[Tutor] dictionary dispatch for object instance attributes question

[Kent Johnson]

Brian van den Broek wrote:
> 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):
> 
>     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
> 
> 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?

Another way to do this is to use dispatch methods. If you have extra processing to do for each tag, 
this might be a good way to go.

I'm going to assume that your data lines have the form 'tag=data'. Then your Node class might have 
methods that look like this:

class Node:
   ...
   def parse_metadata(self, line):
     tag, data = line.split('=', 1)
     try:
       handler = getattr(self, 'handle_' + tag)
     except AttributeError:
       print 'Unexpected tag:', tag, data
     else:
       handler(data)

   def handle_something_tag(self, data):
     self.something = int(data) # for example

   def handle_last(self, data):
     try:
       self.another_thing.append(data) # attribute is a list
     except AttributeError:
       self.another_thing = [data]

and so on. This organization avoids any if / else chain and puts all the processing for each tag in 
a single place.

BTW the try / except / else idiom is used here to avoid catching unexpected exceptions. The naive
way to write it would be

     try:
       handler = getattr(self, 'handle_' + tag)
       handler(data)
     except AttributeError:
       print 'Unexpected tag:', tag, data

The problem with this is that if handler() raise AttributeError you will get an unhelpful error 
message and no stack trace. Putting the call to handler() in an else clause puts it out of the scope 
of the try / except but it will still be executed only if the getattr succeeds.

> 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()

Conditionals on a 'type' flag are a code smell that suggests using subclasses. Maybe you should have 
a TextNode class and an RtfNode class. Then the above becomes just
   self.do_stuff()

and TextNode and RtfNode each have the appropriate implementations of do_stuff().

I'm not saying this is the right choice for you, just something you might consider.

Kent

> 
> Thanks and best to all,
> 
> Brian vdB
> 
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