arcpy MakeFeatureLayer - one workspace question, one output question

Question

I'm trying to run a very simple python script to create a feature layer from a shapefile.

The original script read as follows (I added the two print statements at the end to verify upon completion that:

1) the output I was trying to create was created (Exists(the layer I was trying to create)) and

2) that the script was recognizing the path name by asking if a known folder existed

# Import system modules
import arcpy
from arcpy import env

# Set overwrite option
arcpy.env.overwriteOutput = True


# Set workspace
# arcpy.env.workspace = r"H:\Python\Python_Cookbook\data\TravisCounty"



# Make feature layer to hold selection.
Bldgs = r"H:\Python\Python_Cookbook\data\TravisCounty\BuildingPermits.shp"
Bldg_Lyr = r"H:\Python\Python_Cookbook\data\TravisCounty\Building.lyr"
arcpy.MakeFeatureLayer_management(Bldgs, Bldg_Lyr)


print arcpy.Exists(
    r"H:\Python\Python_Cookbook\data\TravisCounty\Building.lyr")

print arcpy.Exists(
    r"H:\Python\Python_Cookbook\data\TravisCounty")

The result from this first script was that no layer appeared in Catalog, and the print statements returned False and True. So the script recognized the existing folder, but did not find the layer it was supposed to create.

My second attempt was to comment out the workspace and to assign variables to the full paths instead. (see below).

# Import system modules
import arcpy
from arcpy import env

# Set overwrite option
arcpy.env.overwriteOutput = True


# Set workspace
# arcpy.env.workspace = r"H:\Python\Python_Cookbook\data\TravisCounty"



# Make feature layer to hold selection.
Bldgs =     r"H:\Python\Python_Cookbook\data\TravisCounty\BuildingPermits.shp"
Bldg_Lyr = r"H:\Python\Python_Cookbook\data\TravisCounty\Building.lyr"
arcpy.MakeFeatureLayer_management(Bldgs, Bldg_Lyr)


print arcpy.Exists(
r"H:\Python\Python_Cookbook\data\TravisCounty\Building.lyr")

print arcpy.Exists(
r"H:\Python\Python_Cookbook\data\TravisCounty")

This seemed to work better: My print statements returned True and True this time around.

HOWEVER: When I opened the ArcCatalog, I still couldn't find the new layer that was supposed to have been created. Copying the full path (that just returned 'True' to the exists() call) into the Location bar of ArcCatalog returned a message that it was "...and invalid or non-existent directory."

ps. Is there a way to apply the codewrapper without manually entering four spaces before each line of code?

Answer 1

If you really need your layer physically (e.g., saved on your disk), you should follow @Tangnar's advice, but if you want to play with the layer (basically what they are for), there is no such need (since you are testing layer behaviour by a script). Professionally I do not use (never, ever) MakeFeatureLayer_management to create layer in Python scripts but mapping sub-package's Layer method which returns a layer OBJECT. The former generates a layer at the run-time and it names it with your specification (in this case r"H:\Python\Python_Cookbook\data\TravisCounty\Building.lyr", which is a simple STRING object referring to your run-time layer). More clearly, if you change your Bldg_Lyr to "how_are_you Travis" (i.e., Bldg_Lyr = "how_are_you Travis") and use MakeFeatureLayer_management and use arcpy.Exists("how_are_you Travis"), it will return True again. arcpy.Exists looks for not only file locations but also memory items. In this case "how_are_you Travis" exists as a String object as well as reference to a layer object. Confused? I am not surprised. If you are planning to create layers, try this:

Bldgs =     r"H:\Python\Python_Cookbook\data\TravisCounty\BuildingPermits.shp"
my_lyr=arcpy.mapping.Layer(Bldgs)

Now you have a layer OBJECT and simply use my_lyr.save(r"H:\Python\Python_Cookbook\data\TravisCounty\Building.lyr") to have it physically.

Answer 2

Feature layers are temporary... From Arc Help "The layer that is created by the tool is temporary and will not persist after the session ends unless the layer is saved to disk or the map document is saved."

If you are trying to save it, you need to save it as a layer file, or save it as a feature class.

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00170000006p000000