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How to access Field Names

VishalShah2

So I am working on a custom tool that uses one of the existing fields to make a selection by attribute. For example, say I have a feature called Target Feature and in the feature, the fields were called Field1, Field2, Field3, and Field4. I am trying to get a variable to be set equal to arcpy.ListFields('Target Feature') and then to check if a field name is in that feature. The selection I want wasn't working, no selection was being made actually. Here's what I was leaning towards but it didn't work for me:

<SPAN class="keyword token">import</SPAN> arcpy

fieldList <SPAN class="operator token">=</SPAN> arcpy<SPAN class="punctuation token">.</SPAN>ListFields<SPAN class="punctuation token">(</SPAN><SPAN class="string token">'Target Feature'</SPAN><SPAN class="punctuation token">)</SPAN>
fieldName <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">'Field3'</SPAN><SPAN class="punctuation token">]</SPAN>
fieldValue <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">'%45B%'</SPAN><SPAN class="punctuation token">]</SPAN>

<SPAN class="keyword token">if</SPAN> fieldName <SPAN class="keyword token">in</SPAN> fieldList<SPAN class="punctuation token">:</SPAN>
&nbsp;&nbsp;&nbsp; <SPAN class="keyword token">if</SPAN> fieldValue <SPAN class="keyword token">in</SPAN> fieldName<SPAN class="punctuation token">:</SPAN>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; arcpy<SPAN class="punctuation token">.</SPAN>SelectLayerByAttribute_management <SPAN class="punctuation token">(</SPAN><SPAN class="string token">'Target Feature'</SPAN><SPAN class="punctuation token">,</SPAN> NEW_SELECION<SPAN class="punctuation token">,</SPAN> <SPAN class="string token">"Field3 LIKE "</SPAN> <SPAN class="operator token">+</SPAN> <SPAN class="string token">"'"</SPAN> <SPAN class="operator token">+</SPAN> str<SPAN class="punctuation token">(</SPAN>fieldValue<SPAN class="punctuation token">)</SPAN> <SPAN class="operator token">+</SPAN> <SPAN class="string token">"'"</SPAN><SPAN class="punctuation token">)</SPAN>‍‍‍‍‍‍‍‍‍<SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

But this isn't working for me. Any help is appreciated!

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Accepted answers

ClintonDow1

Hello Vishal,

There are some issues with your if statements, so currently the script will never make it to the SelectLayerByAttribute call.

The first conditional is:

fieldList <SPAN class="operator token">=</SPAN> arcpy<SPAN class="punctuation token">.</SPAN>ListFields<SPAN class="punctuation token">(</SPAN><SPAN class="string token">'Target Feature'</SPAN><SPAN class="punctuation token">)</SPAN>
fieldName <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">'Field3'</SPAN><SPAN class="punctuation token">]</SPAN>
fieldValue <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">'%45B%'</SPAN><SPAN class="punctuation token">]</SPAN>

<SPAN class="keyword token">if</SPAN> fieldName <SPAN class="keyword token">in</SPAN> fieldList<SPAN class="punctuation token">:</SPAN>‍‍‍‍‍‍‍‍‍‍<SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

fieldName is a list, since you surrounded it with square brackets. So you are checking if a list holding 'Field3' is in fieldList, not the 'Field3' string itself. The result of arcpy.ListFields contains Field objects which possess a name Property - so we can use a list comprehension to extract them. The result will look something like: ['Field1', 'Field2', 'Field3'], so we can see ['Field3'] isn't inside, however 'Field3' by itself is. This post explains how to search for a list of values within another list, and may help clear it up further.

The second conditional:

&nbsp;&nbsp;&nbsp; <SPAN class="keyword token">if</SPAN> fieldValue <SPAN class="keyword token">in</SPAN> fieldName<SPAN class="punctuation token">:</SPAN>‍‍‍‍‍‍‍‍<SPAN class="line-numbers-rows"><SPAN>‍</SPAN></SPAN>

Has a similar issue - The list ['%45B%'] does not exist inside ['Field3'], which again has no sublists. At this point, instead you may want to check the value of each row inside 'Target Feature', this is where Cursors come in. However, for what you are trying to do in this script, Cursors are unnecessary, I just wanted to mention them for your benefit.

You don't need to use cursors, because the arcpy.SelectLayerByAttribute_management() tool allows you to provide a query, which you are doing in your above example. This loops through the feature class automatically, cursors loop through it manually.

So therefore, we just need a single conditional statement in this case. With these changes, this should be the result:

<SPAN class="keyword token">import</SPAN> arcpy

fieldList <SPAN class="operator token">=</SPAN> arcpy<SPAN class="punctuation token">.</SPAN>ListFields<SPAN class="punctuation token">(</SPAN><SPAN class="string token">'Target Feature'</SPAN><SPAN class="punctuation token">)</SPAN>
fieldName <SPAN class="operator token">=</SPAN> <SPAN class="string token">'Field3'</SPAN>
fieldValue <SPAN class="operator token">=</SPAN> <SPAN class="string token">'%45B%'</SPAN>

<SPAN class="comment token"># list comp to extract field names</SPAN>
<SPAN class="keyword token">if</SPAN> fieldName <SPAN class="keyword token">in</SPAN> <SPAN class="punctuation token">[</SPAN>f<SPAN class="punctuation token">.</SPAN>name <SPAN class="keyword token">for</SPAN> f <SPAN class="keyword token">in</SPAN> fieldList<SPAN class="punctuation token">]</SPAN><SPAN class="punctuation token">:</SPAN>
&nbsp;&nbsp; query <SPAN class="operator token">=</SPAN> <SPAN class="string token">"Field3 LIKE "</SPAN> <SPAN class="operator token">+</SPAN> <SPAN class="string token">"'"</SPAN> <SPAN class="operator token">+</SPAN> str<SPAN class="punctuation token">(</SPAN>fieldValue<SPAN class="punctuation token">)</SPAN> <SPAN class="operator token">+</SPAN> <SPAN class="string token">"'"</SPAN>
&nbsp;&nbsp; arcpy<SPAN class="punctuation token">.</SPAN>SelectLayerByAttribute_management <SPAN class="punctuation token">(</SPAN><SPAN class="string token">'Target Feature'</SPAN><SPAN class="punctuation token">,</SPAN> NEW_SELECION<SPAN class="punctuation token">,</SPAN> query<SPAN class="punctuation token">)</SPAN>‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍<SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

Its also worth mentioning, in case you run into it, that in some cases 'Target Feature' won't be enough and you may need the full path to the dataset on disk.

(Shoutout to vangelo-esristaff‌ for a reminder on the Field objects )

All comments

VishalShah2

But I'd want to check to see if the list that is set to fieldValues is in Field3. So wouldn't it be the other way around from how you have the join listed?

ClintonDow1

In that case it would be appropriate to surround the three values in square brackets to create a list, because you have multiple strings and want to compare for each one of them.

fieldValues <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">'%45B%'</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">'%46A%'</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">'%47C%'</SPAN><SPAN class="punctuation token">]</SPAN>‍‍<SPAN class="line-numbers-rows"><SPAN>‍</SPAN></SPAN>

Then you can use the SQL IN operator instead of LIKE in that case. The syntax would be:

query <SPAN class="operator token">=</SPAN> <SPAN class="string token">"Field3 IN"</SPAN> <SPAN class="operator token">+</SPAN> <SPAN class="string token">"("</SPAN> <SPAN class="operator token">+</SPAN> <SPAN class="string token">', '</SPAN><SPAN class="punctuation token">.</SPAN>join<SPAN class="punctuation token">(</SPAN>fieldValues<SPAN class="punctuation token">)</SPAN> <SPAN class="operator token">+</SPAN> <SPAN class="string token">")"</SPAN>‍
<SPAN class="comment token"># "Field3 IN (‍‍'%45B%', '%46A%', '%47C%')‍‍‍‍</SPAN><SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

VishalShah2

Clinton,

What if fieldValue had more than one input? Say:

fieldValue <SPAN class="operator token">=</SPAN> <SPAN class="string token">'%45B%'</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">'%46A%'</SPAN><SPAN class="punctuation token">,</SPAN> <SPAN class="string token">'%47C%'</SPAN><SPAN class="line-numbers-rows"><SPAN>‍</SPAN></SPAN>

Would that still work the way you have the script written?

VishalShah2

Sorry about that Joshua, my issue was that the selection wasn't being made. Was bogged down by other projects I forgot to mention what wasn't working.

ClintonDow1

Yes good point, that will work and makes it more robust.

RandyBurton

So if the field name is edited in line 4, how about this for line 9:

query <SPAN class="operator token">=</SPAN> fieldName <SPAN class="operator token">+</SPAN> <SPAN class="string token">" LIKE '"</SPAN> <SPAN class="operator token">+</SPAN> str<SPAN class="punctuation token">(</SPAN>fieldValue<SPAN class="punctuation token">)</SPAN> <SPAN class="operator token">+</SPAN> <SPAN class="string token">"'"</SPAN><SPAN class="line-numbers-rows"><SPAN>‍</SPAN></SPAN>

ClintonDow1

Hello Vishal,

There are some issues with your if statements, so currently the script will never make it to the SelectLayerByAttribute call.

The first conditional is:

fieldList <SPAN class="operator token">=</SPAN> arcpy<SPAN class="punctuation token">.</SPAN>ListFields<SPAN class="punctuation token">(</SPAN><SPAN class="string token">'Target Feature'</SPAN><SPAN class="punctuation token">)</SPAN>
fieldName <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">'Field3'</SPAN><SPAN class="punctuation token">]</SPAN>
fieldValue <SPAN class="operator token">=</SPAN> <SPAN class="punctuation token">[</SPAN><SPAN class="string token">'%45B%'</SPAN><SPAN class="punctuation token">]</SPAN>

<SPAN class="keyword token">if</SPAN> fieldName <SPAN class="keyword token">in</SPAN> fieldList<SPAN class="punctuation token">:</SPAN>‍‍‍‍‍‍‍‍‍‍<SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

The second conditional:

&nbsp;&nbsp;&nbsp; <SPAN class="keyword token">if</SPAN> fieldValue <SPAN class="keyword token">in</SPAN> fieldName<SPAN class="punctuation token">:</SPAN>‍‍‍‍‍‍‍‍<SPAN class="line-numbers-rows"><SPAN>‍</SPAN></SPAN>

So therefore, we just need a single conditional statement in this case. With these changes, this should be the result:

<SPAN class="keyword token">import</SPAN> arcpy

fieldList <SPAN class="operator token">=</SPAN> arcpy<SPAN class="punctuation token">.</SPAN>ListFields<SPAN class="punctuation token">(</SPAN><SPAN class="string token">'Target Feature'</SPAN><SPAN class="punctuation token">)</SPAN>
fieldName <SPAN class="operator token">=</SPAN> <SPAN class="string token">'Field3'</SPAN>
fieldValue <SPAN class="operator token">=</SPAN> <SPAN class="string token">'%45B%'</SPAN>

<SPAN class="comment token"># list comp to extract field names</SPAN>
<SPAN class="keyword token">if</SPAN> fieldName <SPAN class="keyword token">in</SPAN> <SPAN class="punctuation token">[</SPAN>f<SPAN class="punctuation token">.</SPAN>name <SPAN class="keyword token">for</SPAN> f <SPAN class="keyword token">in</SPAN> fieldList<SPAN class="punctuation token">]</SPAN><SPAN class="punctuation token">:</SPAN>
&nbsp;&nbsp; query <SPAN class="operator token">=</SPAN> <SPAN class="string token">"Field3 LIKE "</SPAN> <SPAN class="operator token">+</SPAN> <SPAN class="string token">"'"</SPAN> <SPAN class="operator token">+</SPAN> str<SPAN class="punctuation token">(</SPAN>fieldValue<SPAN class="punctuation token">)</SPAN> <SPAN class="operator token">+</SPAN> <SPAN class="string token">"'"</SPAN>
&nbsp;&nbsp; arcpy<SPAN class="punctuation token">.</SPAN>SelectLayerByAttribute_management <SPAN class="punctuation token">(</SPAN><SPAN class="string token">'Target Feature'</SPAN><SPAN class="punctuation token">,</SPAN> NEW_SELECION<SPAN class="punctuation token">,</SPAN> query<SPAN class="punctuation token">)</SPAN>‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍<SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

Its also worth mentioning, in case you run into it, that in some cases 'Target Feature' won't be enough and you may need the full path to the dataset on disk.

(Shoutout to vangelo-esristaff‌ for a reminder on the Field objects )

VinceAngelo

Always be careful to understand what an ArcPy function returns. ListFields returns a list of Field objects, which while they include name, also includes type, isNullable, length and a number of other properties. You'd need to use a list comprehension to map the array of Fields to an array of names.

- V

JoshuaBixby

Can you elaborate on "isn't working for me?" Are you getting errors or unexpected results? If errors, what errors exactly (paste the error message here)? If unexpected results, what are you getting and what do you expect?

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