Python, is there a easier way to add values to a default key?

Question

The program I am working does the following:

• Grabs stdout from a .perl program
• Builds a nested dict from the output

I'm using the AutoVivification approach found here to build a default nested dictionary. I'm using this method of defaultdict because it's easier for me to follow as a new programmer.

I'd like to add one key value to a declared key per pass of the for line in the below code. Is there a easier way to add values to a key beyond making a [list] of values then adding said values as a group?

import pprint

class Vividict(dict):
    def __missing__(self, key):
        value = self[key] = type(self)()
        return value

reg = 'NtUser'
od = Vividict()
od[reg]

def run_rip():
    os.chdir('/Users/ME/PycharmProjects/RegRipper2.8')  # Path to regripper dir
    for k in ntDict:
        run_command = "".join(["./rip.pl", " -r 
                              /Users/ME/Desktop/Reg/NTUSER.DAT -p ", str(k)])  
        process = subprocess.Popen(run_command,
                               shell=True,
                               stdout=subprocess.PIPE,
                               stderr=subprocess.PIPE)
        out, err = process.communicate()  # wait for the process to terminate
        parse(out)
        # errcode = process.returncode // used in future for errorcode checking
        ntDict.popitem(last=False)

def parse(data):
    pattern = re.compile('lastwrite|(\d{2}:\d{2}:\d{2})|alert|trust|Value')
    grouping = re.compile('(?P<first>.+?)(\n)(?P<second>.+?)
                         ([\n]{2})(?P<rest>.+[\n])', re.MULTILINE | re.DOTALL)
    if pattern.findall(data):
        match = re.search(grouping, data)
        global first
        first = re.sub("\s\s+", " ", match.group('first'))
        od[reg][first]
        second = re.sub("\s\s+", " ", match.group('second'))
        parse_sec(second)

def parse_sec(data):
    pattern = re.compile(r'^(\(.*?\)) (.*)$')
    date = re.compile(r'(.*?\s)(.*\d{2}:\d{2}:\d{2}.*)$')
    try:
        if pattern.match(data):
            result = pattern.match(data)
            hive = result.group(1)
            od[reg][first]['Hive'] = hive
            desc = result.group(2)
            od[reg][first]['Description'] = desc

        elif date.match(data):
            result = date.match(data)
            hive = result.group(1)
            od[reg][first]['Hive'] = hive
            time = result.group(2)
            od[reg][first]['Timestamp'] = time

        else:
            od[reg][first]['Finding'] = data

    except IndexError:
         print('error w/pattern match')

run_rip()
pprint.pprint(od)

Sample Input:

bitbucket_user v.20091020
(NTUSER.DAT) TEST - Get user BitBucket values

Software\Microsoft\Windows\CurrentVersion\Explorer\BitBucket
LastWrite Time Sat Nov 28 03:06:35 2015 (UTC)

Software\Microsoft\Windows\CurrentVersion\Explorer\BitBucket\Volume
LastWrite Time = Sat Nov 28 16:00:16 2015 (UTC)

Answer 1

If I understand your question correctly, you want to change the lines where you're actually adding values to your dictionary (e.g. the od[reg][first]['Hive'] = hive line and the similar one for desc and time) to create a list for each reg and first value and then extend that list with each item being added. Your dictionary subclass takes care of creating the nested dictionaries for you, but it won't build a list at the end.

I think the best way to do this is to use the setdefault method on the inner dictionary:

od[reg][first].setdefault("Hive", []).append(hive)

The setdefault will add the second value (the "default", here an empty list) to the dictionary if the first argument doesn't exist as a key. It preempts the dictionary's __missing__ method creating the item, which is good, since we want a the value to be list rather than another layer of dictionary. The method returns the value for the key in all cases (whether it added a new value or if there was one already), so we can chain it with append to add our new hive value to the list.