Design Like a Pro: Scripting Best Practices

Moderator
Don Pearson
Chief Strategy Officer
Inductive Automation

Today’s Agenda
• Introduction to Ignition
• Things to Remember When Scripting
• Scripting Best Practices
• Q&A

About Inductive Automation
• Founded in 2003
• HMI, SCADA, MES, and IIoT software
• Installed in 100+ countries
• Over 1,500 integrators
• Used by 44% of Fortune 100 companies
Learn more at: inductiveautomation.com/about

Ignition: Industrial Application Platform
One Universal Platform for SCADA, MES & IIoT:
• Unlimited licensing model
• Cross-platform compatibility
• Based on IT-standard technologies
• Scalable server-client architecture
• Web-managed
• Web-launched on desktop or mobile
• Modular configurability
• Rapid development and deployment

Presenter
Kevin McClusky
Co-Director of Sales Engineering,
Inductive Automation

Why Scripting Exists
• It’s possible to create complete, powerful projects with HMI/SCADA
software without writing code
• However, many designers find that to complete all project
requirements, they need to use scripting.
• Typically, 80-90% of required functionality can be accomplished
through native features. Scripting exists to fill the other 10-20%.

Why Use Scripting?
• More flexibility
• Extend the logic
• Do more with the product than what’s built into it
• But it has to be used in the right way

When to Use Scripting and When Not To
• Use scripting when you simply can’t accomplish a requirement with
built-in tools.
• Don’t just use scripting for scripting’s sake.
• You may want to ask other users in forums or call tech support.
• Scripting can make it more difficult for another engineer to
understand, so make sure it is applicable and comment your code.

When to Use Scripting: Examples
• Exporting data through scripting that wasn’t built-in to binding or
component
• Custom calculations

When Not to Use Scripting: Examples
• Dynamic SQL query in binding vs. scripting
• Logging data (contextually) from a tag change script when you can
use transaction groups

Which Scripting Language?
Best programming languages to use:
• Use the scripting language that comes with your HMI, SCADA, IIoT
or MES platform.
• Although scripting outside the platform may be possible, it’s best to
use native scripting tools because it eases maintainability and has
the fullest possible integration with the platform.

Understanding Where and When Your Script is Running
• Extremely important to know exactly where and when your code is
running
• Requires knowledge of the software platform you are using
• Avoid unnecessary code from running and performing duplicate
work

Understanding Where and When Your Script is Running
• Where scripts run in Ignition:
• Gateway (server)
• Client
• When scripts run in Ignition:
• Timer
• Tag change
• User in client (button press)
• SFC
• Alarm pipeline
• Etc.

Organization and Standards
• Develop a code standard and be consistent
• Organize well and make sure folders, functions & variables are
named appropriately
• Avoid language shortcuts
• Indent style (tabs or spaces)
• Programming style
• Comment conventions
• Naming conventions

Organization and Standards
• Avoid language shortcuts (example)
1. Example with shortcut:
numbers = [1,2,3,4,5,6]
even = [number for number in numbers if number%2 == 0]
2. Example without:
numbers = [1,2,3,4,5,6]
even = []
for number in numbers:
if number%2 == 0:
even.append(number)

Commenting Code
• A comment is a programmer-readable explanation or annotation in
the source code.
• Added to make the source code easier for humans to understand,
generally ignored by compilers and interpreters.
• You need to do it!
• Helps the next developers and makes code easier to understand
and troubleshoot

Commenting Code (Examples)
• Individual Lines:
# this is a comment
print 'Hello world' # this is also a valid comment
• Blocks of Lines:
'''
This is a lot of text
that you want to show as multiple lines of
comments
Script written by Professor X.
Jan 5, 1990
'''
print 'Hello world'

Commenting Code (Examples)
• Use the Ctrl-/ keyboard shortcut to comment several lines of code at
once. Just highlight one or more lines of code and hold the Ctrl key
and press /

Best Practice: Variables
• Use naming conventions
• Define variables at the top of the script
Example:
name = event.source.parent.getComponent('Name').text
desc = event.source.parent.getComponent('Description').text
building = event.source.parent.getComponent('Building').selectedValue
id = system.db.runPrepUpdate("INSERT INTO machines (machine_name,
description) VALUES (?, ?)", [name, desc])

Best Practice: Define Scripting Functions
• Functions are code that can be called repeatedly from other places
• Can have parameters passed into them, and may return a resulting
value
• Types of functions:
1. Built-in to the language, like len()
2. Part of software package, like system.gui.getMessageBox() for
Ignition
3. Provided by language standard library, like math.sqrt()
4. User-defined functions

Benefits:
• Keeps code organized
• Easier to find and troubleshoot
• Keeps windows and tags cleaner
• Ability to re-use

• Default values for arguments (keyword arguments)
Argument specified by position:
print checkBounds(150, 0, 250)
Argument specified by keyword:
print checkBounds(150, range=250)

• Variable arguments (*args and **kwargs)
Example:
def add(firstValue, *args):
finalValue = firstValue
for value in args:
finalValue = finalValue + value
return finalValue
def checkBounds(value, offset=0, range=200):
if value + offset > range:
return true
else:
return false

Best Practice: Exception Handling
• Exceptions are errors detected during execution
• Not unconditionally fatal
• Most exceptions aren't handled by programs
• You can write programs that handle exceptions
Example:
try:
window = system.gui.getWindow('Overview')
system.gui.closeWindow(window)
except:
system.gui.warningBox("The Overview window isn't open")

Best Practice: Error Console
• One of the most important troubleshooting tools of any package
• Shows a wealth of information about the running state of the
system, system errors, and errors from custom scripts
• Depending on where your script is running, there may be a different
console
• Clear up any known errors

Best Practice: Error Console
Tips:
• Know how long your script takes (Log start, end, and duration)
• Use loggers effectively to troubleshoot your code (debug vs. info vs.
warn vs. error)

Best Practice: Loops
• When to use loops:
- Dealing with multiple items
- Iterating over finite list
• When not to use loops:
- To make your code wait (use a timing mechanism
instead)

Best Practice: Loops
• Stay away from infinite loops (While True or long for loops)
• Use breaks effectively (Don’t do unnecessary work)
Example:
found = False
for row in result:
if row[“name”] == “MyName”:
found = True
break

Best Practice: Timers
• Timers run at a set interval 24x7
• Understanding fixed rate vs. fixed delay:
◦ Fixed delay timer script (default) waits for given delay between
each script invocation. The script's rate will be the delay plus the
time it takes to execute the script. This is the safest option.
◦ Fixed rate scripts attempt to run the script at a fixed rate relative
to the first execution. If script takes too long, or there is too much
background process, this may not be possible.

Best Practice: Timers
• Avoid scripts that run quickly that fill up queues
(such as tag writes)
• Provide ability to enable/disable through a setting
(such as a memory tag)

Best Practice: Tag Change
• Tag change scripts run when the value or quality of a tag
changes.
• Make sure to check the quality of the tag. It is possible the
value didn’t change but the quality did. Don’t run code at
the wrong times.
• You may want to check the value, and whether the value is
different from the previous value.
• Decide whether you want the script to run initially (when
the server reboots)

Best Practice: Avoid Race Conditions
• Race condition or race hazard: the behavior of a system
where the output is dependent on the sequence or timing
of other uncontrollable events
• Expect one script to be completed before running the other
• Becomes a bug when events do not happen in the order
the programmer intended
• Typically due to asynchronous problems (tags, properties)

Best Practice: Avoid Race Conditions
Examples:
• A propertyChange script that refers to two properties but is
only filtered on one. The second property may not have
updated. Instead try to put in a single property, if possible,
or run the code multiple times, looking at all props.
• Writing to a tag and reading the tag on the next line

Best Practice: Understand Threading
• A lot of languages, such as Java, are multi-threaded
• Two or more parts that can run concurrently, and each part
can handle a different task at the same time.

• Are scripts running in a shared or dedicated thread?
• Scripts running in a shared thread will all execute in the
same thread. This is usually desirable, to prevent creating
unnecessary threads. However, scripts that take a long
time to run will block other scripts on the shared thread.

• Rule of thumb: quick-running tasks should run in the
shared thread, and long-running tasks should get their own
thread.
• Understand what else could be affected if running in the
same thread

UI thread vs separate thread (asynchronous)
• UI can hang
• Use progress bars instead (asynchronous)
• Callbacks to the UI thread invokeLater

Other Tips: Restoring Backups on Other Machines
• Scripts could be running in two places
• Make sure to restore disabled or have a code check if it is
a certain machine (by IP or hostname).

Other Tips: Scripting Timesavers
• Software package functions vs. libraries
• Language functions vs. custom code (example: the Python
CSV library)

Recap - Things to Remember When Scripting:
• Understand the basic purpose of scripting
• Know when to use scripting and when not to
• Understand where and when your script is running
• Use organization and standards
• Include comments in your code

Recap of Scripting Best Practices:
• Use variable naming conventions
• Define common functions
• Use exception handling
• Use the error console
• Avoid infinite loops
• Use appropriate threads with timers
• Plan tag change scripts
• Avoid race conditions
• Understand threading

One Last Tip …
Looking for some scripting help? Here are a few good resources:
• User forum at: www.inductiveautomation.com/forum
• Python language tutorial: www.python-course.eu/course.php

Design Like a Pro Series
Available at: inductiveautomation.com/resources

Questions & Comments
Jim Meisler x227
Vannessa Garcia x231
Vivian Mudge x253
Account Executives
Myron Hoertling x224
Shane Miller x218
Ramin Rofagha x251
Maria Chinappi x264
Dan Domerofski x273
Lester Ares x214
800-266-7798 x247
Melanie Moniz
Director of Sales:
Jeff Osterback x207

Design Like a Pro: Scripting Best Practices

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