Calculate field using variables as fields gives ERROR 000539?

Question

When I calculate a field like this:

field_0="field_name0"
arcpy.CalculateField_management("Layer", field_0, '!field_name1! / !field_name2!', "PYTHON_9.3")

It works! The field "field_name0" in the attribute table is populated with the value calculated from fields "field_name1" and "field_name2".

But, if I use variables instead of field names:

field_0="field_name0"
field_1="!field_name1!"
field_2="!field_name2!"
arcpy.CalculateField_management("Layer", field_0, 'field_1 / field_2', "PYTHON_9.3")

I get this error:

Runtime error Traceback (most recent call last): File "", line 3, in File "c:\program files (x86)\arcgis\desktop10.2\arcpy\arcpy\management.py", line 3354, in CalculateField raise e ExecuteError: ERROR 000539: Error running expression: field_1/ field_2 Traceback (most recent call last):
File "", line 1, in NameError: name 'field_1' is not defined Failed to execute (CalculateField).

I want to use loops later on, that's why I want it to work with variables, and not simply with direct field names.

Answer 1

Based on your variable, you just need to make sure variables and strings are not confused

field_1="!field_name1!"
field_2="!field_name2!"

arcpy.CalculateField_management("Layer", field_0, field_1 + " / " + field_2, "PYTHON_9.3")

note that if you have the strings as variables without the "!", it is nicer to use format()

field_1="field_name1"
field_2="field_name2"

arcpy.CalculateField_management("Layer", field_0, """!{}!/!{}!""".format(field_1,field_2), "PYTHON")

Answer 2

You are missing the wrapping exclamation marks, try this:

arcpy.CalculateField_management("Layer", field_0, '!' + field_1 + '!/!' + field_2 + '!', "PYTHON_9.3")

Answer 3

You have embedded the string variable within a string so python sees it as a string.

I would suggest you make you code something like:

field_0="field_name0"
calcString = "!field_name1! / !field_name2!"
arcpy.CalculateField_management("Layer", field_0, calcString', "PYTHON_9.3")