Using dict comprehension to append data

Ignition
, ,
1

does Ignition not support comprehension? this isn't working, but it's valid code:

#
#	'value' is the entire dataset, we only want the assetName at this point
#
assetNames = {}
py_value = system.dataset.toPyDataSet(value)
for item in py_value:        #     ↓ column    ↓ data
	assetNames = {**assetNames,**{'assetName':item['assetName']}}
return [assetNames]	#<--- cuz everything needs to be a list... :)

i am building a table but only require one column from the dataset to populate the table. this should return a dict of key:value pairs (column and data)... but it isn't.

AFAIK, the table parses a data object of key:value pairs; the key is the column name, the value is, well, the value. this should deliver what the table parser is looking for.

okay. rant over. how do i fix this dang script?

2

Doesn't work in Python 2, I'm pretty sure.

3

fffffuuuu.... i keep forgetting that we aren't running Python 3. sorry.

4

Specifically, the issue is multiple sets of kwargs to the dict comprehension in one go.

1 Like
5

the issue is how to do it in Python 2. :smiley: because i haven't used 2 for about a decade...
EDIT: ah... yes. great post. thank you.

6

What were you trying to do exactly ? Make a list of dicts where the keys are the columns names and the values are the values, for each row ?

Something like this ?

headers = value.columnNames
return [dict(zip(headers, row)) for row in system.dataset.toPyDataSet(value)]

But maybe with only specific column/keys ?

7

okay. read the whole post (very good). but i hit a hiccup: i originally had tried this:

assetNames = {}
py_value = system.dataset.toPyDataSet(value)
for item in py_value:
	assetNames.update({'assetName':item['assetName']})
return [assetNames]	#<--- cuz everything needs to be a list... :P

which, ironically, is EXACTLY what the SO post explained (so i'm not actually an idiot after all...). but it only returned a single element (the LAST element): [{assetName: 70456-20}] so i went the comprehension route. so i'm not sure what's going on here.

8

Well for each item in the dataset, it will update the same key in your dict, over and over again. So in the end you only have the last one.

2 Likes
9

He's trying to populate a data property of a table in perspective. So I believe he may actually want a list of dictionaries in that case right?

10

i was, as i mentioned, trying to build a table but only using ONE of the columns of data. so i don't know if your method would work, but i'll give it a shot with an explicitheader definition and see what happens. i am not familiar with the dict method or what it does... so some reading first??? nah. slam it in there!! learn from the pieces!! :smiley:

11

yes. that's correct.

12

Think you may want something like this instead then
return [{"assetName":row["assetName"]} for row in system.dataset.toPyDataSet(value)]

13

:man_facepalming: I HATE PYTHON 2...

14

If the key is always the same, then it HAS to be a list of dicts.

@ian.sebryk : doing things in just one expression is my jam, my peanut butter, my cheese and beer all combined.

return [{column_name: val} for val in ds.getColumnAsList(ds.getColumnIndex(column_name))]

It does the exact same thing in python 3 though.

3 Likes
15

but i haven't used update in forever because i just use comprehensions... :stuck_out_tongue:

EDIT: jam, peanut butter, cheese, and beer... now i'm hungry. :smiley:

16

You can use them in python 2 as well. No issue here.

try the line I posted above.

And if you want to update a dict in just one expression without having to use update, which is a pain to use in comprehension anyway because it doesn't return the updated dict:

new_dict = dict(new_key=some_value, **old_dict)

this can be used in comprehension easily

[dict(k=v, **old_dict) for k, v other_dict.items()]

{k:v for k, v in dict.items()}

absolutely works in python 2. What do you call comprehension ?

2 Likes
17

dict comprehensions didn't exist until Python 3. which is why my original code didn't work: we're running Python 2.

at any rate, it doesn't work, so i'm trying your code now... stdby. :clock10:

EDIT: *boop* that worked.

18

You may not be able to do them with the **someDict but you can certainly make dicts on the fly with comprehensions as @PGriffith did in this post , kind of related as well with going from a ds to a dict

Though maybe this counts as a list comprehension, but if the final result is a dict, its a dict comprehension in my book lol.

19

okay. so what i'm learning here is:

  • data visualization tools in Ignition scripting still suck (Python → prettyPrint, c'mon)
  • i need more prayers to God that my higher-ups let me implement a better IDE strategy soon
  • → i don't need to convert everything to py_dataset ←
  • i may actually commit seppuku before i get my head around this environment

thank you for all your help and comments. very helpful at keeping the blade at bay. :wink:

EDIT: i've always had difficutly visualizing anything, period. once it does click, though, it stays forever. so please bear with as i keep trying to 'see' how this stuff hangs together. very irritating disability for a coder... meditation? forget it.

1 Like
20

Regarding pretty printing of (assuming) dictionaries hopefully eases your pain a bit, I keep this in a project library so I can call it when needed.

import pprint

mydict = {'nest0':'hi',
		'nest1':{'something':1},
		'nest2':{'nest1again':{'somethingelse':'bye'}}
		}

pp = pprint.PrettyPrinter(depth=4)
pp.pprint(mydict)

image

In my experince PyDataSets are most useful when you want access to all or almost all columns in a dictionary. So for things like

for (someCol1, someCol2, someCol3) in system.dataset.toPyDataSet(ds):
    # do something with your extracted columns
    # don't need to fetch them via col1=row['someCol1']
    # or via col1 = ds.getValueAt(row,'someCol1') if a basic dataset

it's very useful.

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