Thursday, August 7, 2014

Tips & Tricks: Using external PY modules in our FDMEE Scripts

Hi all,

some time ago someone (I will call him Agent Brody from now on) asked in the community forum about having multiple BefImport event scripts... Is that possible?

In FDM Classic we could use API function SCRIPTENG.fCompileScript for that purpose. In that way we could have one master script calling functions/subs declared in other "slave" scripts as needed.
Good approach for implementing a custom library function that you could port from one customer to another. Definitely, that solution was helping us to improve our scripting model and maintenance... a lot.

So in our process of forgetting FDM Classic, we may be wondering about same functionality in Jython...

There are different ways but I don't want to go into much detail today. If we think from programming perspective, I would be thinking in having different Jython scripts (py files) so I can call functions declared in script A from script B...that's all.

How can I do this in Jython? Luckily we have the import statement which you can read about in Jython's site.
We will start with a simple custom library called BefImport_Lib.py which has one function declared. This function will show a message to the user. As I will be using the API function fdmAPI.showCustomMessage, I need to pass fdmAPI object as parameter to that function.
As a simple test, I just want to show a message when running the BefImport event script.

What steps do I need to perform?
  1. Import sys module
  2. Append folder with custom library to sys.path (so Jython can find the custom library)
  3. Import our custom library BefImport_Lib.py (so we can use functions declared in it)
  4. Call function customShowMsg(fdmAPI) defined in the custom library

 

You have more details in link above about how Jython works internally when importing modules. In a simply way, the objects within the sys.path list tell Jython where to search for modules.
So let's run our import step in FDMEE...
Let's have a look to the folder where custom library resides:
What is that BefImport_Lib$py.class?
In simple words, Jython compiles the custom py module when the module is imported and the function is called. As result you have this pretty $py.class file :-)

After I change code in my custom library...the old code is still being used!!!
First thing you notice when working with your custom library is that you need to append new code as needed or maybe you are my hero and you get all working with no errors in the first shot!
So I want to change the message (I know, I could have passed a parameter but then I have to spend more time in this blog entry and it's summer time...sun, beach, beer, family, not necessary in that order)
If I run the import again...Houston, Houston! we have a problem! the old message is shown instead of the new one. Then you investigate and you realize that either removing the $py.class file, or compiling the custom library using py_compile module does not help you. It seems that code is cached (memory) after module is imported :-(
Don't think I didn't try to restart FDMEE Service. I did and that works but I didn't want that solution.

Sometimes you need a divine light, and I found it with my colleague Giacomo :-) he provided some suggestions and we found the one working for our case! if we think as we should (I mean with some logic), we would say...what about re-importing the module? Wait, I'm already importing it so how can I re-import the module to get the new code in place? and the correct word is... R_L_AD! (if you take more than 5 seconds then you can read this)
So let's include reload(module) statement after our import module:
And run now the import...
Oleeeeeeeee, we got it! new code is used. That's what we wanted! The cached code is replaced by our new code and we get our new message.

I think this approach can improve our scripting solution and make it clearer and better maintainable. Not only Jython but you can import also custom Java libraries which may give you more flexibility when interacting with Hyperion applications. Don't forget that when you are in Production you don't really need to reload the custom library.

I would like to say thank you to Giacomo. As he says "the import machinery is very implementation-specific"
Thanks to him and all his Python expertise things are easier for me :-)
if you want to learn about Python/Jython and all related, don't forget to visit his http://blog.pythonaro.com/!

And thanks to Brody!

Note: here you have the site I linked for reload explanation in case somebody removes it

Important updates:
  • Adding to sys.path: before adding our folder to list sys.path we need to check if element exists in that list. Otherwise folder path will be added every time the event script is executed

  • It could be that reload(module) does not work as expected. Sometimes it does not work effectively. In fact, it has been removed from Python 3.x and moved to the imp module. Feedback from Python experts is that Python does not provide an effective support for reloading or unloading of previously imported modules as modules references makes it impractical to reload a module because references could exist in many places of our scripts. There is another solution consisting in removing module from memory but initially loaded modules (Python Cookbook). You can execute this code before importing the custom library module.

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