I am attempted to calculate a field in a feature class ("Feat_Class_1", by using the count of selected features within another feature class ("Feat_Class_2") as a numerator, while using the value of a field in Feat_Class_1 as the denominator. I am iteratively selecting each row in Feat_Class_1 to calculate this. The script executes successfully right up until the end, when the selected row retains 0 as its value (I am expecting a proportional value). My script is as follows:

## Select Routes which are less than 800m, count the values and retain as denominator
feat_class_2_subset = str(feat_class_2+"_short")
print feat_class_2_subset
arcpy.MakeFeatureLayer_management(feat_class_2, feat_class_2_subset)
arcpy.SelectLayerByAttribute_management(feat_class_2_subset, "NEW_SELECTION", "Total_Kilometers < 0.8")
result = arcpy.GetCount_management(feat_class_2_subset)
count = str(result.getOutput(0))
print count ##<-to confirm that the subset has been successfully selected
feat_class_1_subset = str(feat_class_1+"_individual_record")
print feat_class_1_subset
arcpy.MakeFeatureLayer_management(feat_class_1, feat_class_1_subset)
selection_sub_expression = str("'"+SA2_String+"'")
print selection_sub_expression
selection_expression = str("SA2_String = "+selection_sub_expression)
print selection_expression
arcpy.SelectLayerByAttribute_management(feat_class_1_subset, "NEW_SELECTION", selection_expression2)
test_results = arcpy.GetCount_management(feat_class_1_subset)
test_count = str(test_results.getOutput(0))
print test_count ##<-to determine if the subset has been selected properly
exp = str(count+"/!Total_Properties!")
print exp
arcpy.CalculateField_management(feat_class_1_subset, "Train_800m_Perc", exp, "PYTHON", "")

This works right up until the "test_count" component but then fails to actually execute the calculation.

Any ideas on where I'm going wrong? I've been careful to ensure my expression is a string for inclusion in the function.

  • 1
    Hey Xavier - On line 16, arcpy.SelectLayerByAttribute(..., you've got a variable called selection_expression2 placed as an argument to the function. Where is this variable defined? I'm looking around the rest of the script and I'm not seeing a selection_expression2 variable defined - this might be the issue? Or, is this variable defined somewhere earlier in the script (not shown here in this question)? This could just be a typo, could the correct variable name just be selection_expression?.
    – Jim
    Jun 20, 2016 at 15:27
  • Hi Jim it should actually be selection_expression (and it is in my script) just a typo here randomly, good spotting thanks! Jun 20, 2016 at 23:23

1 Answer 1


I believe the issue is that you are giving two integers in the CalculateField expression, so an integer is returned. You need to force one to a float or decimal to get a proportional value back. Try:

exp = str(float(count)) + "/!Total_Properties!"
  • 1
    exp = "{} / !Total_Properties!".format(float(count)) - I think always safer to use format() to concatenate rather than + especially when handling numbers
    – Midavalo
    Jun 20, 2016 at 18:23
  • Thanks @Midavalo that worked a treat. I've never had a difficulty forcing integers into float proportions via a string formatted expression, but it seems to have reared its ugly head here. Cheers! Jun 21, 2016 at 0:12

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