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Referencing Shapefiles in Python and Printing Shapefile Entries

RachaelJohnson

I am a complete Python novice and I have been searching and searching and so far, I am unable to find a straightforward answer to my question.

I want to write a statement that checks if a shapefile has a field "Area", and if not, make one. This is what I have:

outline="LRoutline"

if "Area" not in outline:

arcpy.AddField_management(outline, "Area", "FLOAT", 6, 3)

The If statement isn't working. When I run it again, the code gets hung up because there is already a field named "Area". Obviously, the code is not looking if there is a field named "Area" but rather it is looking for that string in the variable. How do I reference the fields inside a shapefile within an If statement? outline.Area doesn't work and neither does !Area!. I have tried the SearchCursor thing using the following code and it outputs <Cursor object at 0x137ed0f0[0x13587f80]>, and I don't know what that means.

arcpy.SearchCursor(outline)

Furthermore, I want to sum the areas in that Area field after they are calculated, so I need to be able to reference the field, right? I looked at the arcpy.Statistics_analysis() function and I need to create a whole separate table to pull one value from the shapefile? That's not required to view the summary statistics in the ArcGIS GUI (right click on field in attribute table-> Summary...). Is there a less cluttered way to sum the entries in a shapefile field in Python?

I have downloaded the shapefile package but that doesn't seem to fit what I need either.

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Accepted answers

DarrenWiens2

Make your list of fields using ListFields. If you're first learning cursors, you should skip to using the arcpy.da.SearchCursor. Both links contain examples at the bottom.

All comments

DanPatterson_Retired

Thanks Blake...I edited my post to reflect this

blake.terhune

Yes, that's correct. I think what Dan is getting at is that although ListFields returns an iterator, the field objects it's iterating can't be compared directly, you have to call the property of the field object; like with .name

RachaelJohnson

>>> 'Id' in arcpy.ListFields(shp_file)

I don't understand why that returns False. Is it because arcpy.ListFields(shp_file) just returns: [<Field object at 0x5498250[0x5444ad0]>, <Field object at 0x5498110[0x5444908]>, <Field object at 0x54981b0[0x5444a40]>, <Field object at 0x5498190[0x5444848]>] ?

blake.terhune

It's referring to an index of the list of values in the row variable. The search cursor returns each row as a tuple (which is a kind of list in Python). The number of indexes in that tuple is equal to the number of fields you used in the arguments to create the cursor. In this case, there's only one field so it returns a tuple with only one value as

(23456.93426,)

The comma in there is important because a single item tuple still has a comma afterwards (unlike a list).

So when I say row[0], it's getting the value of the first item, which is also known as index 0. Index 1 would be the second item, and so on. If I just said row, it would return all field values in the whole row as a tuple instead of just the single value as a number.

DanPatterson_Retired

Rachel .... for a python moment...strings are iterables, just as lists are. To see where the pitfalls can arise, examine the following code segments:

>>> case_1='LRoutline'
>>> for i in case_1: print i
>>> print 'line' in case_1
>>> print 'Area' in case_1

Line 1 case_1 is defined as your input field

Line 2 let's demonstrate that it is an interable...result

>>> for i in case_1: print i
... 
L
R
o
u
t
l
i
n
e
>>>

Line 3 let's see if sequences can be found there... result

>>> print 'line' in case_1
True
>>>

Line 4 let's assume you think it is a file and want the Area field

>>> print 'Area' in case_1
False
>>>

Now for a more practical example...

>>> import arcpy
>>> shp_file = 'c:/temp/cube.shp'
>>> for field in arcpy.ListFields(shp_file): print field.name
... 
FID
Shape
Id
>>> 'Id' in arcpy.ListFields(shp_file)
False
>>>

Now the above produces a false because as pointed out in a subsequent post you need the field.name not the field itself.

>>> shp_file = 'c:/temp/cube.shp' 
>>> field_names = [field.name for field in arcpy.ListFields(shp_file)]
>>> field_names
[u'FID', u'Shape', u'Id']
>>> 'Id' in field_names
True
>>>

Hope this helps for the future.

RachaelJohnson

Thank you! Would you mind helping me understand that code? What is the purpose of the row[0]? Is it telling the computer to start with the first entry in the "Area" field? Or is it creating a list? Or something else?

blake.terhune

The regular (non da) cursors are also older and will eventually get depreciated by Esri. The new ones are more future proof.

blake.terhune

To expand on what James Crandall posted and accomplish your second task of getting the total area for all features in the shapefile, you could use a SearchCursor like Darren Wiens mentioned, or the Summary Statistics that you mentioned yourself. If you go with the Summary Statistics, you could use in-memory workspace for the output table, but you'd still have to use a SearchCursor to get the values out of it, so you might as well just do your own adding.

import arcpy

fc = r'PathToLayer\LayerNamel'
field_names = [f.name for f in arcpy.ListFields(fc)]
if 'Area' not in field_names:
    arcpy.AddField_management(fc, "Area", "FLOAT", 6, 3)

arcpy.CalculateField_management(
    fc,
    "Area",
    "!SHAPE.area!",
    "PYTHON_9.3",
)

fcAreas = [row[0] for row in arcpy.da.SearchCursor(fc, ["Area"])]
print sum(fcAreas)

EDIT:

After thinking about this, it might actually be better if you used Summary Statistics instead of calculating the field because if the geometry of the features change, the Area field will not get updated unless you run this code again. The values in the Area field could be misleading.

DarrenWiens2

Data module cursors are much faster and provide access to geometry tokens. Probably other unknown benefits.

RachaelJohnson

Thank you so much for pointing me to ListFields and arcpy.da.SearchCursor! How is da.SearchCursor different from regular SearchCursor?

Anyway, I will see what I can do with these two commands.

JamesCrandall

fc = r'PathToLayer\LayerNamel'
field_names = [f.name for f in arcpy.ListFields(fc)]
if 'Area' not in field_names:
    arcpy.AddField_management(fc, "Area", "FLOAT", 6, 3)

Resources:

List Fields ArcGIS Help (10.2, 10.2.1, and 10.2.2)

AddFields ArcGIS Help (10.2, 10.2.1, and 10.2.2)

RachaelJohnson

I actually just copy/pasted from PyCharm and it looked like that without any effort! But thank you.

blake.terhune

It looks like you spent a lot of time formatting your Python code to highlight syntax like the IDE. There's a handy way to let the GeoNet forums do that for you!

Posting Code blocks in the new GeoNet

DarrenWiens2

Make your list of fields using ListFields. If you're first learning cursors, you should skip to using the arcpy.da.SearchCursor. Both links contain examples at the bottom.

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