Re: Overriding iadd for dictionary like objects
On Sep 1, 3:00 am, a...@pythoncraft.com (Aahz) wrote:
> In article
> ,
>
>
>
>
>
> RunThePun wrote:
> >On Aug 30, 10:33=A0pm, a...@pythoncraft.com (Aahz) wrote:
> >> In article >.com>,
> >> RunThePun =A0 wrote:
>
> >>>I made a DictMixin where the keys are filenames and the values are the
> >>>file contents. It was very simple and easy to do thanks to DictMixin.
>
> >>>For example this code writes "abc" in a file named "temp.txt" and
> >>>prints the contents of the file named "swallow", these files are
> >>>looked up/created/deleted in the directory "spam":
> >>>>>> d =3D3D FilesDict('spam')
> >>>>>> d['temp.txt'] =3D3D 'abc'
> >>>>>> print(d['swallow'])
>
> >>>My problem arose when I wanted to append a string to a file which
> >>>using open(..., 'ab') would have been miles more efficient because I
> >>>wouldn't have to read the entire file (__getitem__) and then write the
> >>>entire file back (__setitem__). The files are expected to be as big as
> >>>600 KB which will be appended 30 bytes at a time about 3 times a
> >>>second. Performance-wise the system would probably work without open
> >>>(..., 'ab') but it would be a real thrashing so the current solution
> >>>uses a method "AddTo" as Robert suggested, sacrificing the neat
> >>>getitem/setitem syntax.
>
> >> You can do mostly what you want, I think, by having __setitem__()
> >> convert string values into FileProxy() objects that have an appropriate
> >> __iadd__() method. =A0That brings a whole new set of problems, of course.
>
> >I'm guessing you meant __getitem__, which is what Jan Kaliszewski
> >suggested, but as you noted, would be a bit cumbersome in this case.
>
> Actually, what I meant was __setitem__. The idea is that you create the
> proxy item when you add the data to the dict (wrapping the original
> data), and the proxy has an __iadd__ method, which would allow you to do
> the file append.
> --
> Aahz (a...@pythoncraft.com) <*> http://www.pythoncraft.com/
>
> "I support family values -- Addams family values" --www.nancybuttons.com
But you do mean that __getitem__ would return a wrapped object as
well, right? Otherwise I don't see how the iadd would be relevant
because:
d['a'] += 3
is equivalent to:
d.__setitem__('a', d.__getitem__('a').__iadd__(3))
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Re: Overriding iadd for dictionary like objects
On Aug 30, 10:33 pm, a...@pythoncraft.com (Aahz) wrote:
> In article
> ,
>
>
>
>
>
> RunThePun wrote:
>
> >I made a DictMixin where the keys are filenames and the values are the
> >file contents. It was very simple and easy to do thanks to DictMixin.
>
> >For example this code writes "abc" in a file named "temp.txt" and
> >prints the contents of the file named "swallow", these files are
> >looked up/created/deleted in the directory "spam":
> >>>> d =3D FilesDict('spam')
> >>>> d['temp.txt'] =3D 'abc'
> >>>> print(d['swallow'])
>
> >My problem arose when I wanted to append a string to a file which
> >using open(..., 'ab') would have been miles more efficient because I
> >wouldn't have to read the entire file (__getitem__) and then write the
> >entire file back (__setitem__). The files are expected to be as big as
> >600 KB which will be appended 30 bytes at a time about 3 times a
> >second. Performance-wise the system would probably work without open
> >(..., 'ab') but it would be a real thrashing so the current solution
> >uses a method "AddTo" as Robert suggested, sacrificing the neat
> >getitem/setitem syntax.
>
> You can do mostly what you want, I think, by having __setitem__()
> convert string values into FileProxy() objects that have an appropriate
> __iadd__() method. That brings a whole new set of problems, of course.
> --
> Aahz (a...@pythoncraft.com) <*> http://www.pythoncraft.com/
>
> "I support family values -- Addams family values" --www.nancybuttons.com
I'm guessing you meant __getitem__, which is what Jan Kaliszewski
suggested, but as you noted, would be a bit cumbersome in this case.
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Re: Overriding iadd for dictionary like objects
more efficient because I
wouldn't have to read the entire file (__getitem__) and then write the
entire file back (__setitem__). The files are expected to be as big as
600 KB which will be appended 30 bytes at a time about 3 times a
second. Performance-wise the system would probably work without open
(..., 'ab') but it would be a real thrashing so the current solution
uses a method "AddTo" as Robert suggested, sacrificing the neat
getitem/setitem syntax.
Just so I don't leave out any information, I actually also made a
DirectoryDict for having multiple 'tables'. In this DictMixin, keys
are directory names and values are FilesDict instances. So to write
"European or African" to the file "/root/tmp/velocity" one would be
just:
>>> d = DirectoryDict("/root")
>>> d["tmp"]["velocity"] = "European or African"
So now I hope it's clearer why and how I wanted a special
__item_iadd__ for the dictionary syntax,
RunThePun
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Re: Overriding iadd for dictionary like objects
On Aug 27, 6:58 am, Robert Kern wrote:
> On 2009-08-26 20:00 PM, Jan Kaliszewski wrote:
>
>
>
>
>
> > 27-08-2009 o 00:48:33 Robert Kern wrote:
>
> >> On 2009-08-26 17:16 PM, RunThePun wrote:
> >>> I'd like to build a database wrapper using DictMixin and allow items
> >>> to be appended by my own code. The problem is += is always understood
> >>> as setitem and getitem plainly.
>
> >>> d = MyDict()
> >>> d['a'] = 1
>
> >>> # this is the problem code that's I'd like to override. It's always
> >>> setitem('a', getitem('a') + 3)
> >>> d['a'] += 3
> >>> # i wanted to do something like my own 'appenditem' function which for
> >>> example could be useful if getitem is an expensive operation which can
> >>> be avoided.
>
> >>> I hope that was clear enough of a request, it's really late at night
> >>> here...
>
> >> I'm sorry, this is just part of the syntax of Python. You cannot
> >> override it.
>
> > Though
> > d['a'] = 3
> > is equivalent to:
> > d.__setitem__('a', 3)
>
> > The
> > d['a'] += 3
> > *is not* equivalent to:
> > d.__setitem__('a', d.__getitem__('a') + 3)
> > *but is* equivalent to:
> > d.__getitem__('a').__iadd__(3)
>
> > Then you can override __getitem__() of MyDict in such a way that it
> > returns prepared (wrapped) object with overriden __iadd__() as you
> > want to.
>
> You could, but then you will almost certainly run into problems using the
> wrapped object in places that really expect the true object.
>
> --
> Robert Kern
>
> "I have come to believe that the whole world is an enigma, a harmless enigma
> that is made terrible by our own mad attempt to interpret it as though it
> had
> an underlying truth."
> -- Umberto Eco
Exactly my problem Robert. I'm actually going to be using this dict
for storing and retrieving strings so wrapping str and replacing the
iadd would cause alot of craziness.
Anybody have any more ideas? I think python should/could havev a
syntax for overriding this behaviour, i mean, obviously the complexity
of supporting all operators with the getitem syntax could introduce
alot of clutter. But maybe there's an elegant solution out there...
---RP
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Overriding iadd for dictionary like objects
I'd like to build a database wrapper using DictMixin and allow items
to be appended by my own code. The problem is += is always understood
as setitem and getitem plainly.
d = MyDict()
d['a'] = 1
# this is the problem code that's I'd like to override. It's always
setitem('a', getitem('a') + 3)
d['a'] += 3
# i wanted to do something like my own 'appenditem' function which for
example could be useful if getitem is an expensive operation which can
be avoided.
I hope that was clear enough of a request, it's really late at night
here...
thanks,
RunPun
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