CalculateField by dividing with a variable that holds a number

Question

I'm trying to calculate a field (using idle python to write my scipt and working with ArcMap 10.3) by dividing two fields then dividing by a variable that holds a number, which the number was calculated by dividing 2 sums.

So essentially I'm trying to divide two ratios but I'm having trouble going about that. Basically the equation is ( a / b ) / ( sum(x) / sum(y)).

I have the variable set to hold the number of the second ratio of (sum(x) / sum(y)) but can't figure out how to incorporate the variable into the calculate field expression or how to set up a double division in the expression. As you can see I used TableToNumPyArray to figure out the sum ratio.

field = arcpy.da.TableToNumPyArray (fc, ["E_Total", "E__Black"], skip_nulls=True)
LQC = 1.0 *(field["E__Black"].sum()) / (field["E_Total"].sum())
#Fields are in long format, 1.0* forces it to be in float format and this section of the code works!



field = arcpy.da.TableToNumPyArray (fc, ["E_Total", "E__Black"], skip_nulls=True)
Lratio = 1.0 *(field["E__Black"]) / (field["E_Total"])
expression = '(!'+Lratio+'!/!'+LQC+'!)'
arcpy.CalculateField_management(lyr, "LQ_Black", expression, "PYTHON")

Answer 1

I don't think your code is doing quite what you think. In CalculateField_management, surrounding a name with ! indicates a field. But as you've set it up here, Lratio and LQC both are holding float values. So, your expression is going to end up looking like (!1.0!/!32.6!), which is probably not what you want. Since you've already calculated these values, just insert the result of the calculation as the expression, i.e.

arcpy.CalculateField_management(lyr, "LQ_Black", "{}".format(Lratio / LQC), "PYTHON")

If you have a third variable that you want to divide by, for example, var3, then

arcpy.CalculateField_management(lyr, "LQ_Black", "{}".format(Lratio / LQC / var3), "PYTHON")

If you have two fields in lyr that you want to divide by the value of LQC, you could write your expression as

arcpy.CalculateField_management(lyr, "LQ_Black", "!field1! / !field2! / {}".format(LQC), "PYTHON")