Passing string field value to another field in same Feature Class using ArcPy?

Question

All the literature I have read suggests that to pass a string value to a another field in a feature class via ArcPy you need to do the following. I am running it in ArcMap 10.6.

arcpy.CalculateField_management(New_Labels_layer, "FC_Field", "'Word_Variable'", "VB", "") will pass a word variable. While it successfully runs it does not pass the variable into the field. I did an exist function. So I know it exists and is being identified. The field I want to pass to is a string field.

arcpy.CalculateField_management(New_Labels_layer, "Area", Numeric_Variable, "VB", "") successfully passes a numeric variable.

I have tried the following variations:

• 'Word_Variable'
• "Word_Variable"
• "'%Word_Variable%'"
• "%Word_Variable%"
• !Word_Variable!

None of these seems to work. Most allow me to run the script but do not pass anything.

Answer 1

Check the syntax for the arcpy.CalculateField() function:

CalculateField_management(in_table, field, expression, {expression_type}, {code_block})

In the expression parameter you have to pass a string referencing a field. However, the way you reference another field will depend on the expression_type. In your code you are specifying VB. In the tool's documentation is stated that:

For VB calculations, field names must be enclosed in square brackets ([fieldname]).

In case you want to pass a Python expression you have to specify 'PYTHON' as your expression_type. Furthermore:

For Python calculations, field names must be enclosed in exclamation points (!fieldname!).

None of your examples would work by passing a VB expression. however, your last example should work if you change the expression_type to 'PYTHON'.

• Damned_Novice: # Process: Calculate Field arcpy.CalculateField_management(New_Labels_layer, "Area", !Word_Variable!, "VB", "") give me the following error Traceback (most recent call last): File "C:\Documents\Type_SelectV1.py", line 208, in <module> arcpy.CalculateField_management(New_Labels_layer, "Area", [Word_Variable], 'VB', "") File "c:\program files (x86)\arcgis\desktop10.6\arcpy\arcpy\management.py", line 3654, in CalculateField raise e RuntimeError: Object: Error in executing tool Failed to execute (TypeSelections).
• Damned_Novice: # Process: Calculate Field arcpy.CalculateField_management(New_Labels_layer, "Area", [Word_Variable], "PYTHON", "") give me the following error SyntaxError: invalid syntax (Type_SelectV1.py, line 208) Failed to execute (TypeSelections).
• Marcelo Villa: Yeah, you are mixing it up. For 'PYTHON' use '!Word_Variable!', for 'VB' use '[Word_Variable]'
• Marcelo Villa: Furthemore, make sure you are passing a string. If you pass [Word_Variable] Python will interpret it a list.
• Marcelo Villa: It is working fine for me. The only thing that occurs to me is that you declared Word_Variable somewhere in your script and it is a string. If the field you are trying to calculate is called Area and your text field is called my_field the code would be as follows: arcpy.CalculateField_management(New_Labels_Layer, 'Area', '!my_field!', 'PYTHON')

Answer 2

When you put the name of your variable into quotes, you are essentially just passing the name of the variable as a string. What you want to do is pass the variable itself, as it is a reference to a string.

Do it just like you do for the numeric variable and pass the variable name without quotes and it should work as expected.

arcpy.CalculateField_management(New_Labels_layer, "FC_Field", Word_Variable, "VB", "")