sublcassing dict without losing functionality

[Steven Bethard]

I'd like to subclass dict to disallow overwriting of keys, something like:
[color=blue][color=green][color=darkred]
>>> class SafeDict(dict):[/color][/color][/color]
.... def __setitem__(sel f, key, value):
.... if key in self:
.... raise KeyError('canno t assign value %r to key %r, value %r'
.... 'already exists' % (value, key, self[key]))
.... super(SafeDict, self).__setitem __(key, value)
....

The problem is, dict doesn't appear to call __setitem__ in any of the
__init__ forms, so none of the following raise errors as I'd like them
to:
[color=blue][color=green][color=darkred]
>>> SafeDict({'one' :1}, one=2)[/color][/color][/color]
{'one': 2}[color=blue][color=green][color=darkred]
>>> SafeDict([('one', 1), ('one', 2)])[/color][/color][/color]
{'one': 2}[color=blue][color=green][color=darkred]
>>> SafeDict([('one', 1), ('one', 2)], one=3)[/color][/color][/color]
{'one': 3}[color=blue][color=green][color=darkred]
>>> SafeDict(('one' , x) for x in (1, 2))[/color][/color][/color]
{'one': 2}

etc. Is there a simple way to override this behavior in dict without
having to rewrite __init__? There are so many cases in dict.__init__
that I'm hesitant to try to reproduce them all...

Steve
--
You can wordify anything if you just verb it.
- Bucky Katt, Get Fuzzy

[Mathias Waack]

Re: sublcassing dict without losing functionality

Steven Bethard wrote:
[color=blue]
> I'd like to subclass dict to disallow overwriting of keys,
> something like:[/color]

<snip>
[color=blue]
> The problem is, dict doesn't appear to call __setitem__ in any of
> the __init__ forms, so none of the following raise errors as I'd
> like them to:[/color]

<snap>
[color=blue]
> etc. Is there a simple way to override this behavior in dict
> without
> having to rewrite __init__? There are so many cases in
> dict.__init__ that I'm hesitant to try to reproduce them all...[/color]

How about starting with UserDict (source comes with your python
distribution) and modifying it for your needs?

Mathias

[Steve Holden]

Re: sublcassing dict without losing functionality

Mathias Waack wrote:
[color=blue]
> Steven Bethard wrote:
>
>[color=green]
>>I'd like to subclass dict to disallow overwriting of keys,
>>something like:[/color]
>
>
> <snip>
>[color=green]
>>The problem is, dict doesn't appear to call __setitem__ in any of
>>the __init__ forms, so none of the following raise errors as I'd
>>like them to:[/color]
>
>
> <snap>
>[color=green]
>>etc. Is there a simple way to override this behavior in dict
>>without
>>having to rewrite __init__? There are so many cases in
>>dict.__init __ that I'm hesitant to try to reproduce them all...[/color]
>
>
> How about starting with UserDict (source comes with your python
> distribution) and modifying it for your needs?
>[/color]

Well, here's one reason I can think of:
[color=blue][color=green][color=darkred]
>>> from UserDict import UserDict
>>> issubclass(User Dict, object)[/color][/color][/color]
0[color=blue][color=green][color=darkred]
>>> issubclass(dict , object)[/color][/color][/color]
1[color=blue][color=green][color=darkred]
>>> d = UserDict()
>>> type(d)[/color][/color][/color]
<type 'instance'>

UserDict is an old-style class. Simply converting it to use object as a
metaclass won't get you anywhere near the speed of a true dict.

regards
Steve
--

http://www.holdenweb.com

http://pydish.holdenweb.com

Holden Web LLC +1 800 494 3119

[Steven Bethard]

Re: sublcassing dict without losing functionality

Mathias Waack <M.Waack <at> gmx.de> writes:[color=blue]
>
> How about starting with UserDict (source comes with your python
> distribution) and modifying it for your needs?[/color]

I'm trying to avoid a huge code bloat for something that should be relatively
simple. Modifying and using UserDict would give me some 60+ lines of code to do
something I would hope could be done in much less. A somewhat better option
might be to copy/paste and slightly modify the __init__ and update code from
UserDict which do more or less what I want... But of course, this has the
problem of any copy/paste solution -- it will inevitably turn into a pain to
sync. What happens if dict acquires a new keyword argument? I have to update
my class again...

The code is not hard to write, but it *is* already written for the dict object.
If there's any way to take advantage of that fact, I'd like to.

Steve

[Peter Otten]

Re: sublcassing dict without losing functionality

Steven Bethard wrote:
[color=blue]
> simple. Modifying and using UserDict would give me some 60+ lines of code[/color]

Not tested beyond what you see:
[color=blue][color=green][color=darkred]
>>> import UserDict as userdict
>>> class Dict(userdict.D ictMixin, dict):[/color][/color][/color]
.... def __setitem__(sel f, key, value):
.... if key in self: raise ValueError("key already exists")
.... super(Dict, self).__setitem __(key, value)
.... def __init__(self, other=None, **kw):
.... super(Dict, self).__init__( )
.... self.update(oth er, **kw)
....[color=blue][color=green][color=darkred]
>>> d = Dict({"one": 1}, one=2)[/color][/color][/color]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 7, in __init__
File "UserDict.p y", line 155, in update
self.update(kwa rgs)
File "UserDict.p y", line 147, in update
self[k] = v
File "<stdin>", line 3, in __setitem__
ValueError: key already exists[color=blue][color=green][color=darkred]
>>> d["one"] = 3[/color][/color][/color]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 3, in __setitem__
ValueError: key already exists[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Peter