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): 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
breakmetadata_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
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