 2 lists 
> to 2 attributes.
> 
> Try instead self.update(dict_of_datum_objects).

In general, when you create a subclass, the subclass __init__() method 
should call the base class __init__(). Otherwise the base class is not 
initialized. That is what you see - you get an empty dict.

The syntax for the call to the base class __init__() is a little 
strange, it looks like this:
   base.__init__(self, args)
where base is the name of the base class and args are the arguments for 
the base class __init__() method.

As Bob points out, you are also making unwarranted assumptions about the 
way dict stores its data.

Try this:
 def __init__(self, dict_of_datum_objects, required=None):
 dict.__init__(self, dict_of_datum_objects)
 self.required = required

Kent

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Re: [Tutor] when I create an instance of a class that inherits from Python dictionary, my data disappears

[Bob Gailer]

[EMAIL PROTECTED] wrote:
> hi all, I would like to create a class that specializes Python 
> dictionary.  I would like an instance of this class to store objects 
> representing html form data, and I would like to have an instance of 
> this Data_Set class be able to use the Python dictionary method pop to 
> remove objects as I see fit.  I defined the following:
>
> class Data_Set(dict):
> def __init__(self, dict_of_datum_objects, required=None):
> self.keys = dict_of_datum_objects.keys
> self.values = dict_of_datum_objects.values
> self.required = required
>
> For some reason, whenever I create an instance of this class with 
> data, all I get is an empty dictionary.  As soon as I redefine my 
> class to no longer inherit from Python dictionary, then I get expected 
> behavior, but now I don't have benefit of Python dictionary methods 
> such as pop and has_key:
>
> sdso = Data_Set({'full_name':'full_name_obj', 
> 'address_line_1':'address_line_1_obj'})
> >>> sdso
> {}
> >>> class Data_Set:
> def __init__(self, dict_of_datum, required=None):
> self.keys = dict_of_datum.keys
> self.values = dict_of_datum.values
> self.items = dict_of_datum.items
> self.required = required
>
> 
> >>> sdso = Data_Set({'full_name':'full_name_obj', 
> 'address_line_1':'address_line_1_obj'})
> >>> sdso
> <__main__.Data_Set instance at 0x419690ac>
> >>> sdso.keys()
> ['full_name', 'address_line_1']
> >>> sdso.pop('full_name')
>
> Traceback (most recent call last):
>   File "", line 1, in -toplevel-
> sdso.pop('full_name')
> AttributeError: Data_Set instance has no attribute 'pop'
>
> Am I doing something wrong ?
Err, yes. You are assuming that assigning to self.keys and self.values 
creates dictionary entries. All that actually does is assign the 2 lists 
to 2 attributes.

Try instead self.update(dict_of_datum_objects).

-- 
Bob Gailer
just up the road in El Cerrito
510-978-4454

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