Overriding iadd for dictionary like objects

Aahz aahz at pythoncraft.com
Tue Sep 1 02:00:31 CEST 2009

In article <b11a8a0e-03ca-41c9-b0d0-c5180b6a2af9 at p15g2000vbl.googlegroups.com>,
RunThePun  <ubershmekel at gmail.com> wrote:
>On Aug 30, 10:33=A0pm, a... at pythoncraft.com (Aahz) wrote:
>> In article <e09276e8-8152-4002-8366-4c12705a8... at l35g2000vba.googlegroups=
>> RunThePun =A0<ubershme... at gmail.com> 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 (aahz at pythoncraft.com)           <*>         http://www.pythoncraft.com/

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