Call field from zip() into Field Calculator expression ArcGIS

Question

I've been trying to find a way to call in field names from a zip(). Basically I have 12 fields called "Flw_days1", "Flw_days2" etc and corresponding "Depth" fields that need to be calculated using the inputs of the "Flw_days" fields. I.e. "Depth1" is calculated using "Flw_Days1" and so on.

I've used the zip() function to match these together and then tried to call these into a codeblock and expression for use in the field calculator.

See example below:

depth_fields = ["Depth1", "Depth2", "Depth3", "Depth4", "Depth5", "Depth6", "Depth7", "Depth8", "Depth9", "Depth10", "Depth11", "Depth12"]
flow_fields = ["!Flw_Day1!", "!Flw_Day2!", "!Flw_Day3!", "!Flw_Day4!", "!Flw_Day5!", "!Flw_Day6!", "!Flw_Day7!", "!Flw_Day8!", "!Flw_Day9!", "!Flw_Day10!", "!Flw_Day11!", "!Flw_Day12!"]

zipped = zip(sorted(depth_fields), sorted(flow_fields))

for input in zipped:
  codeblock = """
def depth(flow, kfc, rf):
  if kfc == 0 or rf == 0:
    return ""
  else:
    return (flow * (kfc / rf) + 2)
"""
  expression = "depth(input[0], !K_FC!, !Ret_Fact!)"
  arcpy.CalculateField_management(CMPP_fc, input[1], expression, "PYTHON_9.3", codeblock)

As you can see I have tried to call the "Flw_Days" field into the expression and the "Depth" field into the field calculator.

The error I get is:

Runtime error <class 'arcgisscripting.ExecuteError'>: ERROR 000539: Error running expression: depth(input[0], 0, 0) <type 'exceptions.TypeError'>: 'builtin_function_or_method' object is unsubscriptable Failed to execute (CalculateField).

So it seems to be calling the input[0] into the expression that is causing the issue.

I am using ArcGIS Desktop 10.

Answer 1

Using an UpdateCursor lets you avoid the codeblock and directly work with the row data, instead of dealing with the Field Calculator. For example, in 10.1:

depth_fields = # list of depth fields
flow_fields = # list of flow fields

zipped = zip(sorted(depth_fields), sorted(flow_fields))

for input in zipped:
    fields = (input[0], input[1], 'K_FC', 'Ret_Fact')
    with arcpy.da.UpdateCursor(CMPP_fc, fields) as cursor:
        for row in cursor:
            if row[2] == 0 or row[3] == 0:
                # error message?
            else:
                row[0] = row[1] * (row[2] / row[3]) + 2
                cursor.updateRow(row)

For 10.0, which doesn't have DA cursors, you can use a regular Update Cursor. The basic idea is the same, but it needs to call the fields specifically since it's not limiting its initial query:

depth_fields = # list of depth fields
flow_fields = # list of flow fields

zipped = zip(sorted(depth_fields), sorted(flow_fields))

for input in zipped:
    fields = (input[0], input[1], 'K_FC', 'Ret_Fact')
    cursor = arcpy.UpdateCursor(CMPP_fc)
    for row in cursor:
        flow = row.getValue(fields[1])
        kfc = row.getValue(fields[2])
        rf = row.getValue(fields[3])
        # pulled values from rows into variables to simplify following code; not required
        if kfc == 0 or rf == 0:
            row.setValue(fields[0],'')
        else:
            depth = flow * (kfc / rf) + 2
            row.setValue(fields[0], depth)
            cursor.updateRow(row)
    del row, cursor

Answer 2

Although this can be accomplished with one update cursor, I find it easier to separate the two. Here's the somewhat tested code to get you started with cursors. I created a mock feature class with 26 fields filled with random values and I picked two of them to compare with field calculator and it seemed to be correct. Feel free to ask for clarification.

import arcpy
#Create two lists of input and output fields.    
flow_fields, depth_fields = zip(*(("Flw_Day{}".format(i),
                                "Depth{}".format(i)) for i in xrange(1,13)))

#Create list of lists containing each record's attributes for each field.
flow_data = [[r for r in row] for row in arcpy.da.SearchCursor(CMPP_fc, flow_fields)]

other_data = [[r for r in row] for row in arcpy.da.SearchCursor(CMPP_fc, ['K_FC', 'Ret_Fact'])]

#Bulk update each field
with arcpy.da.UpdateCursor(CMPP_fc, depth_fields) as rows:
    #The first counter loops over each feature
    for j,row in enumerate(rows):
        #The second counter loops over each field for every feature
        for k,_ in enumerate(depth_fields):
            if other_data[j][0] == 0 or other_data[j][1] == 0:
                row[k] = ""
            else:
                row[k] = flow_data[j][k] * other_data[j][0] / other_data[j][1] + 2
            rows.updateRow(row)

There's definitely a more efficient method, since you're checking K_FC and Ret_Fact for each field instead of once per feature. But unless you're running this code often or on massive datasets, it should work well enough. It might work to bring the if statement before the second for loop....