topic Re: Appending data to an empty dictionary in Python Questions

Appending data to an empty dictionary

VishalShah2 — Sun, 12 Dec 2021 00:26:35 GMT

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.

Re: Appending data to an empty dictionary

RandyBurton — Sun, 12 Dec 2021 00:26:38 GMT

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?

Re: Appending data to an empty dictionary

DanPatterson_Retired — Wed, 28 Mar 2018 01:41:26 GMT

a 'dictionary comprehension' or just 'dict comprehension'

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

Re: Appending data to an empty dictionary

VishalShah2 — Thu, 29 Mar 2018 13:14:07 GMT

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.

Re: Appending data to an empty dictionary

DanPatterson_Retired — Thu, 29 Mar 2018 14:43:33 GMT

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

Re: Appending data to an empty dictionary

VishalShah2 — Sun, 12 Dec 2021 00:26:40 GMT

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.

Re: Appending data to an empty dictionary

BlakeTerhune — Sun, 12 Dec 2021 00:26:43 GMT

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"
}‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

Re: Appending data to an empty dictionary

DanPatterson_Retired — Sun, 12 Dec 2021 00:26:46 GMT

do you have them in the right order ?

r = [[1, 'a'], [2, 'b'], [3, 'c']]

valueDict = {v[0]:(v[1:]) for v in r}

valueDict
{1: ['a'], 2: ['b'], 3: ['c']}

# ---- switch it up

valueDict2 = {v[1]:(v[0]) for v in r}

valueDict2
{'a': 1, 'b': 2, 'c': 3}‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

Re: Appending data to an empty dictionary

VishalShah2 — Thu, 29 Mar 2018 15:14:39 GMT

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'}‍

Re: Appending data to an empty dictionary

DanPatterson_Retired — Thu, 29 Mar 2018 15:19:56 GMT

Unicode ...

Re: Appending data to an empty dictionary

BlakeTerhune — Thu, 29 Mar 2018 15:24:59 GMT

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

Re: Appending data to an empty dictionary

VishalShah2 — Thu, 29 Mar 2018 15:38:17 GMT

Awesome and that doesn't interfere with my definition query that I am applying afterwards. Thank you so much for your help Blake!

Re: Appending data to an empty dictionary

VishalShah2 — Thu, 29 Mar 2018 15:38:29 GMT

Thank you so much Dan!

Re: Appending data to an empty dictionary

BlakeTerhune — Sun, 12 Dec 2021 00:26:49 GMT

Typically, no, it does not interfere. However, it can depend on how you're creating your query expression. Assuming my example valueDict above:

for i in valueDict:
    expression = "NAME = '{}'".format(valueDict[i])
    print expression‍‍‍‍‍‍‍‍‍‍‍‍

Output:

NAME = 'Some Name'
NAME = 'Another Name'
NAME = 'Name Example'
NAME = 'And Another‍‍‍‍'‍‍‍‍‍‍‍‍‍‍‍‍

This makes perfectly valid SQL.

However, If you do something like this:

names = tuple(valueDict.values())
expression = "NAME in{}".format(names)
print expression‍‍‍‍‍‍‍‍‍

Output:

NAME in(u'Some Name', u'Another Name', u'Name Example', u'And Another')‍‍‍

It will keep the u designation and it won't be valid SQL.

You would have to do something like:

names = tuple(str(i) for i in valueDict.values())
expression = "NAME in{}".format(names)
print expression‍‍‍‍‍‍‍‍‍

Output:

NAME in('Some Name', 'Another Name', 'Name Example', 'And Another')‍‍‍

But if you have exactly one name record in the dictionary, a single item tuple still has a comma at the end:

NAME in('Some Name',)‍‍‍‍

Which is not valid SQL.

The best would be something like:

names = ",".join("'{}'".format(i) for i in valueDict.values())
expression = "NAME in({})".format(names)
print expression‍‍‍‍‍‍‍‍‍

Output:

NAME in('Some Name','Another Name','Name Example','And Another')‍‍‍

EDIT:

Or, you could get rid of the u designation by converting row[0] to a plain string when you create the dictionary:

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

Re: Appending data to an empty dictionary

VishalShah2 — Sun, 12 Dec 2021 00:26:51 GMT

I didn't even think of the first way you had it set up.

What I used was this:

hubFeatures = r'hub_locations'

hubDict = {}

with arcpy.da.SearchCursor(hubFeatures, 'HubName') as cursor:
     for enum, row in enumerate(cursor):
         hubDict[enum] = row[0]

for hub in hubDict:
     valueHub = dict.get(hubDict, hub)
     hubQuery = "NAME = " + "'" + str(valueHub) + "'"
     hubLayer = arcpy.mapping.ListLayers(mxd, 'hub_locations')[0]
     hubLayer.definitionQuery = hubQuery
‍‍‍‍‍‍‍‍‍‍‍‍‍

Re: Appending data to an empty dictionary

BlakeTerhune — Thu, 29 Mar 2018 20:25:35 GMT

On line 10, you've got the wrong syntax for get(). Should be:

valueHub = hubDict.get(hub)‍‍

But since you are iterating over the keys in the dictionary (as hub), you know the key exists so you don't need to use get()