What this, the Slashie, mean is you consider me the best actor slash model and not the other way around.

How do you run your Python scripts on PSSE? Do you open the PSSE program and run from the “Run Program Automation File..” menu? I know some of my colleagues have created macro buttons that sit on their customised toolbar. The macro button is linked to a Python file and when you click the button that Python file runs.

Both of these exemplify the case where PSSE is run first, and then your Python script is executed. PSSE is the boss and your Python script is the worker that is told when to run.

I’d like to take you through another way to run your Python scripts. The examples I’ll show you will enable your Python script to be the boss and for it to tell PSSE to run.

Why would I care who is the boss?

Many times, PSSE is just one small cog in your overall program. You need it to do all the heavy computations for your latest energy congestion forecast but you still want to send the results by email to your boss in Singapore, or publish your results on the interwebs (Yes you can do that, I’ll explain how in a later post). Then consider testing, debugging, calling your script from another program.

There are lots of cases where it absolutely makes sense to have Python firmly seated on the driver’s side with two hands on the steering wheel and its foot pushing the accelerator through the floor. PSSE is a great tool and running your programs from inside it is fine for some, but sometimes you need to let your Python scripts run free.

Show me the code

Ok, you asked and I’m not one to hold out on you. Here is some code that is suggested in the PSSE manual to get an External Interpreter Environment. I’ll take you through what it does, so you can see through the magic. I will suggest methods that will make including this in your own work a piece of cake.

First Python is its own language, complete with a full library of useful tools. The only way Python knows to find those useful tools is to look in every single directory listed in its path variable. Python does not know about PSSE at all. You need to tell it where to look to find the PSSE library files (psspy, redirect et al).

The first parts of the program, beginning with sys.path.append and os.environ['PATH'] are adding to the Python and system path variables. Only now does Python know where to look to find psspy. If you don’t believe me, try running the line import psspy from your Python shell (not inside PSSE). You should get an ImportError. Now tell Python where it can find psspy using the first three lines in the example and try import psspy again.. No error!

There are a couple of important other lines in the code before we are done, the rest are not helpful.

If you want output that PSSE would normally print while it is running to show up on the command line for your Python script use this

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import redirect
redirect.psse2py()

Those lines are a matter of taste, your program won’t break without them.

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import psspy

This was the whole point of the exercise, to get this psspy library. That is where the interaction with PSSE occurs. So here is the minimum that you need:

A word of warning about os.chdir

The best place for these commands is near the top of your Python script file or in another file in the same directory as your Python script file which you import. No one expects for a library they are importing to change the working directory, especially not to c:\work_dir so don’t use os.chdir.

Putting it all together

Here is a basic script that will open up the save case of your choice and run a fully coupled Newton-Raphson iterative solve. Just a starting point skeleton file to give you an idea of how all of this sits together:

Comments

Very helpful! thank you

this is brilliant!

How do you get that standard ForeignKey select box with the “+” add another icon next to it on your form? This post will take you through the steps that I took to get the ForeignKey working like it does in the Django admin.

How does the Django admin create a ForeignKey select field

Django creates a forms.ModelChoiceField with queryset and to_field_name arguments as the default form field for a foreignkey. To see for yourself, have a look at django.db.models.fields.related.py under the ForeignKey class and method formfield here.

Now we know to use the ModelChoiceField. But what about those arguments queryset and to_field_name, and how does the “+” add icon appear?

The queryset argument should be a queryset or manager for the model your ForeignKey is to. For the following example:

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class SpaceMen(models.Model):
  company = models.ForeignKey('SpaceCompany')

We are trying to build a form.Form with a ForeignKey type field to SpaceCompany we might use:

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queryset = SpaceCompany._default_manager

I don’t know what to_field_name does. So I’ve left it out of this discussion. Don’t worry though, to_field_name, will appear later on.

How Django admin adds that “+” add icon

Django actually replaces the default select widget on the ModelChoiceField with its own django.contrib.admin.widgets.RelatedFieldWidgetWrapper. This swap occurs in django.contrib.admin.options near the top of the file in formfield_for_dbfield method of the BaseModelAdmin class. This code is interesting for two reasons:

• It checks if the current user has permission to add a new SpaceCompany
• The RelatedFieldWidgetWrapper requires knowledge of the admin_site

But where can I make this widget swap occur? and how do I get an instance of admin_site?

This will largely depend on how and why you are using the forms.Form in the first place. I was using the forms.Form as one of the FormWizard’s forms. So in render_template I was able to create a hook that checked the form for any ModelChoice fields and did the switch, and checked for permissions. I had a copy of the admin_site on my FormWizard thanks to this article onFormWizard in the Django admin. If you were not using ModelChoiceField in the admin, then you probably don’t need that “+” add another icon, because it is created using the reverse of admin_site.name amongst other things (more details in django.contrib.admin.widgets.py class RelatedFieldWidgetWrapper)

I recently needed a link checker to create a csv formatted list of all links (especially hosted pdfs) on a client site.

There is a tool called webcheck by arthur de jong which does a great job of checking all of the links on a website and creating a pretty html report.

This got me most of the way there, I could see that in the output there was a page dedicated to a list of every url that was encountered during the search, which looked like what I wanted but was formatted as html

I wrote a small file which will use webcheck’s own code to read in its stored .dat file and write all of the links to a csv file with the format:

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path, extension, internal, errors

Where path is the url, extension is the url ending (for example .pdf, .html, ..), internal is a boolean True or False if the link is an internal link and errors is the error (for example 404, ..) if any for that link.