# Global Variable Usage

**URL:** <https://forum.inductiveautomation.com/t/global-variable-usage/41921>\
**Category:** Ignition\
**Tags:** ignition81\
**Created:** [January 4, 2021, 12:23pm UTC](https://forum.inductiveautomation.com/t/global-variable-usage/41921 "2021-01-04T12:23:32Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![fabio.gambarin](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/fabio.gambarin/32/20449_2.png) [@fabio.gambarin](https://forum.inductiveautomation.com/u/fabio.gambarin)\
**Post date:** [January 4, 2021, 12:23pm UTC](https://forum.inductiveautomation.com/t/global-variable-usage/41921/1 "2021-01-04T12:23:32Z")

</div>

I am trying to define and use a global variable with no luck.  
I need to have a shared array that can be accessed from anywhere.

Actually I am using a memory tag, I want to replace that tag with a global variable.

I tryed to define the variable on the gateway startup script and on a function in project library that is called on the startup script.

from the script console:  
g = system.util.getGlobals()[‘Array’]  
print g

All I get is:

Traceback (most recent call last):  
File “”, line 1, in   
KeyError: ‘Array’

How can I achieve this?

Thanks

---

<div class="post-metadata">

**Author:** ![JordanCClark](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/jordancclark/32/83362_2.png) [@JordanCClark](https://forum.inductiveautomation.com/u/JordanCClark)\
**Post date:** [January 4, 2021, 12:52pm UTC](https://forum.inductiveautomation.com/t/global-variable-usage/41921/2 "2021-01-04T12:52:42Z")

</div>

Consider storing it in a dataset or document tag.

---

<div class="post-metadata">

**Author:** ![fabio.gambarin](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/fabio.gambarin/32/20449_2.png) [@fabio.gambarin](https://forum.inductiveautomation.com/u/fabio.gambarin)\
**Post date:** [January 4, 2021, 1:11pm UTC](https://forum.inductiveautomation.com/t/global-variable-usage/41921/3 "2021-01-04T13:11:07Z")

</div>

How can I store on a global dataset?  
I think that the problem will persist.

What is a document tag?

What I am trying to do is populate an array (dataset could be ok too) on some tag change event  
and with a timer script elaborate all the data with a pop function.

---

<div class="post-metadata">

**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:** [January 4, 2021, 2:50pm UTC](https://forum.inductiveautomation.com/t/global-variable-usage/41921/4 "2021-01-04T14:50:26Z")

</div>

Since system.util.getGlobals() returns a dictionary, when you use the square bracket notation, the system tries to read the value from that key, if that key doesn’t exist then you will a KeyError exception as seen.

You first need to set the value of the global prior to it being called.

```auto
#This will create the key if it doesn't exist and set the value
system.util.getGlobals()['myArray'] = [0,1,2,3,4]

#This will print the value of the key, so long as the key exists in the globals dictionary
print system.util.getGlobals()['myArray']

```

Honestly, a tag is probably the best option here, though depending on the use case a client tag may be more appropriate.

---

<div class="post-metadata">

**Author:** ![fabio.gambarin](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/fabio.gambarin/32/20449_2.png) [@fabio.gambarin](https://forum.inductiveautomation.com/u/fabio.gambarin)\
**Post date:** [January 4, 2021, 3:42pm UTC](https://forum.inductiveautomation.com/t/global-variable-usage/41921/5 "2021-01-04T15:42:52Z")

</div>

> [@lrose](#):
>
> Honestly, a tag is probably the best option here, though depending on the use case a client tag may be more appropriate.

Thanks,  
I tryed mainly 2 things:

1. On Gateway's startup script I defined a variable:  
Array = [1,2,3]

2. On a Project Library I defined a Library  
def onStartup():  
Array\_Event = [1,2,3]  
and I call the function on the gateway's startup script

In both way I can not get the array with the getGlobals function

Actually I am using a memory tag like You suggested.  
With some test looks like sometime I miss some event if the event come when the scheduled task is running.  
I believe that this is because of the 2 task asyncrounism  
(one on valuechange and one every 1 second)  
or the delay between the system.tag.read() and system.tag.write() functions on the 1 second task where I read the array I elaborate the value and then I write the modified array again

I'm Open to suggestion

---

<div class="post-metadata">

**Author:** ![JordanCClark](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/jordancclark/32/83362_2.png) [@JordanCClark](https://forum.inductiveautomation.com/u/JordanCClark)\
**Post date:** [January 4, 2021, 5:10pm UTC](https://forum.inductiveautomation.com/t/global-variable-usage/41921/6 "2021-01-04T17:10:48Z")

</div>

A short example:

```python
# Path to document tag
globalsPath = '[default]Test/Globals'

# Initialize dictionary
globalVarsOut = {}
#Make up some stuff to add to the dictionary
globalVarsOut['List1'] = [1,2,3]
globalVarsOut['Dict1'] = {'A':101 , 'B':102, 'C':103}

# Encode to JSON
jsonOut = system.util.jsonEncode(globalVarsOut)

# Write to tag
system.tag.writeBlocking([globalsPath], [jsonOut])

# Read the tag
globalVars = system.tag.readBlocking([globalsPath])[0].value.toDict()

# Print some values
print globalVars['List1']
print globalVars['Dict1']
print globalVars['Dict1']['B']

```

Output:

```nohighlight
>>> 
[Good]
[1L, 2L, 3L]
{'A': 101L, 'B': 102L, 'C': 103L}
102
>>> 

```