How to Calculate Field in joined table using ArcPy?

Question

I have to calculate a field based on another field in a joined table, but an error occurs:

"Parameters are not valid. ERROR 000728: Field !Table_A.field_A! does not exist within table"

My code:

import arcpy
from arcpy import env
env.workspace = "C:/TMP/BASELINE.gdb"
Table_A = "dist"
Table_B= "matrix"
arcpy.AddJoin_management(Table_A, "id_grid", Table_B, "id_matrix")
arcpy.CalculateField_management(Table_A, "!" + Table_A + ".field_A!", "!" + Table_B + ".field_B!","PYTHON")

Both tables are in the BASELINE.gdb. The Join occurs without any problem.

I forgot to say that, of course, "field_A" exists in "Table_A" and "field_B" exists in "Table_B".

Last but not least, if I run the same calculation in ArcMap, using Field Calculator, no error occurs.

Answer 1

Read the help for Add Join. In particular:

"The input must be a feature layer, a table view, or a raster layer that has an attribute table; it cannot be a feature class or table."

Of course, you are passing a table, but a join does not alter a table (i.e. the database), only a layer or a table view (i.e. in memory). That means by the time you call CalculateField_management, Table_A--a table or feature class--still does not have any more columns than it had before.

I think your best solution is to do what the sample in the help does: call MakeFeatureLayer_management first. If Table_A is just a table and not a feature class, call MakeTableView_management instead.

arcpy.MakeTableView_management(Table_A, "table_a_view")
arcpy.AddJoin_management("table_a_view", "id_grid", Table_B, "id_matrix")
arcpy.CalculateField_management(Table_A, Table_A + ".field_A", "!" + Table_B + ".field_B!","PYTHON")

Then "table_a_view" will be a table view, which means the join should work.