# Understanding qualified values **URL:** \ **Category:** Ignition\ **Tags:** ignition81\ **Created:** [November 1, 2024, 8:07pm UTC](https://forum.inductiveautomation.com/t/understanding-qualified-values/98064 "2024-11-01T20:07:49Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![JeremyMills](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/jeremymills/32/70121_2.png) [@JeremyMills](https://forum.inductiveautomation.com/u/JeremyMills)\ **Post date:** [November 1, 2024, 8:07pm UTC](https://forum.inductiveautomation.com/t/understanding-qualified-values/98064/1 "2024-11-01T20:07:49Z")
I don't think I understand what's happening when we fill a variable with a system.tag.readBlocking("tagPath") call and I want to. I'm using the following line of code to fill a variable with a single value rather than the whole qualified value list, but I don't know why it works. `value = system.tag.readBlocking("tagPath")[0].value` Why do we use the .value for the system.tag.readBlocking() function when python lists and dictionaries both use [param] to specify a particular object within them? Why would I not use: `value = system.tag.readBlocking("tagPath")[0]["value"]` While I have a working script because I can copy and paste, I would like to know why the script is working with the current syntax. Thoughts? ---
**Author:** ![amarks](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/amarks/32/45610_2.png) [@amarks](https://forum.inductiveautomation.com/u/amarks)\ **Post date:** [November 1, 2024, 8:10pm UTC](https://forum.inductiveautomation.com/t/understanding-qualified-values/98064/2 "2024-11-01T20:10:57Z")
I believe the answer here is because the qv is an _object_ ? So, `.value` is getting the defined "value" attribute of that object. `[0]` is, of course, the index of the qv object in the read qv list. ---
**Author:** ![lrose](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/lrose/32/62724_2.png) [@lrose](https://forum.inductiveautomation.com/u/lrose)\ **Post date:** [November 1, 2024, 8:46pm UTC](https://forum.inductiveautomation.com/t/understanding-qualified-values/98064/3 "2024-11-01T20:46:18Z")
> [@JeremyMills](#): > > Why do we use the .value for the system.tag.readBlocking() function when python lists and dictionaries both use [param] Lists use an index, and Dictionaries use a Key. The `system.tag.readBlocking()` function returns a `List` of `QaulifiedValue` _Objects_ as @amarks has already said. So in this line: ```python value = system.tag.readBlocking(['tagPath'])[0].vaule ``` There are a few things going on. A `QualifiedValue` object has three attributes, the first and most used is the `value`, the second the `quality`, and the third is the `timestamp`. There are various times when you might want to use any of those attributes. The object also has 3 getter functions defined that generally take the shape of `getValue()`. However, because Ignition uses Jython, there is some fancy footwork done in the background that makes it so that most getter and setter functions can be referenced as a "pythonic" attribute, this is why `.value` works, what is really happening is `QualifiedValue.getValue()`. So, the call to `readBlocking()` returns a list of `QualifiedValues` and `[0]` dereferences the list to the object at index 0, and `.value` returns the value of the value attribute of the `QualifiedValue`. Your next question might be "Why does it return a list when I'm only reading a single tag?". Well, even though you can provide a single string to the function, it was actually designed to and takes a `List` of tag paths. This is an important feature because reading 2 or more calls in a single call to `readBlocking()` is more performant than calling the function multiple times. The larger the number of tags you read at once the greater this performance enhancement becomes. (The same goes for `system.tag.writeBlocking()`. The returned list of QualifiedValues is guaranteed to match the order of the tag paths provided, so the QualifiedValue at Index 0 will always be the value for the tag path at index 0. This allows you to do things like this: ```python tagPaths = ['tagPath1','tagPath2',....'tagPathN'] tagValues = {path:qv.value for path,qv in zip(tagPaths,system.tag.readBlocking(tagPaths)} tagValues['tagPath1'] = 'value of tag path 1' tagValues['tagPath2'] = 'value of tag path 2' .... tagValues['tagPathN'] = 'value of tag path N' ``` Hopefully that provides a little bit of insight into what is going on. ---
**Author:** ![kvane](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kvane/32/22228_2.png) [@kvane](https://forum.inductiveautomation.com/u/kvane)\ **Post date:** [November 1, 2024, 9:08pm UTC](https://forum.inductiveautomation.com/t/understanding-qualified-values/98064/4 "2024-11-01T21:08:14Z")
Louis is correct here with this writeup, and if you are curious where to find sources for this info: All Ignition scripting function in the User Manual will link to any Ignition specific objects they use, typically in the "Returns" section. Such as: [system.tag.readBlocking | Ignition User Manual](https://www.docs.inductiveautomation.com/docs/8.1/appendix/scripting-functions/system-tag/system-tag-readBlocking) Which references: [Scripting Object Reference | Ignition User Manual](https://www.docs.inductiveautomation.com/docs/8.1/appendix/reference-pages/scripting-object-reference#qualifiedvalue) and the qualified value object. ---
**Author:** ![JeremyMills](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/jeremymills/32/70121_2.png) [@JeremyMills](https://forum.inductiveautomation.com/u/JeremyMills)\ **Post date:** [November 1, 2024, 9:10pm UTC](https://forum.inductiveautomation.com/t/understanding-qualified-values/98064/5 "2024-11-01T21:10:43Z")
THAT is what I was looking for. Thank you for the thorough response and depth of insight.