topic Re: how to check if cursor object is empty? in Python Questions

how to check if cursor object is empty?

kyleturner — Thu, 02 Feb 2012 14:15:50 GMT

It seems an easy task, but I can't figure out how to check if the searchcursor object is empty.
I know that it is empty because the query fails....i.e. arcpy.searchcursor(FC, query).
However, even if it (arcpy.searchcursor..) fails, it still returns a geoprocessing cursor object...

rows = gp.SearchCursor(crosswalk_dbf,query)

print rows ---> returns
<geoprocessing cursor object object at 0x029C7CC0>

I can't check if it's empty, because, well, it's not.
And for reasons I obviously don't understand.......

rows = gp.SearchCursor(crosswalk_dbf,query)
for row in rows:
if not row.isNull(fieldname)

------------

is always 'true'.

Please help.

I'm using Arc10 SP3, win 7 64bit.

Many thanks.

Re: how to check if cursor object is empty?

MathewCoyle — Sat, 11 Dec 2021 07:26:26 GMT

I am sure there is a more elegant way, but you could do something simple like...

checkvar = "cursor empty"
rows = arcpy.SearchCursor(fc, query)
for row in rows:
    if checkvar == "cursor empty":
        checkvar = "cursor full"
print checkvar

Here's something probably better, early morning brain not working yet.

row = None
rows = arcpy.SearchCursor(fc, query)
for row in rows:
    #cursor things
if not row:
    print "cursor was empty"

Re: how to check if cursor object is empty?

ChrisSnyder — Sat, 11 Dec 2021 07:26:29 GMT

Maybe this would help?

arcpy.MakeFeatureLayer_management(indxTileFC, indxTileFL, "HCPUNIT_NM <> 'OESF'); showGpMessage()
if int(arcpy.GetCount_management(indxTileFL).getoutput(0)) == 0:
   print "Query returned no records!"
else: #if there are records returned by the query, then run a search cursor and look for Null field values in MY_FIELD
   searchRows = arcpy.SearchCursor(indxTileFC, "HCPUNIT_NM <> 'OESF')
   for searchRow in searchRows:
      if searchRow.MY_FIELD == None: #Another way: if searchRow.isNull("MY_FIELD") == True:
         print "MY _FIELD is Null!"
   del searchRow, searchRows

Re: how to check if cursor object is empty?

kyleturner — Thu, 02 Feb 2012 16:15:28 GMT

Thanks for the help Mathew. (Chris I haven't tried your suggestion yet.)

Still no go.
I even tried a very basic search from the Python cursor. Here's what I tried:

> row, rows = None, None
> query = "Fieldname = '" + "'" variable + "'" --- The quotes are right in my code I know for sure, maybe not here tho.
> table = "C:/path/to/data.dbf
> rows = row.searchcursor(table, query)
> for row in rows:
        if row == None:
           print 'asdf'
        if row == 'None':    ### just to be sure
           print 'jkl'
        if not row == 'None':
          print 'not asdf'
        if not row == None:
          print 'not jkl'

And nothing is printed.   Nothing at all.

What's the deal.

Thanks

Re: how to check if cursor object is empty?

MathewCoyle — Thu, 02 Feb 2012 16:24:42 GMT

You need to copy and paste the exact indentation you are using, that is probably where your error is.

Re: how to check if cursor object is empty?

kyleturner — Sat, 11 Dec 2021 07:26:32 GMT

All:

To help expedite this matter, I attached a very brief snippet of my code along with the necessary table (the table is zipped up because I can't attach a dbf).
This is really starting to make me crazy.

Chris: Your suggestion looks like it's the ticket, but can you 'dumb it down' a bit.

import arcgisscripting
gp = arcgisscripting.create(9.3)
table = 'C:/Users/lilaandmax/Downloads/AttributeCrosswalk.dbf'
####### the following three0FC works 
three0FC = 'AirfieldSurface'
######## but if you do this, three0FC = 'XAirfieldSurface'  ## nothing happens 
query = "Three0FCla = '" + three0FC + "'"
row, rows = None, None
rows = gp.SearchCursor(table, query)

for row in rows:
    if row.isNull("Three0FCla"):
        print "isNull"
    elif row == None:
        print "None"
    elif row == "":
        print "emptyspace"
    else:
        print row.getValue("Three0FCla")

Thanks

Re: how to check if cursor object is empty?

MathewCoyle — Thu, 02 Feb 2012 18:59:59 GMT

If your cursor is empty, it will never step in. Nothing in the loop

for row in rows:

will execute because there is no row in rows, if you follow.

So I tested your code and came back with 4 rows, is that right? i am confused, I thought you wanted to test if the cursor object was empty?

Re: how to check if cursor object is empty?

ChrisSnyder — Sat, 11 Dec 2021 07:26:35 GMT

Sure - see (code comments denoted by the # symbol):

#STEP1: Execute a SQL query (the MakeFeatureLayer tools is probably the quickest way to do this)
arcpy.MakeFeatureLayer_management(indxTileFC, indxTileFL, "HCPUNIT_NM <> 'OESF')
#STEP2: Are any records returned? If 0 records satisfy the SQL query, then let us know! If 0 records are returned, the SQl query is probably incorrect!!!
if int(arcpy.GetCount_management(indxTileFL).getoutput(0)) == 0:
   print "Query returned no records!"
#STEP3: If there are records returned by the query, then run a search cursor and look for Null field values in MY_FIELD
else:
   #Create a search cursor object using the same query as the one in the MakeFeatureLayer 
   searchRows = arcpy.SearchCursor(indxTileFC, "HCPUNIT_NM <> 'OESF')
   #For each row in the collection of search cursor rows
   for searchRow in searchRows:
      #if the value in the "MY_FIELD" field is Null let us know... If theer are no Null values in the MY_FIELD field, then no message will be printed.
      if searchRow.MY_FIELD == None: #Another way: if searchRow.isNull("MY_FIELD") == True:
         print "MY _FIELD is Null!"
   #Delete the search cursor objects so the read lock is released on the search cursor table
   del searchRow, searchRows

Re: how to check if cursor object is empty?

ChrisSnyder — Thu, 02 Feb 2012 19:20:10 GMT

Yes, like Mathew says, your issue is that the SQL statement you provide in your cursor is probably incorrect and is returning 0 records (which is why you get no cursor row object). My code will only run the cursor if your query returns 1 or more records.

Re: how to check if cursor object is empty?

kyleturner — Thu, 02 Feb 2012 19:26:07 GMT

If your cursor is empty, it will never step in. Nothing in the loop
for row in rows:
will execute because there is no row in rows, if you follow.

So I tested your code and came back with 4 rows, is that right? i am confused, I thought you wanted to test if the cursor object was empty?

Duh...I should have tested for row == None outside the for/loop, being sure to set row = None before the call to Searchcursor....duh.
Yes, the code as shown will find something, because "AirfieldSurface" is in the dbf. However, change the FCls30 variable to something like, ZZZ, and see what happens.

Thanks again.

Re: how to check if cursor object is empty?

kyleturner — Fri, 03 Feb 2012 07:20:41 GMT

Thanks for your help gentlemen.

Chris, I tried your getcount_management option, but it seems that you can only use it on a layer/fc. I tried count = int(gp.getcount_management(rows).getoutput(0))......which fails.
I suppose I could have made a layer, but I figured out another way.

Here's how I fixed it:
rows = gp.SearchCursor(fc,query)
row = rows.next()
if not (row == None):
do what you need to do

elif (row == None):
well, then someone needs to fix the dataset.

Thanks again gentlemen for your time.
Cheers.

Re: how to check if cursor object is empty?

GeorgeNewbury — Sat, 11 Dec 2021 07:26:38 GMT

Be careful when you do the 'row = rows.next()' because then you are moving the cursor forward, which could throw off any subsequent looping.

I prefer the method Matthew described:

    row, rows= None, None
    rows= arcpy.SearchCursor(inputFC)
    for row in rows:
        -- do work---
            
    del row, rows

Of course you can put your query clause in there to or do any sorting. If you wanted to know how many records were in there you could add a count:

    rowCount = 0
    row, rows= None, None
    rows= arcpy.SearchCursor(inputFC)
    for row in rows:
        rowCount += 1
        -- do work---
            
    del row, rows

If the count is 0 then rows was empty. It used be to be that you would use the '.next()' method for a while loop, but now with the new syntax ('row in rows') I try to avoid the next method.

-George

Re: how to check if cursor object is empty?

PhilMorefield — Sat, 11 Dec 2021 07:26:40 GMT

Here's another way...

import types
import arcpy as ap

rows = ap.SearchCursor(myTable, sql_statement)
row = rows.next()

if type(row) != types.NoneType:
    # do a bunch of work

Re: how to check if cursor object is empty?

ChrisSnyder — Sat, 11 Dec 2021 07:26:43 GMT

An even shorter way:

if arcpy.SearchCursor(myTable, sql_statement).next() == None:
   print "Empty cursor!"

The only trick though is to figure out if your SQL is returning any records or not... Which could be accomlished through a cursor statment like the one above or a combo of the MakeFeatureLayer/MakeTableView and the GetCount tool. Which one is faster? I dunno... It would also improve performance if the fields in your SQL were properly indexed also.

Re: how to check if cursor object is empty?

BlakeTerhune — Sat, 11 Dec 2021 07:26:45 GMT

I know I'm playing necromancer and raising this discussion from the dead but I was looking for an answer to this question and found my own solution that I thought I would post. Maybe someone else has come up with something better by now.

Thanks to Joshua Bixby‌ for his blog post on cursors, here's what I came up with:

with arcpy.da.SearchCursor(in_table, "*") as s_cursor:
    if sum(1 for row in s_cursor):  ## Counts items in cursor, returns 0 if empty
        s_cursor.reset()  ## Return cursor back to the first row after counting
        for row in s_cursor:
            # Process rows
            print(row)
    else:
        print("Cursor is empty!")‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

The check for rows will iterate through the cursor leaving nothing for the .next() item so you have to .reset() the cursor to start from the beginning again. In my case, I couldn't just count the rows with GetCount_management() in the original feature class because I was using a where clause in the search cursor.

Re: how to check if cursor object is empty?

XanderBakker — Sat, 11 Dec 2021 07:26:48 GMT

To check if a applying a where clause returns no results can also be done with the GetCount tool. Just make a featurelayer and check the result. Below some examples including using the search cursor on empty results, which really is no problem at all.

def main():
    import arcpy
    fc1 = r'D:\Xander\GeoNet\Ellipse\data.gdb\Ellipses'
    fc2 = r'D:\Xander\GeoNet\Ellipse\data.gdb\Ellipses_empty'

    # get count on a featureclass
    cnt1 = getCount(fc1)
    cnt2 = getCount(fc2)

    print "fc1 has {0} features".format(cnt1)
    print "fc2 has {0} features".format(cnt2)

    # Cursor on a featureclass with features
    max_oid = None
    with arcpy.da.SearchCursor(fc1, ('OID@')) as curs:
        for row in curs:
            oid = row[0]
            if max_oid is None:
                max_oid = oid
            elif oid > max_oid:
                max_oid = oid
    print "max_oid for fc1:", max_oid

    # Cursor on a featureclass without features
    max_oid = None
    with arcpy.da.SearchCursor(fc2, ('OID@')) as curs:
        for row in curs:
            oid = row[0]
            if max_oid is None:
                max_oid = oid
            elif oid > max_oid:
                max_oid = oid
    print "max_oid for fc2:", max_oid


    # get count on a feature layer (with where clause)
    where = "Ellipse = 1"
    fl1 = arcpy.MakeFeatureLayer_management(fc1, "flay1", where)
    fl2 = arcpy.MakeFeatureLayer_management(fc2, "flay2", where)
    cnt1 = getCount(fl1)
    cnt2 = getCount(fl2)

    print "fl1 has {0} features".format(cnt1)
    print "fl2 has {0} features".format(cnt2)


   # Cursor on a featurelayer (where clause) with features
    max_oid = None
    with arcpy.da.SearchCursor(fl1, ('OID@')) as curs:
        for row in curs:
            oid = row[0]
            if max_oid is None:
                max_oid = oid
            elif oid > max_oid:
                max_oid = oid
    print "max_oid for fl1:", max_oid


    # Cursor on a featureclass (where clause) with features, but without results
    where = "Ellipse = -1" # there are no features witch match this where clause
    max_oid = None
    with arcpy.da.SearchCursor(fc1, ('OID@'), where) as curs:
        for row in curs:
            oid = row[0]
            if max_oid is None:
                max_oid = oid
            elif oid > max_oid:
                max_oid = oid
    print "max_oid for fc1 (with where clause, no match):", max_oid


def getCount(fc):
    return int(arcpy.GetCount_management(fc).getOutput(0))

if __name__ == '__main__':
    main()
‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

This will print:

fc1 has 722 features
fc2 has 0 features
max_oid for fc1: 722
max_oid for fc2: None
fl1 has 361 features
fl2 has 0 features
max_oid for fl1: 361
max_oid for fc1 (with where clause, no match): None‍‍‍‍‍‍‍‍

Re: how to check if cursor object is empty?

JoshuaBixby — Sat, 11 Dec 2021 07:26:51 GMT

As Xander Bakker‌ points out, the geoprocessing tool approach to addressing your issue involves creating a feature layer and counting that, and then passing the feature layer to the search cursor if there are results.

Looking at it from purely a Python perspective, i.e., not involving geoprocessing tools, the major issue with the code provided is that it completely consumes the iterable to determine if there is just one or more records. There are several ways in Python to check if an iterable is empty and then do one thing or another depending on the results.

One approach that doesn't fully consume the iterable to check for the existence of items uses next() with a sentinel value:

s_cursor = arcpy.da.SearchCursor(in_table, "*")
with s_cursor:
    if next(s_cursor, None) is not None:
        s_cursor.reset()
        for row in s_cursor:
            print row
    else:
        print "Cursor is empty!"‍‍‍‍‍‍‍‍

Another approach sets the variable for holding what is returned by the iterable to a sentinel value and then checks after processing the iterable to see if anything was processed:

row = None
with s_cursor:
    for row in s_cursor:
        print row
    if row is None:
        print "Cursor is empty!"‍‍‍‍‍‍

Re: how to check if cursor object is empty?

BruceHarold — Thu, 25 Aug 2016 21:43:15 GMT

Another one is wrapping the cursor in a generator function and seeing if StopIteration is raised when you request a row.

Re: how to check if cursor object is empty?

JoshuaBixby — Thu, 25 Aug 2016 21:50:54 GMT

Since the cursor itself will raise a StopIteration error if there isn't a row to request, I guess I don't understand the value of wrapping it in a generator function to raise a StopIteration error.

Re: how to check if cursor object is empty?

BruceHarold — Fri, 26 Aug 2016 17:15:38 GMT

Using a generator function is a pattern I find useful so I thought I would mention it.

Handling StopIteration lets you automate returning something you design, like below I return a dictionary if the cursor is empty or not.

# Define row generator
def rowGenerator(table,fields,sortField=None):
    if sortField is not None:
        sql = [None,'ORDER BY ' + sortField]
        cursor = arcpy.da.SearchCursor(table,fields,sql_clause=sql)
    else:
        cursor = arcpy.da.SearchCursor(table,fields)
    with cursor:
        for row in cursor:
            yield row

# Create row generator
if postalField:
rowGen = rowGenerator(inTable,cursorFields,postalField)
else:
rowGen = rowGenerator(inTable,cursorFields)

# Define recordset builder
def recordsetBuilder(rowgen):
    records = []
    for i in range(0,1000):
        try:
            data = next(rowgen)
            attributes = {'attributes':{recordFields:data
                                        for i in range(len(data))}}
            records.append(attributes)
        except StopIteration:
            break
    return {'records':records}