[GIS] Calculate Field using variable in arcpy

arcpyfield-calculatorpython-parser

I've got a problem with my script and hope that someone can help me.

What I´d like to do is using the field calculator to calculate values for a field using values from another field and a variable in the expression. The script should be as follows:

var_a = 6.2
arcpy.CalculateField_management (in_table, in_field,"[FIELD_A]*100/var_a")

[FIELD_A] contains the values of another field in the same table.

I only know how to integrate var_a in the expression using

arcpy.CalculateField_management (in_table, in_field, "'"+str(var_a)+"'", "PYTHON")

but then I can´t use the other values for the calculation.

Best Answer

Try passing your variable to your string:

var_a = 6.2
arcpy.CalculateField_management (in_table, in_field,"[FIELD_A]*100/{}".format(var_a))

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I think you could go either way with this code. Your first code block would run correctly.
I imagine there are some speed differences between looping through rows and doing a calculate values, so the latter is probably much faster.

Try this for the field calculator expression:

expression = "!Field_B! * constant"

arcpy.CalculateField_management(InTable, "Field_A", expression, "PYTHON_9.3")

You did not have the required field delimiter for the field name in the expression string. Fields have to be wrapped with a "!", just like they would appear in the Field Calculator window in ArcMap when using a python expression. Also note that I added PYTHON_9.3 to the end, which will invoke the more current version of the geoprocessor. Read the Calculate Field (Data Management) help entry, for more information.

Here is a screenshot of the Field Calculator window for reference:

[GIS] How to Calculate Field in joined table using ArcPy

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.

Best Answer

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