Why [0] is needed in row[0] when using ArcPy (Data Access) cursors?

Question

when running the following 2 sample codes, "[0]" is needed to be added after "row" in order to make code run well. I was confused. can anyone help explain? I know that it might be too easy for most of you from here.

sample code #1:

sf = r"C:\Data\Exercise07\alaska.shp"
cursor = arcpy.da.UpdateCursor(sf,["PERIMETER"])
for row in cursor:
...     if row[0] == 0.224:
...         print "find"

sample code #2:

cursor =arcpy.da.SearchCursor("alaska",["PERIMETER"],'"PERIMETER"=0.224')
for row in cursor:
...     print row[0]

Answer 1

In both your examples cursor is a Cursor object that consists of many Row objects.

You use the for loop to iterate through the Row objects which you have called row.

The Row object returned by arcpy.da.UpdateCursor() is a list of values. Understanding lists is a Python rather than ArcPy topic but think of it as comma-separated values enclosed in square brackets so if you print yours it will look like [123.456] (for one row) because it is a list of one floating point item.

To see this try running this code that I just tested at ArcGIS 10.4:

import arcpy

sf = r"C:\Data\Exercise07\alaska.shp"
cursor = arcpy.da.UpdateCursor(sf,["PERIMETER"])
for row in cursor:
    print row

If you loaded three fields (text, integer, text) into your cursor then your printed row might look something like ["A",23,"Road"].

Whenever you have a list (or a tuple, which is very similar) you use an index to get at its items.

If your row has only one item then you get at that using an index value of 0 i.e. row[0]. In the "list of 3" example to get at the second and third values I would use row[1] and row[2] respectively.

As identified by @AlexTereshenkov Search Cursors return tuples whereas Update Cursors return lists so note that my example uses Update Cursors but is quite similar to Search Cursors.

Answer 2

row[0] simply refers to the first field in your list of fields in the cursor. Since you only have one field in your list, "PERIMETER", then row[0] refers to that field. If you had multiple fields, ["PERIMETER","AREA","POPULATION"] that were being searched or updated in your cursors, then

row[0] would be "PERIMETER"
row[1] would be "AREA"
row[2] would be "POPULATION"

It is simply how python access elements in a list.

Answer 3

Great answers. But it looks as if the row itself was not shown in the answers.

Another thing that is good to know is that when a single field is supplied, a singleton tuple is returned (row in this case):

with arcpy.da.SearchCursor(fc,"PROPERTY_ID") as search_cur:
    for row in search_cur:
        print row
        break

 >>> (5001,)

It's easy to understand that this is a singleton tuple by looking at the trailing comma it has. By the way, you don't need to supply a list of fields when you have just one field. This line would also work for you:

cursor = arcpy.da.SearchCursor("alaska","PERIMETER",'"PERIMETER"=0.224')

Please use the with statement when constructing the cursors; this will handle cleaning up and deleting the objects when they've been used and are no longer needed. Take a look at the Esri samples here and other samples here.

A handy way to access all of the features within a feature class without using the for iteration, is a list comprehension:

feats = [feat for feat in arcpy.da.SearchCursor(fc,"PROPERTY_ID")]
#printing first three elements
print feats[:3]
>>> [(5001,), (5002,), (1003,)]

The feats can be referred to as a list of singleton tuples. Fairly often, you would like to get a list of those values without having tuples at all. Again, list comprehension comes in very handy.

feats = [feat[0] for feat in arcpy.da.SearchCursor(fc,"PROPERTY_ID")]
#printing first three elements
print feats[:3]
>>> [5001, 5002, 1003]

The feat[0] returns a first item in the singleton tuple.