Getting difference between field and constant value in ArcPy?

Question

I need to calculate the difference between two input parameters within a table. the first parameter is a field in the table, and the second is a constant i want to subtract from this field, both of which are asked for in a script/tool.

currently my code looks like this:

heightfield = arcpy.GetParameterAsText(4) # field (float) in table inObsPtswHeight
heightOffset = arcpy.GetParameterAsText(5) # linear unit, constant value
arcpy.CalculateField_management(inObsPtswHeight, heightfield, heightfield - heightOffset, "PYTHON")

This is run in an arcgis script toolwindow and returns the error:

arcpy.CalculateField_management(inObsPtswHeight, heightfield, heightfield - heightOffset, "PYTHON")
TypeError: unsupported operand type(s) for -: 'unicode' and 'unicode'

EDIT: Bera have given around 70 % of the answer: the first argument (which is a field) cannot be changed to float because it is a field name. the second argument however can.

heightfield = arcpy.GetParameterAsText(0)
#line above: field in table inObsPtswHeight - this is left unchanged
heightOffset = float((re.findall("\d+\.\d+",arcpy.GetParameterAsText(1)))[0]) 
# linear unit, constant value - a linear unit returns a string (e.g. "102.9 meters") 
# which has to be stripped into only the number/float. this line returns a float.


arcpy.CalculateField_management(inObsPtswHeight, heightfield, "!{0}! - {1}".format(heightfield, heightOffset), "PYTHON")
# as Bera pointed out, the smartest is putting the expression into a string. 
#to call on the field in a python field calculator expression, you have to use !fieldname!, while the float can be used directly. 

Answer 1

arcpy.GetParameterAsText return strings so you are trying to subtract two strings which will not work. Try converting them to float:

heightfield = float(arcpy.GetParameterAsText(4)) # field (float) in table inObsPtswHeight
heightOffset = float(arcpy.GetParameterAsText(5)) # linear unit, constant value

Also i think you are supposed to enclose the expression in quotes:

arcpy.CalculateField_management(inObsPtswHeight, heightfield,'{0} - {1}'.format(heightfield,heightOffset), "PYTHON")