Setting field equal to raster Name ID for batch process in ArcPy

Question

I am wanting to set an existing field called RasterID to the name of the raster for which the attribute table belongs to, but for a batch of rasters. Is there a way to do this with my current code structure and by setting 'expression' equal to the raster name? If I simply set expression = desc.baseName, I get an error.

This is how I am trying to implement arcpy.Describe():

import os,arcpy,glob
from arcpy import env
arcpy.env.workspace = r"C:\VMshared\small_example_valley3\SnowDepth4"
#arcpy.env.overwriteOutput = True
fieldName = "RasterID"

for rasterFile in arcpy.ListRasters("*.tif"):
    desc = arcpy.Describe(rasterFile)
    print "Base Name: " + desc.baseName 
    expression = desc.baseName
    print expression
    arcpy.CalculateField_management(rasterFile, fieldName, expression, "PYTHON_9.3") #something wrong with this line?
print "done"

Base Name: snowdepthN0001 snowdepthN0001 Runtime error Traceback (most recent call last): File "", line 12, in
File "c:\program files (x86)\arcgis\desktop10.1\arcpy\arcpy\management.py", line 3128, in CalculateField raise e ExecuteError: ERROR 000539: Error running expression: snowdepthN0001 Traceback (most recent call last): File "", line 1, in NameError: name 'snowdepthN0001' is not defined Failed to execute (CalculateField).

Answer 1

I can see that multiple comments aren't being understood. This is what I meant:

import os,arcpy

from arcpy import env
mxd = arcpy.mapping.MapDocument("CURRENT")

#arcpy.env.overwriteOutput = True
fieldName = "RasterID"
RowsField = "unknown" # the field (numeric) for the number of rows
ColsField = "unknown" # the field (numeric) for the number of columns

for lyr in arcpy.mapping.ListLayers(mxd):
    desc = arcpy.Describe(lyr)
    expression = desc.baseName
    desc = arcpy.Describe(desc.catalogPath) # describe the data that the layer points to

    arcpy.AddMessage("Base Name: " + expression )
    arcpy.AddMessage("Rows: " + str(desc.height) + ", Cols:" + str(desc.width))

    arcpy.CalculateField_management(lyr, fieldName, "\"" + expression + "\"", "PYTHON_9.3")
    arcpy.CalculateField_management(lyr, RowsField, desc.height,"PYTHON_9.3")
    arcpy.CalculateField_management(lyr, ColsField, desc.width,"PYTHON_9.3")
arcpy.AddMessage("done")

When I say that you need to quote the value it is for the field calculator to know that you want to calculate a constant string value expression and not a field called expression, to do this you need to surround the expression by quote marks thus: "\"" + expression + "\""; in python (and many other languages) if you want to include a quote mark in a string you need to escape it, this allows characters that you wouldn't normally be able to put into the string like \" (quote), \n (new line) and \t (tab). Because the \ character is used to escape if you want one in a string you need to put two \\ which python interprets as \, for example a path name : "C:\\some\\folder" is interpreted as C:\some\folder.

This only applies for string fields, all other fields that you can calculate (short, long, double etc..) do not need to be quoted.

With some error checking:

import os,arcpy

from arcpy import env
mxd = arcpy.mapping.MapDocument("CURRENT")

fieldName = "RasterID"
RowsField = "unknown" # the field (numeric) for the number of rows
ColsField = "unknown" # the field (numeric) for the number of columns

for lyr in arcpy.mapping.ListLayers(mxd):
    desc = arcpy.Describe(lyr) # describe the layer
    expression = desc.baseName
    arcpy.AddMessage( "Base Name: " + expression )
    desc = arcpy.Describe(desc.catalogPath) # describe the data that the layer points to
    gotRowsCols = True # assume true unless they can't be retrieved
    try:
        arcpy.AddMessage("Rows: " + str(desc.height) + ", Cols:" + str(desc.width))
    except:
        arcpy.AddWarning("Cannot retrieve rows-cols property, layer may not be a raster")
        gotRowsCols = False # unable to get the rows-cols from the layer

    # look for the RasterID field and if it exists try to calculate the name of the raster
    # and then rows and columns if those fields are found
    field = arcpy.ListFields(lyr,fieldName)
    if field:
        arcpy.CalculateField_management(lyr, fieldName, "\"" + expression + "\"", "PYTHON_9.3")

        # if the rows and columns values can be retrieved then try find the fields and calc
        if gotRowsCols:
            # try to find the RowsField and if it exists then calc the height of the raster
            field = arcpy.ListFields(lyr,RowsField)
            if field:
                arcpy.CalculateField_management(lyr, RowsField, desc.height,"PYTHON_9.3")

            # try to find the ColsField and if it exists then calc the width of the raster
            field = arcpy.ListFields(lyr,ColsField)
            if field:
                arcpy.CalculateField_management(lyr, ColsField, desc.width,"PYTHON_9.3")
    else:
        # field not found in this layer, issue a warning and move on.
        arcpy.AddWarning("Field not found " + fieldName)
arcpy.AddMessage( "done" )

Answer 2

As @Michael Miles-Stimson has commented your calculate field method will overwrite all row values each loop through the raster list method. So, the table will only have one raster name for each record. In addition, it is unclear from your question if the update table has an equal number of rows for rasters that exist within the directory.

If you are simply wanting to add new rows to a table I would recommend using an insert cursor. If you are wanting to update an existing table/rows you could use a update cursor (make sure you put the expression statement within the update cursor loop so each row will get a different raster name).

You could use the calculate field method, but you would have to first make the update table a feature layer and perform a select by attribute on one row at a time before the calculate field statement (...iterating selected row and calculate field.....iterating selected row and calculating field....etc).