topic Re: If else statement in Python Questions

If else statement

JamesSmith7 — Wed, 28 Aug 2013 11:39:13 GMT

I am modifying a script suggested in a prior thread. http://forums.arcgis.com/threads/90242-Polygon-Centroid?highlight=jsmith0705

I want to be able to select a line, determine if the ARCLENGTH field has a value greater than zero, return that value if greater than zero, if not return the value from the Shape_Length field.

The script runs with no errors or results.

import arcpy  mxd = arcpy.mapping.MapDocument ("CURRENT") df = arcpy.mapping.ListDataFrames (mxd)[0]  lyr = arcpy.mapping.ListLayers(mxd, "Lot_Lines", df)[0]  for lyr in arcpy.mapping.ListLayers(mxd):  tlyr = lyr  dsc = arcpy.Describe(tlyr)  sel_set = dsc.FIDSet  if dsc.shapeType == "Line":   if len(sel_set) > 0: #If ARCLENGTH value > 0           arcpy.AddMessage(str(ARCLENGTH)) #Return ARCLENGTH value   else:    arcpy.AddMessage(str(Shape_Length)) #Return Shape_Length value

Re: If else statement

MichaelVolz — Wed, 28 Aug 2013 12:15:08 GMT

Try adding some print statements to understand what is actually occurring with your script.

print("The describe statement is returning " + dsc) - You mau need to convert dsc to a string if it is not by default a string

If that returns an expected item then I would continue to add print statements at each critical step to ensure you are getting the expected results.

Please provide results of your print statements so I can further assist you.

Re: If else statement

JakeSkinner — Sun, 12 Dec 2021 08:22:42 GMT

Hi James,

If this is a feature class, try the following below. If this is a shapefile, you will have to update the SearchCursor to use FID instead of OBJECTID.

for lyr in arcpy.mapping.ListLayers(mxd):
    tlyr = lyr
    dsc = arcpy.Describe(tlyr)
    sel_set = dsc.fidSet
    if dsc.shapeType == "Polyline":
        rows = arcpy.SearchCursor(tlyr, "OBJECTID = " + sel_set)
        for row in rows:
            arcLength = row.ARCLENGTH
            shapeLength = row.shape.length
        if arcLength > 0:
            print arcLength
        else:
            print shapeLength
        del row, rows

Re: If else statement

JamesSmith7 — Sun, 12 Dec 2021 08:22:44 GMT

Hi James,

If this is a feature class, try the following below. If this is a shapefile, you will have to update the SearchCursor to use FID instead of OBJECTID.

for lyr in arcpy.mapping.ListLayers(mxd):
    tlyr = lyr
    dsc = arcpy.Describe(tlyr)
    sel_set = dsc.fidSet
    if dsc.shapeType == "Polyline":
        rows = arcpy.SearchCursor(tlyr, "OBJECTID = " + sel_set)
        for row in rows:
            arcLength = row.ARCLENGTH
            shapeLength = row.shape.length
        if arcLength > 0:
            print arcLength
        else:
            print shapeLength
        del row, rows

Jake and Michael,

I am using a feature class. The script runs with no errors or results. I am going to try and adjust the SearchCursor and see what happens.

Re: If else statement

JamesSmith7 — Wed, 28 Aug 2013 12:58:21 GMT

Why would changing the SearchCursor to arcpy.da.SearchCursor return an SQL error, whereas, arcpy.SearchCursor does not? I thought the only difference was arcpy.da.SearchCursor was more efficient?

Re: If else statement

JakeSkinner — Wed, 28 Aug 2013 13:02:48 GMT

Take a look at the help links. The arcpy.da.SearchCursor is formatted differently.

arcpy.SearchCursor
http://resources.arcgis.com/en/help/main/10.2/index.html#//018v00000050000000

arcpy.da.SearchCursor
http://resources.arcgis.com/en/help/main/10.2/index.html#//018w00000011000000

Can you post the exact code you're using in ArcMap?

Re: If else statement

JamesSmith7 — Sun, 12 Dec 2021 08:22:47 GMT

import arcpy

mxd = arcpy.mapping.MapDocument ("CURRENT")
df = arcpy.mapping.ListDataFrames (mxd)[0] 
lyr = arcpy.mapping.ListLayers(mxd, "Lot_Lines", df)[0]

for lyr in arcpy.mapping.ListLayers(mxd):
 tlyr = lyr
 dsc = arcpy.Describe(tlyr)
 sel_set = dsc.fidSet
 if dsc.shapeType == "Polyline":
  rows = arcpy.da.SearchCursor(tlyr, "OBJECTID = " + sel_set)
  for row in rows:
   arcLength = row.ARCLENGTH
   shapeLength = row.Shape_Length
  if arcLength > 0:
   print arcLength
  else:
   print shapeLength
  del row
  del rows

The error is in line 32, for row in rows: RuntimeError: An invalid SQL statement was used. I know the select Object ID works, because it changes values depending on the feature selected.

Re: If else statement

JakeSkinner — Wed, 28 Aug 2013 13:15:45 GMT

Check the name of your OBJECTID field (right-click on layer > Properties > Fields tab). It may be OID, or OBJECTID1.

Re: If else statement

JakeSkinner — Wed, 28 Aug 2013 13:20:48 GMT

Also, change arcpy.da.SearchCursor, to arcpy.SearchCursor.

Re: If else statement

JamesSmith7 — Wed, 28 Aug 2013 13:22:10 GMT

Check the name of your OBJECTID field (right-click on layer > Properties > Fields tab). It may be OID, or OBJECTID1.

The field is named OBJECTID. Changing the cursor back to arcpy.SearchCursor eliminates the SQL Error. But, the script runs with no results.

Re: If else statement

MichaelVolz — Wed, 28 Aug 2013 13:41:04 GMT

Have you checked to see if some of your earlier statements are getting the desired results such as the Describe?

Can you get a length of your sel_set to see if that is returning values?

Re: If else statement

JamesSmith7 — Wed, 28 Aug 2013 13:48:24 GMT

Have you checked to see if some of your earlier statements are getting the desired results such as the Describe?

Can you get a length of your sel_set to see if that is returning values?

Michael,

I have used the arcpy.AddMessage. I noticed that when the AddMessage is inserted after the variables it returns the value. But, when I use the AddMessage after the sel_set = dsc.fidSet, the value is returned three times. Which is odd. I need to research why the value is returned three times.

Re: If else statement

RichardFairhurst — Sun, 12 Dec 2021 08:22:50 GMT

import arcpy

mxd = arcpy.mapping.MapDocument ("CURRENT")
df = arcpy.mapping.ListDataFrames (mxd)[0] 
lyr = arcpy.mapping.ListLayers(mxd, "Lot_Lines", df)[0]

for lyr in arcpy.mapping.ListLayers(mxd):
 tlyr = lyr
 dsc = arcpy.Describe(tlyr)
 sel_set = dsc.fidSet
 if dsc.shapeType == "Polyline":
  rows = arcpy.da.SearchCursor(tlyr, "OBJECTID = " + sel_set)
  for row in rows:
   arcLength = row.ARCLENGTH
   shapeLength = row.Shape_Length
  if arcLength > 0:
   print arcLength
  else:
   print shapeLength
  del row
  del rows

The error is in line 32, for row in rows: RuntimeError: An invalid SQL statement was used. I know the select Object ID works, because it changes values depending on the feature selected.

When you switched back to the original cursor type you got no records because your where clause needs to use the IN operator not the = operator.

The error using the new arcpy.da cursor is due to the cursor declaration syntax being different. With the arcpy data access (arcpy.da) cursor you have to supply a mandatory field name list prior to defining an optional where clause. Your code has no field list. At a minimum the field list would have to include OBJECTID, ARCLENGTH and Shape_Length. So this code:

rows = arcpy.da.SearchCursor(tlyr, "OBJECTID = " + sel_set)

should be changed to this code:

rows = arcpy.da.SearchCursor(tlyr, ["OID@", "ARCLENGTH", "SHAPE@LENGTH"], "OBJECTID IN (" + sel_set +")")

OID@ makes sure you get the correct ObjectID field name reference, regardless of the database. Possibly SHAPE@LENGTH should be Shape_Length, depending on whether you want to access a geodatabase length field or the actual shape geometry length. You also do not have to use field names again after getting the cursor. You can use numbers to represent the fields. So that means the code beyond that line could also change to:

  rows = arcpy.da.SearchCursor(tlyr, ["OID@", "ARCLENGTH", "SHAPE@LENGTH"], "OBJECTID IN (" + sel_set + ")")
  for row in rows:
   arcLength = row.[1]
   shapeLength = row.[2]
  if arcLength > 0:
   print arcLength
  else:
   print shapeLength
  del row
  del row

Re: If else statement

JamesSmith7 — Sun, 12 Dec 2021 08:22:52 GMT

Why are your rows numbered 1 and 2? Should they be 0 and 1. Since, python normally begins the numbering sequence with zero.

When you switched back to the original cursor type you got no records because your where clause needs to use the IN operator not the = operator.

The error using the new arcpy.da cursor is due to the cursor declaration syntax being different. With the arcpy data access (arcpy.da) cursor you have to supply a mandatory field name list prior to defining an optional where clause. Your code has no field list. At a minimum the field list would have to include OBJECTID, ARCLENGTH and Shape_Length. So this code:

rows = arcpy.da.SearchCursor(tlyr, "OBJECTID = " + sel_set)

should be changed to this code:

rows = arcpy.da.SearchCursor(tlyr, ["OID@", "ARCLENGTH", "SHAPE@LENGTH"], "OBJECTID IN (" + sel_set +")")

OID@ makes sure you get the correct ObjectID field name reference, regardless of the database. Possibly SHAPE@LENGTH should be Shape_Length, depending on whether you want to access a geodatabase length field or the actual shape geometry length. You also do not have to use field names again after getting the cursor. You can use numbers to represent the fields. So that means the code beyond that line could also change to:
  rows = arcpy.da.SearchCursor(tlyr, ["OID@", "ARCLENGTH", "SHAPE@LENGTH"], "OBJECTID IN (" + sel_set + ")")
  for row in rows:
   arcLength = row.[1]
   shapeLength = row.[2]
  if arcLength > 0:
   print arcLength
  else:
   print shapeLength
  del row
  del row

Re: If else statement

JamesSmith7 — Sun, 12 Dec 2021 08:22:55 GMT

Thanks for the explanations, information, and suggested script.

I want to use the Shape_Length field, which is the actual geometry length.

The script runs with added arcpy.AddMessages throughout. Yet, the script does not print the ARCLENGTH or Shape_Length value.

When you switched back to the original cursor type you got no records because your where clause needs to use the IN operator not the = operator.

The error using the new arcpy.da cursor is due to the cursor declaration syntax being different. With the arcpy data access (arcpy.da) cursor you have to supply a mandatory field name list prior to defining an optional where clause. Your code has no field list. At a minimum the field list would have to include OBJECTID, ARCLENGTH and Shape_Length. So this code:

rows = arcpy.da.SearchCursor(tlyr, "OBJECTID = " + sel_set)

should be changed to this code:

rows = arcpy.da.SearchCursor(tlyr, ["OID@", "ARCLENGTH", "SHAPE@LENGTH"], "OBJECTID IN (" + sel_set +")")

OID@ makes sure you get the correct ObjectID field name reference, regardless of the database. Possibly SHAPE@LENGTH should be Shape_Length, depending on whether you want to access a geodatabase length field or the actual shape geometry length. You also do not have to use field names again after getting the cursor. You can use numbers to represent the fields. So that means the code beyond that line could also change to:
  rows = arcpy.da.SearchCursor(tlyr, ["OID@", "ARCLENGTH", "SHAPE@LENGTH"], "OBJECTID IN (" + sel_set + ")")
  for row in rows:
   arcLength = row.[1]
   shapeLength = row.[2]
  if arcLength > 0:
   print arcLength
  else:
   print shapeLength
  del row
  del row

Re: If else statement

MichaelVolz — Wed, 28 Aug 2013 14:53:04 GMT

How many features do you have in your target feature class?

If you print out the fidset, are all of these features included in the semicolon-delimited string?

Can you post the results of printing the fidset from your dsc object?

Re: If else statement

RichardFairhurst — Wed, 28 Aug 2013 14:56:17 GMT

How many features do you have in your target feature class?

If you print out the fidset, are all of these features included in the semicolon-delimited string?

Can you post the results of printing the fidset from your dsc object?

If the fidset is delimited with semicolons it will have to be changed so that the fid values are delimited by commas for the where clause SQL to work. The SQL IN operator only works with lists of values separated by commas. Nothing in SQL works with semicolon separated lists.

Re: If else statement

JamesSmith7 — Wed, 28 Aug 2013 15:07:55 GMT

How many features do you have in your target feature class?

If you print out the fidset, are all of these features included in the semicolon-delimited string?

Can you post the results of printing the fidset from your dsc object?

The feature class has over a million records.

Printing that many records will take quite a while.

The arcpy.AddMessage("Continue") value was returned.

Re: If else statement

MichaelVolz — Wed, 28 Aug 2013 15:23:05 GMT

James:

Can you create a feature class with a small subset of features (say 10) and then print out the fidset?

It appears from Richard's last post that the semi-colon delimited string will need to be converted to a comma separated list. You shouldbe able to find python methods to easily perform that process.

Re: If else statement

RichardFairhurst — Wed, 28 Aug 2013 15:48:48 GMT

The feature class has over a million records.

Printing that many records will take quite a while.

The arcpy.AddMessage("Continue") value was returned.

If there are a million FID values placed within the IN clause I don't know that there won't be an error. Try it on a much smaller set first. I suspect the cursor won't accept a string that big that it could pass to the query parser and performance would be horrible.

If it is going to process the entire table don't include a where clause. If it is a selection based on some grouped attribute values include those fields in the field list and write a logical SQL expression that gets the selection. If you are doing it based on a spatial query and get a huge list, perhaps try feeding portions of the OID list in a for loop to the query expression if an error gets thrown or performance drops too drastically.