ArcPy and ArcMap – Calculating Fields from Layer Outputs Explained

arcmaparcpymodelbuilderparameterspython-script-tool

I'm working in Pycharm Python IDLE under ArcPy, to later work on setting up a toolbox in ArcGIS Desktop 10.7.

I have a large script running in Python that works fine without parameters. Now, I want to create a tool for ArcGIS Desktop 10.7, and have this problem configuring Script Properties. I can't make appear in the next window -'Parameter Properties' > 'Property' = 'Obtained From'-, tables or layers that has been calculated during my tool process, and become like intermediate variables.

I need to recalculate fields in several layers from intermediate data (i need one field from that data), but doesn't appears in the mentioned property like a value, only appears input layers that already get in my hard disk before running process. When I run my tool from ArcMap, in field calculator doesn't appear because, obviously the referenced layer still not processed. But same situation in ModelBuilder it's allowed or better preconfigured.

Do I need to edit my Python code, or is it a configuration problem from 'Script Properties' from ArcMap?

Pair of images, from my script tool, and same situation in ModelBuilder:

My script tool, line blue, that three layers are inputs. Don't show any intermediate data.

Here, what I need to get, those layers with blue symbol processing, but translated in Python and in the script tool

Because in image 2 appears the info from previous layer (loaded fields)

Best Answer

When you wire up a script to a tool interface, the interface will have no knowledge of any layers created by the code itself, unless its an output parameter you have specified. As you are creating these layers in your code I would suggest you edit your code to utilise them as it is not possible to access something that does not yet exist.

So to answer your question I would say this is a configuration problem and you need to adapt your code.

Related Solutions

[GIS] Creating custom tool using Python script exported from ModelBuilder

Here is a list of things to consider before exporting models to Python:

If your model uses an iterator, the iteration logic will not be exported and will have to be replaced with looping statements in Python.
If your model used a Model only tool such as Merge Branch, Collect Values, or Calculate Value, these tools will not run properly in Python; they may execute without error, but nothing happens and your script doesn't work as expected. In some cases, the Python script will fail at a later point. You need to implement the equivalent Python functionality these Model only tools provide.
If your model used inline variable substitution such as %workspace%, %scratchworkspace%, %n%, %i%, or %variable name%, you will have to replace these with the correct value in the Python script.
If your model used layers or table views from ArcMap or ArcGlobe, those layers or table views will have to be created or the data will need to be fully referenced in the script.
If your model used layers or table views that are not created within the original model, those layers or table views will have to be created in the script using tools such as MakeFeatureLayer and MakeTableView.
If your model included sub-models, the contents within those sub-models will not be exported. Instead, the toolbox will have to be referenced and the model tool will need to be called inside the script.
If you expect to be overwriting data, set the overwriteOutput property to True. arcpy.env.overwriteOutput = True
Variables in your model are converted directly into Python variables in the exported script. You need to avoid naming your model variables the same as Python's reserved keywords (for example, class, global, return). For a full list of reserved keywords, use Python's keyword module.
If your model has an input variable type that is a FeatureSet or RecordSet, it will be exported as an in-memory feature class such as "in_memory\{E83BC42B-3578-4D20-A817-5F2AF3F5D05E}". You need to update this variable in Python with a path to a valid dataset or create a script tool where the input data type is a feature set.

[GIS] Using Python Script tool in ModelBuilder to check if field exists, if not, create one

The Check Is Not Actually Required If You are Going to Create The Field Anyhow

If you are going to create the field if it doesn't exist, you don't even need to check for its existence. If the field already exists, and you run arcpy.AddField_management() for that same field, it will do nothing.

Similarly in ModelBuilder if you run the 'Add Field' tool, it will output a message warning that the field already exists, but it will continue on to the next step in the model just fine. (NB: If you do it with a different data type, it will fail, but so long as you use the same data type, it is fine.) Subsequent tools in the model that depend on that field should then be able to validate successfully, because the model can determine that the field will actually exist. Creating the field with a script tool will not allow the model to validate the existence of that field. So with your script tool method the model may be able to run successfully if you run it directly (ie, not in edit mode) but it won't be able to validate and run in edit mode.

I can't see this behaviour of the 'Add Field' tool and arcpy.AddField_management() function documented anywhere, but I noticed it a while back and have found it to be useful.

Of course you would need to be confident that if the field already exists, that it will be of the correct data type, length, etc.

To confirm, I just ran the following code:

print "Adding new field first time"
arcpy.AddField_management(fc, "NewField", "TEXT")
print "Adding new field second time"
arcpy.AddField_management(fc, "NewField", "TEXT")
print "Adding new field third time"
arcpy.AddField_management(fc, "NewField", "TEXT")

and got the following output:

Adding new field first time
Adding new field second time
Adding new field third time

The new field was added to the fc feature class and no errors were reported.

I then ran a model in ModelBuilder that adds a field that already exists and then does a Calculate Field on that same field and it worked fine. I got the output:

Executing (Add Field): AddField C:\Users\<username>\Documents\ArcGIS\Default.gdb\fc NewField TEXT # # # # NULLABLE NON_REQUIRED #
Start Time: Tue Aug 21 11:01:38 2018
WARNING 000012: NewField already exists
Succeeded at Tue Aug 21 11:01:38 2018 (Elapsed Time: 0.08 seconds)
Executing (Calculate Field): CalculateField C:\Users\<username>\Documents\ArcGIS\Default.gdb\fc NewField 'blah' PYTHON #
Start Time: Tue Aug 21 11:01:38 2018
Succeeded at Tue Aug 21 11:01:38 2018 (Elapsed Time: 0.12 seconds)

It's up to you whether you want to be extra safe and do the check anyhow, but at least in version 10.3.1 and 10.5.1 that I've used recently, it works fine this way. It's quite convenient behaviour (at least for this kind of scenario).

If a Field Existence Check Really Is Required

There are some situations when you really do need to check for the existence of a field (eg, you don't want to create a new field, but you do want to do something different depending on if the field exists or not). In this case, I use something like:

if len(arcpy.ListFields(fc, fieldName)):
    #  Field exists.  Do something.
else:
    #  Field does not exist.  Do something else.

Best Answer

Related Solutions

[GIS] Creating custom tool using Python script exported from ModelBuilder

[GIS] Using Python Script tool in ModelBuilder to check if field exists, if not, create one

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