topic Re: New to Python in Python Questions

New to Python

ChrisGlenn — Tue, 14 Mar 2023 21:21:01 GMT

I am having trouble with a file path or the format of the destination I believe. I am very new to Python and have pieced this together with a lot of questions without a strong understanding of ArcGIS yet.

# Imports import arcpy # Workspace trail_data = r"C:\PythonPro\Final\Trail" county_data = r"C:\PythonPro\Final\Counties\Oregon_Counties.shp" # Get a list of unique county names counties = [] with arcpy.da.SearchCursor(county_data, "County_Name") as cursor: for row in cursor: if row[0] not in counties: counties.append(row[0]) # Print the list of counties and ask the user to choose one print("Choose a county: ") for i, county in enumerate(counties): print(f"{i+1}. {county}") selection = int(input("> ")) - 1 selected_county = counties[selection] print("Measuring...") # Use the selected county to clip the trail data clip_output = "C:/data/clipped_trails.shp" where_clause = f"County_Name = '{selected_county}'" arcpy.Clip_analysis(trail_data, county_data, clip_output, where_clause) # Calculate the total distance of bike trails in the clipped data total_distance = 0 with arcpy.da.SearchCursor(clip_output, "SHAPE@LENGTH") as cursor: for row in cursor: total_distance += row[0] # Display the total distance to the user print(f"The total distance of bike trails in {selected_county} is {total_distance:.2f} meters.")

The error code I am getting is: RuntimeError: Cannot find field 'County_Name'

I have tried to change the source and the configuration of the files on my PC. I suspect that the problem is something simple that I am missing, but I am banging my head against a wall and can't see it clearly. Any help is appreciated. Thanks.

Chris

Re: New to Python

DavidPike — Tue, 14 Mar 2023 20:00:11 GMT

Please format your code for readability, it's also lost its indentation. https://community.esri.com/t5/python-blog/code-formatting-the-community-version/ba-p/1007633

Do you know what line it's erroring on? that would save a lot of debugging time. Perhaps comment out each section and run it sequentially uncommenting lines to see where the error is.

##what is trail data? Looks like a workspace/folder but you're using it as a clip input? trail_data = r"C:\PythonPro\Trail" arcpy.Clip_analysis(trail_data, county_data, clip_output, where_clause)

Re: New to Python

DanPatterson — Tue, 14 Mar 2023 20:01:52 GMT

Code formatting ... the Community Version - Esri Community

"county_data" is a folder, so if it is a geodatabase, then you need to provide the gdb name and the featureclassname. If it is a shapefile you need the *.shp

eg county_data = r"C:\PythonPro\Counties.gdb\MyFeatureclass"

Re: New to Python

Mahdi_Ch — Tue, 14 Mar 2023 21:42:17 GMT

I think since it is not obvious that what type of file you are reading (shape file, geodatabase, etc) it is not easy to go further in troubleshooting as that might be the initial issue.

Then the second issue might be the field (column header) name that you are passing does not exist. You can check for that like this:

fields = arcpy.ListFields(country_data) # list of objects field_names = [i.name for i in fields] # list of field names #check if 'Country_Name' is in the list of names # instead of one field name you can also pass a list of them. if 'County_Name' in field_names: counties = [] with arcpy.da.SearchCursor(county_data, "County_Name") as cursor: for row in cursor: if row[0] not in counties: counties.append(row[0]) else: print('County_Name not in the fields')

Re: New to Python

Mahdi_Ch — Wed, 15 Mar 2023 04:33:59 GMT

By the way , assuming the 'County_Name' exists, if you are looping to get the unique values of that field, there is a faster (at least faster to write) cleaner Pythonic way, by using set.

A set is an unordered collection with no duplicate elements. So basically, instead of going line by line and assigning the values after checking if they already exist, you can use a "set comprehension" to add all the values to the set function. And then Python set() do the work behind the scene and returns only the unique values for you.

with arcpy.da.SearchCursor(county_data, "County_Name") as cursor: # extracting unique values in the field using Python set function and set comprehension counties= sorted({row[0] for row in cursor})

Then the sorted function -obviously- sorts the values and returns them in a list.

Please follow what @DavidPike sent you to make your code readable.

Re: New to Python

DavidPike — Tue, 14 Mar 2023 20:53:58 GMT

@Mahdi_Ch I think it would be useful if you explained in the code comments the logic of writing to a set, would be a good thing to understand for beginners.

Re: New to Python

ChrisGlenn — Tue, 14 Mar 2023 21:10:27 GMT

Re: New to Python

ChrisGlenn — Tue, 14 Mar 2023 21:16:10 GMT

Apologies for the incorrect format. Here is what I have. I've tried to download the layer I am using as a .shp and .gdb and have had similar issues.

Line 17 is where the error code comes from.

Re: New to Python

DavidPike — Tue, 14 Mar 2023 21:40:37 GMT

Line 17 doesn't make much sense. the error is probably coming from your search cursor. Are you sure county_name exists in the data rather than being some result of a join on layer object? Are you also sure county_name is the field name rather than an alias?

Re: New to Python

Mahdi_Ch — Tue, 14 Mar 2023 21:57:45 GMT

David, you are totally right. I edited my post and included the comments. Thank you for mentioning that.

Re: New to Python

Anonymous User — Wed, 15 Mar 2023 00:54:11 GMT

@Mahdi_Ch
A set is an unordered collection with no duplicate elements. So basically, instead of going line by line and assigning the values after checking if they already exist, you can use a "set comprehension" to add all the values to the set function. And then Python set() do the work behind the scene and returns only the unique values for you.

Nice explanation! Now one line it! 🙂

counties = list(set([row[0] for row in arcpy.da.SearchCursor(fc, "County_Name")]))

Re: New to Python

DavidPike — Wed, 15 Mar 2023 00:57:32 GMT

It's already a set, and sort() produces a list from the set.

Re: New to Python

Anonymous User — Wed, 15 Mar 2023 01:26:37 GMT

I'm aware- sorry if that was misguided... I was just showing an all inclusive version of the cursor with list comprehension.

Re: New to Python

Mahdi_Ch — Wed, 15 Mar 2023 05:07:30 GMT

@Anonymous User that was hilarious! So, waiting for the next person to do that in zero line! 😂

Jokes aside, that's cool. And I love list comprehensions. I used to write pretty long ones but after a while I realized that there are certain problems associated with long list comprehensions.

First, after a certain point it negatively affects the readability and it becomes harder to read and understand the logic.
Second, in case of errors, since a lot is happening inside the list -without making intermediate variables available to us- that makes it very hard to debug.

I thought it worth mentioning for those who just learned about this concept.

BTW, If you want to go rogue on that, you can throw some ifs and lambdas there and then watch it burn. 😁