Using Search Cursor to calculate fields with records having a specific value

Question

I'm a total beginner at programming and Python. I'm trying to select features by attributes and then field calculate in another field.

In VBA it might look like this:

    If [Z1COMM] = "R" then
    [Z1UNITS] = [Z1RUNTS] and
    [Z1SIZE] = [Z1RSIZE]
    ElseIf [Z1COMM] = "O" then
    [Z1UNITS] = [Z1OUNTS] and
    [Z1SIZE] = [Z1OSIZE]
    Else [Z1UNITS] = [Z1RUNTS] and
    [Z1SIZE] = [Z1SIZE]



    End If

I could probably do this pretty easily in VBA using the code block in the field calculator but I want to start grasping Python more. Here's what I got from trying to make a list of the values as a first step (taken and modified from http://support.esri.com/en/knowledgebase/techarticles/detail/41442):

    # Import modules

   import arcpy, os , sys, string

   # Create environmental variables

   arcpy.env.overwriteOutput = True
   arcpy.env.workspace = r"J:\CO_004"

    # Set variables

    input = "testfile.shp"
    lyr = "tst_layer"
    arcpy.MakeFeatureLayer_management(input,lyr)

    outFile = open(r"J:\CO_004\test.text", "w")

    # Build  search cursor
    fcSearch = arcpy.da.SearchCursor(lyr, ["Z1COMM"])
    for fcRow in fcSearch:
        # Process: Select by attributes
        arcpy.SelectLayerByAttribute_management(lyr, "NEW_SELECTION", "Z1COMM" == 'O')

        txtSearch = arcpy.da.SearchCursor(lyr, ["Z1COMM"])
        outFile.write("Commodity: " + str(fcRow[0]) + "\n")
        for txtRow in txtSearch:
            zval = str(txtRow[0])
            outFile.write(zval + "\n")

    outFile.close() #This closes the text file
    del input, lyr, fcRow, txtRow, txtSearch, fcSearch, outFile, zval

And this is my latest error: ExecuteError: ERROR 000358: Invalid expression Failed to execute (SelectLayerByAttribute).

I originally tried to use a feature class for the input to avoid this part of the code: lyr = "tst_layer" arcpy.MakeFeatureLayer_management(input,lyr)

But got an error that feature classes could not be used. If I have to make a separate shapefile so be it but it's be more convenient to work straight from a feature class in a file GDB.

I'm mainly confused by this part:

    arcpy.SelectLayerByAttribute_management(lyr, "NEW_SELECTION", "Z1COMM" == 'O')

where the last parameter I thought should be the selection criteria.

Also, this code just makes a txt file, it doesn't actually calculate anything but I thought once I figured this out I could take the next step.

Answer 1

Instead of:

"Z1COMM" == 'O'     # will always evaluate to False because "Z1COMM" != 'O'

Try this:

""""Z1COMM" = '{0}'""".format("O")

There are a number of ways this can be done.