Appending data to an empty dictionary

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03-27-2018 01:36 PM
VishalShah2
Occasional Contributor II

So I am currently working on a tool that the user needs to apply a query in order to the run the tool for the queried data. I am trying to get around that by creating a blank dictionary in the script and then appending data to it. The data comes from a field in a feature class. I have never done this before but in theory it should work the same as appending to lists correct?

hubFeatures = r'hub_locations'

hubDict = {}

These are my working variables for this. The field that I want the data from to append to the dictionary is called 'NAME' in the hubFeatures FC. I was thinking of using cursors to achieve this. Unless there is an alternative approach. The idea is to then use the items in the dictionary to apply a query to the necessary layers and complete the functionality of the tool using a for loop. That part I can do. Need some assistance in setting up the dictionary.

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BlakeTerhune
MVP Regular Contributor

If I'm understanding your request, I think this will get you what you need. I learned it's easy to enumerate a cursor for an index or key field. Here's the expanded code so you can see what's happening:

valueDict = {}
with arcpy.da.SearchCursor(sourceFC, ["NAME"]) as cursor:
    for enum, row in enumerate(cursor):
        valueDict[enum] = row[0]‍‍‍‍‍‍‍‍

This will build a dictionary like this:

{
    0: "Some Name",
    1: "Another Name",
    2: "Name Example",
    3: "And Another"
}‍‍‍‍‍‍‍‍‍‍‍‍

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15 Replies
RandyBurton
MVP Regular Contributor

For an example of populating a dictionary, see Turbo Charging Data Manipulation with Python Cursors and Dictionaries (starting with example 1).  You will see code like this:

# Use list comprehension to build a dictionary from a da SearchCursor  
valueDict = {r[0]:(r[1:]) for r in arcpy.da.SearchCursor(sourceFC, sourceFieldsList)}  

You mention using the NAME field.  If it is to be used as the key, what will you use as the value?

DanPatterson_Retired
MVP Esteemed Contributor

a 'dictionary comprehension' or just 'dict comprehension'

dictionary order is now preserved in python 3.6 (ie if you are using PRO)

VishalShah2
Occasional Contributor II

I want to use the NAME field's values to be the value in the dictionary and a numeric value starting at 0 as the key if that is possible?

I am unfortunately still using ArcGIS for Desktop and thus do not have access to python 3.6.

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DanPatterson_Retired
MVP Esteemed Contributor

Did you try Randy's suggestion? It should work without problem

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VishalShah2
Occasional Contributor II

I did. While it does create a dictionary and populates it with a value, the NAME field value becomes the key and the value portion is empty. I'm trying to make it where the key is a numeric integer and the NAME field value is the value in the dictionary.

hubFeatures = r'hub_locations'

fieldList = "NAME"

hubDict = {r[0]:(r[1:]) for r in arcpy.da.SearchCursor(hubFeatures, fieldList)}

using:

print hubDict

I get:

{u'Hub Name': ()}

My desired output:

{1: 'Hub Name'}

How do I get to that? Reason I want to get that to be the output is because I want to use this for applying queries before doing the core function of the tool, which is conducting a network analysis.

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BlakeTerhune
MVP Regular Contributor

If I'm understanding your request, I think this will get you what you need. I learned it's easy to enumerate a cursor for an index or key field. Here's the expanded code so you can see what's happening:

valueDict = {}
with arcpy.da.SearchCursor(sourceFC, ["NAME"]) as cursor:
    for enum, row in enumerate(cursor):
        valueDict[enum] = row[0]‍‍‍‍‍‍‍‍

This will build a dictionary like this:

{
    0: "Some Name",
    1: "Another Name",
    2: "Name Example",
    3: "And Another"
}‍‍‍‍‍‍‍‍‍‍‍‍
VishalShah2
Occasional Contributor II

Blake that seems to get it in the format I want it in.

Do you know why the u appears before the value from the NAME field?

{0: u'Hub Name'}
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DanPatterson_Retired
MVP Esteemed Contributor

Unicode ...

BlakeTerhune
MVP Regular Contributor

Indeed.

The 'u' in front of the string values means the string has been represented as unicode. Letters before strings here are called "String Encoding declarations". Unicode is a way to represent more characters than normal ascii can manage.

python - What does the 'u' symbol mean in front of string values? - Stack Overflow