Looping fields in an update cursor

Question

I'm running an update cursor on an in_memory feature class to remove blank (but not null) values as part of a larger process.

I wanted by update cursor to loop through my list of fields and if the value was '' or ' ' just replace it with a Null, however while it is finding the correct fields it's not updating the value.

If I explicitly refer to each item in the row e.g. row[1] it works as expected, but not if I try to loop through row.

This works:

with arcpy.da.UpdateCursor(AddressLayer, fieldList) as cursor:
    for row in cursor:
        arcpy.AddMessage(row)
        u = 0
        if i > 1000:
            break

        ##  THIS BIT DOESN'T WORK! ##
        #for r in row:
        #   if r == '' or r == ' ':
        #       arcpy.AddMessage("OID = {}, value = '{}'".format(row[0], r))
        #       r = None
        #       u = 1
        ## THE ABOVE DOESN'T WORK! ##

        ## ADDED BECAUSE THE ABOVE DOESN'T WORK ##
        r = 1
        while r < 9:
            if row[r] == '' or row[r] == ' ':
                arcpy.AddMessage("OID = {}, value = '{}'".format(row[0], row[r]))
                row[r] = None
                u = 1
            r+=1
        ## END OF ADDED BIT ##

        if u == 1:
            arcpy.AddMessage("Updating")
            cursor.updateRow(row)
        i+=1

But this (the commented section above) doesn't:

for r in row:
    if r == '' or r == ' ':
        arcpy.AddMessage("OID = {}, value = '{}'".format(row[0], r))
        r = None
        u = 1

What am I missing about the second code snippet that means it wouldn't work?

Answer 1

Your commented code doesn't update anything because you are only changing the temporary value (r) rather than the list item (row[r])

Compare...

>>> list = ['a','b','c']
... for item in list:
...     if item == 'b':
...         item = None # changes to None, but immediately changes to next value in list
... print list
... 
['a', 'b', 'c']

with...

>>> list = ['a','b','c']
... for i, item in enumerate(list):
...     if item == 'b':
...         list[i] = None # changes the list item
... print list
... 
['a', None, 'c']

Answer 2

Something like this should do the trick:

with arcpy.da.UpdateCursor(AddressLayer, fieldList) as cursor:
    for row in cursor:
        rows.updateRow(map(lambda x: x if unicode(x).strip() != '' else None, row)) #explicitly check for blank strings, cast to unicode first to avoid type errors

I have had all kinds of problems with this in the past and found it is best to explicitly check for empty values as whitespace by stripping.

Haven't tested this exact snippet in a cursor, but it did pass this test:

>>> row = (' ', 'a', 2, 3, 4334.9, 'alla', 'rack', '', 'ball', '     ')
>>> map(lambda x: x if unicode(x).strip() != '' else None, row)
[None, 'a', 2, 3, 4334.9, 'alla', 'rack', None, 'ball', None]