[Tutor] dictionary dispatch for object instance attributes question

Liam Clarke cyresse at gmail.com
Wed Feb 16 00:08:41 CET 2005

Hi Brian, why not take it the next step and 

>         for key in metadata_dict:
>             if data.startswith(key):
>                  exec('''self.%s = """%s"""''' %(metadata_dict[key],
>                       data[len(key):]))
>                 # triple quotes as there may be quotes in metadata
>                  # values
>                 break

self.foo = {}

for key in metadata_dict.keys():  #? I got confused, so I guessed.
   if data.startswith(key):

And then instead of self.x (if metadata_dict[key] = x] You just call

A bit more obfuscated, but it would seem to remove the exec, although
I'm not sure how else it impacts your class.

I came across something similar when using Pythoncard, because
Pythoncard stores widgets as a dictionary.

To crate a widget via a method in your card, you'd use - 
self.components.myButton = {'properties'}
But to generate widgets based on data input, I'd use a dictionary
generated by the data -

a = raw_input('name? ')

widgDict = {a: {dict of properties}}

for (key, val) in widgDict.items():

(Which incidentally is why Pythoncard rocks)

So yeah, hope that helps a wee bit.


Liam Clarke

On Tue, 15 Feb 2005 17:19:37 -0500, Brian van den Broek
<bvande at po-box.mcgill.ca> wrote:
> Hi all,
> I'm still plugging away at my project of writing code to process
> treepad files. (This was the task which I posted about in the recent
> "help with refactoring needed -- which approach is more Pythonic?"
> thread.)
> My present problem is how best to reorganize a long (20 elements) elif
> chain. The file format I am dealing with organizes itself with a file
> header, and then a series of nodes. Each node has a header filled with
> up to 20 different metadata elements, followed by the node content
> proper. These metadata elements can be in arbitrary order, and need
> not all be present.
> My Node class defines a _parse method which separates out the node
> header, and sends those lines to a _parse_metadata method. This is
> where the elif chain occurs -- each line of the metadata starts with a
> tag like "dt=" and I need to recognize each tag and set the
> appropriate Node object attribute, such as .document_type. (I do not
> want to rely on the unhelpful names for the tags in the file format,
> preferring more self-documenting attribute names.)
> I've come up with *a* way to use a dictionary dispatch, but I'll wager
> a great deal it isn't the *best* way.
> Here is a minimal illustration of what I have come up with:
> <code>
> class A:
>      def __init__(self):
>         self.something = None
>         self.something_else = None
>         self.still_another_thing = None
>      def update(self, data):

> metadata_dict = {'something_tag=': 'something',
>                   '2nd_tag=': 'something_else',
>                   'last=': 'still_another_thing'}
> a = A()
> print a.still_another_thing
> a.update('last=the metadata value for the "last=" metadata tag')
> print a.still_another_thing
> </code>
> <output>
>  >>>
> None
> the metadata value for the "last=" metadata tag
> </output>
> So, it works. Yay :-)
> But, should I be doing it another way?
> Also, I know the general security concerns about things like exec.
> They make me nervous in using it, even though I am (as yet) the sole
> user. Am I right in thinking that the constrained way I am using it
> here protects me? My code uses most of the attributes as a simple
> storage container for later rewriting of the file, but in a few cases
> they enter into (safe seeming) conditionals like:
> if 'text' == self.document_type:
>     self.do_text_stuff()
> if 'RTF' == self.document_type:
>     self.do_RTF_stuff()
> Thanks and best to all,
> Brian vdB
> _______________________________________________
> Tutor maillist  -  Tutor at python.org
> http://mail.python.org/mailman/listinfo/tutor

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