Python: Expanding Complicated Tree DataStructure

问题

I am exploring a data structure which get expands to sub-elements and resolves to a final element. But I only want to store top two levels.

Example: Lets say I start with New York which breaks into Bronx, Kings, New York, Queens, and Richmond as counties but then finally somehow they resolve to USA.

I am not sure if this is a good example but just to make it clear here is more clear explanation of the problem.

A (expands to) B,C,D -> B (expands to) K,L,M -> K resolves to Z 

I initially wrote it in series of for loops and then use the recursion but in recursion I am loosing some of the elements that get expand and due to that I don't drill down each of the expanded element. I have put the both recursive version and non-recursive. I am looking for some advise on building this data structure, and what is the best way to do.

I call a data base query for every element in the expanded version which returns a list of items. Go until it resolves to single element. With out recursion I don't loose drilling all the way till the final element that others resolve to. But with recursion its not the same. I am also new to python so hopefully this is not a bad question to ask in a site like this.

returnCategoryQuery is a method that returns list of items by calling the database query.

With out recursion

#Dictionary to save initial category with the rest of cl_to
baseCategoryTree = {};
#categoryResults = [];

# query get all the categories a category is linked to
categoryQuery = "select cl_to from categorylinks cl left join page p on cl.cl_from = p.page_id where p.page_namespace=14 and p.page_title ='";
cursor = db.cursor(cursors.SSDictCursor);

    for key, value in idTitleDictionary.iteritems():
        for startCategory in value[0]:
            #print startCategory + "End of Query";
            categoryResults = [];
            try:
                categoryRow = "";
                baseCategoryTree[startCategory] = [];
                print categoryQuery + startCategory + "'";
                cursor.execute(categoryQuery + startCategory + "'");
                done = False;
                while not done:
                    categoryRow = cursor.fetchone();
                    if not categoryRow:
                        done = True;
                        continue;
                    categoryResults.append(categoryRow['cl_to']);
                for subCategoryResult in categoryResults:
                    print startCategory.encode('ascii') + " - " +  subCategoryResult;
                    for item in returnCategoryQuery(categoryQuery + subCategoryResult + "'"):
                        print startCategory.encode('ascii') + " - " + subCategoryResult + " - "  + item;
                        for subItem in returnCategoryQuery(categoryQuery + item + "'"):
                            print startCategory.encode('ascii') + " - " + subCategoryResult + " - "  + item + " - " + subItem;
                            for subOfSubItem in returnCategoryQuery(categoryQuery + subItem + "'"):
                                 print startCategory.encode('ascii') + " - " + subCategoryResult + " - "  + item + " - " + subItem + " - " + subOfSubItem;
                                 for sub_1_subOfSubItem in returnCategoryQuery(categoryQuery + subOfSubItem + "'"):
                                      print startCategory.encode('ascii') + " - " + subCategoryResult + " - "  + item + " - " + subItem + " - " + subOfSubItem + " - " + sub_1_subOfSubItem;
                                      for sub_2_subOfSubItem in returnCategoryQuery(categoryQuery + sub_1_subOfSubItem + "'"):
                                          print startCategory.encode('ascii') + " - " + subCategoryResult + " - "  + item + " - " + subItem + " - " + subOfSubItem + " - " + sub_1_subOfSubItem + " - " + sub_2_subOfSubItem;
            except Exception, e:
                traceback.print_exc();

With Recursion

def crawlSubCategory(subCategoryList):
    level = 1;
    expandedList = [];
    for eachCategory in subCategoryList:
        level = level + 1
        print "Level  " + str(level) + " " + eachCategory;
        #crawlSubCategory(returnCategoryQuery(categoryQuery + eachCategory + "'"));
        for subOfEachCategory in returnCategoryQuery(categoryQuery + eachCategory + "'"):
            level = level + 1
            print "Level  " + str(level) + " " + subOfEachCategory;
            expandedList.append(crawlSubCategory(returnCategoryQuery(categoryQuery + subOfEachCategory + "'")));
    return expandedList;


#Dictionary to save initial category with the rest of cl_to
baseCategoryTree = {};
#categoryResults = [];

# query get all the categories a category is linked to
categoryQuery = "select cl_to from categorylinks cl left join page p on cl.cl_from = p.page_id where p.page_namespace=14 and p.page_title ='";
cursor = db.cursor(cursors.SSDictCursor);

for key, value in idTitleDictionary.iteritems():
    for startCategory in value[0]:
        #print startCategory + "End of Query";
        categoryResults = [];
        try:
            categoryRow = "";
            baseCategoryTree[startCategory] = [];
            print categoryQuery + startCategory + "'";
            cursor.execute(categoryQuery + startCategory + "'");
            done = False;
            while not done:
                categoryRow = cursor.fetchone();
                if not categoryRow:
                    done = True;
                    continue;
                categoryResults.append(categoryRow['cl_to']);
            #crawlSubCategory(categoryResults);
        except Exception, e:
            traceback.print_exc();
        #baseCategoryTree[startCategory].append(categoryResults);
        baseCategoryTree[startCategory].append(crawlSubCategory(categoryResults));

回答1:

Are you trying to lookup "Queens" and learn that it is in the USA? Have you tried encoding your tree in XML, and using lxml.etree to find an element and then use getpath to return the path in XPath format?

This would meaning adding a fourth top level to your tree, namely World, and then you would search for Queens and learn that the path to Queens is World/USA/NewYork/Queens. The answer to your question would always be the second item in the XPath.

Of course you could always just build a tree from the XML and use a tree search algorithm.

来源：https://stackoverflow.com/questions/8031509/python-expanding-complicated-tree-datastructure