topic Re: Intersecting Lists Problem in Python Questions

Intersecting Lists Problem

AlanDodson — Tue, 19 Mar 2019 22:08:11 GMT

First post here, please be gentle...

I use ArcMap 10.3.1 and I'm having trouble with part of a Python script which will always be run from the Current mxd and active data frame.

I have to pass a list of layers to a function. I have given the script a list of all potential layers for this function (List 1). Some of those potential layers will actually be in my active data frame (always at least 1), along with other layers that are not compatible with the next function (List2). I want to only work with the potential layers that are actually present in my active data frame (List3)

I've tried the following:

import arcpy

mxd = arcpy.mapping.MapDocument("CURRENT")

df = mxd.ActiveDataFrame

List 1 = ["September", "October", "November", "December"]

List 2 = arcpy.mapping.ListLayers(df) # layers are named September and December

List 3 = list(set.(List1).intersection(List2))

The result is an empty list when it should be September, December.

I think the syntax roughly works, but the problem is that List2 returns something similar to <layer u'September'> so the two layers with identical names are not recognized as such.

(For context, I'll then be using List3 to print unique messages depending on the layer name returned... In case this helps)

Can anyone help me out?

Re: Intersecting Lists Problem

PavanYadav — Sun, 12 Dec 2021 16:48:00 GMT

arcpy.mapping.ListLayers(df)

is returning a list of Layer objects. Something like this:

[<map layer u'September '>, <map layer u'December'>]

How about the following:

List2a = []
for l in List2:
    List2a.append(str(l.name))

Re: Intersecting Lists Problem

JoeBorgione — Sun, 12 Dec 2021 04:53:57 GMT

I found this with a google search: Python | Intersection of two lists - GeeksforGeeks

and it suggests in method 2 something close to what you are trying:

def intersection(lst1, lst2): 
    return list(set(lst1) & set(lst2)) 
  

lst1 = [15, 9, 10, 56, 23, 78, 5, 4, 9] 
lst2 = [9, 4, 5, 36, 47, 26, 10, 45, 87] 
print(intersection(lst1, lst2)) ‍‍‍‍‍‍‍

Re: Intersecting Lists Problem

RandyBurton — Sun, 12 Dec 2021 04:54:01 GMT

As Pavan suggests, you need to use the layer's name property in your comparison. Try:

import arcpy 
mxd = arcpy.mapping.MapDocument("CURRENT")
df = mxd.ActiveDataFrame

List1 = ["September", "October", "November", "December"]
List2 = arcpy.mapping.ListLayers(df)  # layers are named September and December
List3 = []

for lyr in List2:
    if lyr.name in List1:
        List3.append(lyr.name)‍‍‍‍‍‍‍‍‍‍‍

EDIT: As Joshua Bixby‌ shows in his code example, line 6 in my code should read as below. See: ListLayers.

# ListLayers (map_document_or_layer, {wildcard}, {data_frame})

List2 = arcpy.mapping.ListLayers(mxd, data_frame=df)‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

Re: Intersecting Lists Problem

AlanDodson — Wed, 20 Mar 2019 01:49:15 GMT

Ah, this looks promising. I didn't get to the layer name before but this looks like it could solve that. I will try it out first thing in the morning and report back.

Do you know if this method preserves the list order?

Re: Intersecting Lists Problem

RandyBurton — Wed, 20 Mar 2019 03:35:24 GMT

List3 should be in the List2 (or ListLayers) order. It is likely that it would match the Table of Contents order.

Re: Intersecting Lists Problem

AlanDodson — Wed, 20 Mar 2019 11:57:03 GMT

Unfortunately I'm getting an error on the line 'if lyr.name in List2:'

File "<string>" - AttributeError: 'str' object has no attribute 'name'

When I print List2 the entries are still "<layer u'September'>" or something like that so it's not matching List1 "September". My List3 result is a list of lots of duplicates of items in List2 with the "<layer u'September'>" formatting.

Re: Intersecting Lists Problem

PavanYadav — Wed, 20 Mar 2019 16:03:19 GMT

Did you try str(), like this:

    List2a.append(str(l.name))

Re: Intersecting Lists Problem

JoshuaBixby — Sun, 12 Dec 2021 04:54:03 GMT

This works for me:

>>> mxd = arcpy.mapping.MapDocument("CURRENT")
>>> df = mxd.activeDataFrame
>>> 
>>> List1 = ["September", "October", "November", "December"]
>>> List2 = [lyr.name for lyr in arcpy.mapping.ListLayers(mxd, data_frame=df)]
>>> List3 = list(set(List1).intersection(List2))
>>> List3
[u'September', u'December']
>>> ‍‍‍‍‍‍‍‍‍

Re: Intersecting Lists Problem

AlanDodson — Wed, 20 Mar 2019 17:27:11 GMT

This works for me, thanks so much. If anyone else reading this requires a string it requires the following change: On line 5 change [lyr.name for .... to [str(lyr.name) for ....

Re: Intersecting Lists Problem

JoshuaBixby — Sun, 12 Dec 2021 04:54:06 GMT

Since lyr.name returns a unicode,

>>> type(lyr.name) 
<type 'unicode'>
>>> ‍‍‍

I am interested in what you are doing that requires conversion to str, since Unicode and byte strings can be compared directly.

>>> u = u"September"
>>> type(u)
<type 'unicode'>
>>> 
>>> s = "September"
>>> type(s)
<type 'str'>
>>> 
>>> u == s
True
>>> ‍‍‍‍‍‍‍‍‍‍‍

Re: Intersecting Lists Problem

AlanDodson — Wed, 20 Mar 2019 19:36:20 GMT

I am a bit lost to be honest! I'd love to explain but at the moment I don't fully understand what my script is doing and how I can go from there. Perhaps this problem stems from me typing in the first list and hard-coding that into the script, along with each of the subsets of that list. The only real arcpy listlayers list is the one I get from my active data frame. I couldn't figure out any other way to do it - especially when I have to make the subset lists to weed out features.

To recap, I have a list of all potential layer names which I have hardcoded like List1 = ["Quarries", "Ponds", "Gullies", "Runways"].

Not all of these layers are present in each map, so I want to run a function with only the layers from (List1) that are actually in my map (List3 - Ponds & Runways). I have other layers in my dataframe that I cannot fun the function on because the result is not needed and/or I cannot predict what will happen if other users add other datatypes to the dataframe. Every layer in the dataframe makes up List2 (Features, Annotation, Grids, Rasters, user-specific layers etc).

So using the snippet you shared with me I was able to weed out the unnecessary layers, but if I try to print List3 as a check nothing happens, and if I try to get the selection set after my Select By Location loop runs (referenced in another post you have graciously replied to) nothing happens - the script completes properly with no errors, but I do not get the result that I am looking for, 'getSelectionSet > 0' and FIDSet != '' queries do not return anything. Because they do not return anything I can't pass the names of the layers which contain a selection to a bunch of if / elif statements to output specific instructions if that layer meets my conditions...

I hope to get a handle on things with our on-site ESRI help but he is stretched thin these days.

I do appreciate your help though, it's made things better And I'm learning.

Re: Intersecting Lists Problem

JoshuaBixby — Sun, 12 Dec 2021 04:54:08 GMT

It appears you want a list of layers based on a predefined list of layer names. Does this work for you:

>>> mxd = arcpy.mapping.MapDocument("CURRENT")
>>> df = mxd.activeDataFrame
>>> List1 = ["September", "October", "November", "December"]
>>> List3 = [lyr for lyr in arcpy.mapping.ListLayers(mxd, data_frame=df) if lyr.name in List1]
>>> List3
[<map layer u'December'>, <map layer u'September'>]
>>> ‍‍‍‍‍‍‍

Re: Intersecting Lists Problem

AlanDodson — Wed, 20 Mar 2019 23:31:07 GMT

If I'm reading that right - List1 is typed-in, hardcoded, predefined etc; List3 creates a true list of layer objects based whether or not the layer object name of everything in the dataframe (pseudo-List2) appears in List1; List3 therefore becomes the resulting list of true layer objects that can be recognized by getSelectionSet and any other method that recognizes layers... If I'm interpreting that correctly I think it would be exactly right..... I'll have to wait to try it out in the morning but this looks promising... Thank you again for walking me through this! I'll let you know how it turns out

Re: Intersecting Lists Problem

AlanDodson — Thu, 21 Mar 2019 17:46:59 GMT

This works even better for my purposes than the first message I marked as correct! Thank you, you've unlocked the key to my problem and I'm happily on my way now.... Until the next inexperienced coder hiccup! Cheers.