# Create dictionary with segmentid as key, calculated length as value1, measured length as value2
fiberCableLengths = defaultdict(list)
with arcpy.da.SearchCursor(fiberCable, ['segmentid', 'calculatedlength', 'measuredlength']) as cursor:
    for row in cursor:

# Create slackloop dictionary with segmentidfkey as key, total up the length values of each slackloop under that segmentidfkey, and store as the dictionary value
slackloopLengths = defaultdict(list)
with arcpy.da.SearchCursor(slackLoop, ['segmentidfkey', 'length']) as cursor:
    for row in cursor:
        if row[1] is not None:

sumValueDict = {k: sum(v) for k, v in slackloopLengths.items()}

# For each fibercable segmentid in dictionary, If calculated length + total slackloop length = measured length – Add to good list, Else – Add to bad list
goodList = []
badList = []
with arcpy.da.SearchCursor(fiberCable, ['segmentid', 'calculatedlength', 'measuredlength']) as cursor:
    for row in cursor:
        for k, v in sumValueDict.items():
            if row[0] == k:
                if row[1] and row[2] is not None:
                    if int(row[1]) + int(v) == int(row[2]):
                        print "Good List: " + row[0]
                        print int(row[1]) + int(v)
                        print int(row[2])
                        print "Bad List: " + row[0]
                        print int(row[1]) + int(v)
                        print int(row[2])
print "Good List Size: " + str(len(goodList))
print "Bad List Size: " + str(len(badList))

The code above creates a dictionary where each item looks like the following: '1f169eae-5d7c-4ebb-90cd-abbe0e1b72dd': [50.0, 50.0, 50.0, 50.0]

How do I get it to sum the values for each key? The output I'd be looking for here would be: 4ebb-90cd-abbe0e1b72dd': [200]

Resources I've looked at:

https://stackoverflow.com/questions/4880960/how-to-sum-all-the-values-in-a-dictionary/4881100 https://stackoverflow.com/questions/34849651/how-to-add-the-values-of-dictionary-with-same-keys-in-python https://stackoverflow.com/questions/34849651/how-to-add-the-values-of-dictionary-with-same-keys-in-python

  • What is your next step? You could use summary statistics
    – BERA
    Jun 24, 2019 at 15:56

1 Answer 1


Iterate over the dict items and construct a new dict from the key and sum of the values:

>>> slackloopLength = {'abc': [1,2,4,7], 'def': [1,3,5,7,9]}
>>> {k: sum(v) for k, v in slackloopLength.items()}
{'abc': 14, 'def': 25}

The way to read this construction is:

We're going to make a dict:


and each element will have key k and value sum(v):

   {k: sum(v)

where k and v are each of the key-value pairs of elements of slackloopLength:

   {k: sum(v) for k, v in slackloopLength.items()}       
  • Would you mind taking that out of list comprehension format? I've yet to wrap my mind around that format.
    – GIS_GOD
    Jun 24, 2019 at 15:23
  • 1
    How else would you like it? That's probably the most "pythonic" way of doing this and wrapping it in a "for" loop would be inefficient and bad style. I've spelt it out now in the way I read these things.
    – Spacedman
    Jun 24, 2019 at 15:52
  • The code above creates a temporary output that sums the values of the original slackloopLengths dictionary container? How would I get it to update the values in my original dictionary?
    – GIS_GOD
    Jun 24, 2019 at 16:30
  • 2
    Reassign it: slackloopLength = {blah blah}
    – Spacedman
    Jun 24, 2019 at 17:33

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