topic Re: Nested Cursor- why is it bad? in Transportation Questions

Nested Cursor- why is it bad?

Anonymous User — Mon, 22 Apr 2013 00:07:41 GMT

Original User: luke.kaim

Hi Everyone,

I have heard nested cursors are bad. Why is that? If I want to test if a point is in a particular polygon wouldn't I need nested cursors?
Something like this would work well. Why is nested cursors not a good idea? Thank you for your help

for row1 in arcpy.SearchCursor(shapefile1):     for row2 in arcpy.SearchCursor(shapefile2):         if row1.shape.contains(row2.shape):             return True

Luke Kaim

Re: Nested Cursor- why is it bad?

DanPatterson_Retired — Mon, 22 Apr 2013 01:59:52 GMT

you ruled out a spatial join?

Re: Nested Cursor- why is it bad?

Anonymous User — Mon, 22 Apr 2013 02:22:27 GMT

Original User: luke.kaim

I did not rule out a spatial join. When I solved this issue with tables I did use a spatial join. But joins do take a long time and I thought a cursor could be faster. Now I want to solve this issue programmatically. Why are nested cursors a bad idea?

Re: Nested Cursor- why is it bad?

curtvprice — Mon, 22 Apr 2013 11:19:15 GMT

I did not rule out a spatial join. When I solved this issue with tables I did use a spatial join. But joins do take a long time and I thought a cursor could be faster. Now I want to solve this issue programmatically. Why are nested cursors a bad idea?

I don't think there's a problem with nested cursors, as long as the datasets accessed inside the loops don't participate in the cursors. A cursor opens a file lock on the table you opened, so if you run a tool that accesses that dataset you are accessing it twice at the same time -- this is what can get you into trouble. Your example does not do this.

Feel free to try this, but if there's a tool that will do the same thing, it's unlikely a cursor would be faster because most standard tools are written in C++, which will usually be much faster than Python.

In my experience, the most effective way to speed up a really slow geoprocessing workflow is to try a different approach that more efficiently solves your problem.