What's the voodoo magic behind bpy GUI programming?

wynk · January 3, 2012, 6:10am

Hello. I’ve been using Python for some time now, I’ve also used Blender 2.4 API and I must say the GUI part of the new Blender API is nothing I’ve ever seen. I read the docs and I still don’t get the deal.

Can anyone explain me what gives? I’ve also used other libraries like wxPython and this is honestly the first API I see that requires you to create a class for GUI elements.

In Blender 2.4, you could just do “filename = Blender.Window.FileSelector” to open a file browser. In wxPython GUI library it’s similar: "filename = wx.FileSelector(“Description”).
How can you open a file browser in the new API? I’ve seen some codes making subclasses of Panel which none works with 2.6, btw.

Can anyone post a small snippet which opens a file browser please?
I’ve been searching for info the last hour and honestly I am still clueless.

wynk · January 3, 2012, 9:54am

Yes, like I said I’ve found some codes but they don’t teach much.

I think you mean these particular parts of code:


class ImportSIC(bpy.types.Operator):
    '''Load a sound File'''
    bl_idname = "sound.import"
    bl_label = "Import sound"

    filename_ext = ".mp3"
    filter_glob = StringProperty(default="*.mp3;*.ogg;*.wav;*.avi;*.mp4;*.wma", options={'HIDDEN'})
    filepath = StringProperty(subtype="FILE_PATH")
    filename = StringProperty()
    files = CollectionProperty(name="File Path",type=bpy.types.OperatorFileListElement)
    directory = StringProperty(subtype='DIR_PATH')

    
    def execute(self, context):
        print(self.filename)
        if self.files :
            for sfile in self.files:
                 context.window_manager.playlist.append(self.directory + sfile.name)
        else:
            context.window_manager.playlist.append(self.filepath)
        return {'FINISHED'}

    def invoke(self, context, event):
        context.window_manager.fileselect_add(self)
        return {'RUNNING_MODAL'}

So yeah, I’ve read in a tutorial that GUI widgets are created as classes, subclasses of Operator.

But all I see is bunch of string properties and two functions. That doesn’t tell me much.

wynk · January 3, 2012, 9:57am

Yes, I’ve seen codes like that, but they dont explain the theory.

I think you mean this classes:


class ImportSIC(bpy.types.Operator):
    '''Load a sound File'''
    bl_idname = "sound.import"
    bl_label = "Import sound"

    filename_ext = ".mp3"
    filter_glob = StringProperty(default="*.mp3;*.ogg;*.wav;*.avi;*.mp4;*.wma", options={'HIDDEN'})
    filepath = StringProperty(subtype="FILE_PATH")
    filename = StringProperty()
    files = CollectionProperty(name="File Path",type=bpy.types.OperatorFileListElement)
    directory = StringProperty(subtype='DIR_PATH')

    
    def execute(self, context):
        print(self.filename)
        if self.files :
            for sfile in self.files:
                 context.window_manager.playlist.append(self.directory + sfile.name)
        else:
            context.window_manager.playlist.append(self.filepath)
        return {'FINISHED'}

    def invoke(self, context, event):
        context.window_manager.fileselect_add(self)
        return {'RUNNING_MODAL'}

All I see is a class, inheriting from Operator, which has two methods and bunch of string properties. That doesn’t tell much to me…

RickyBlender · January 3, 2012, 10:04am

it’s magic !

look in blender folders for scripts there are many example of panels with properties ect.

also check out the wiki API there are some intro to this API ect

also check out the PDF tut here

http://wiki.blender.org/index.php/Dev:2.5/Py/Scripts/Cookbook/Code_snippets

good luck

happy 2.6

flokkievids · January 3, 2012, 10:16am

the basics is that you have a class for example:

class operator(bpy.types.Operator)

this class includes a execute function to execute the operator you’ve made and a draw class to draw buttons.

class operotor(bpy.types.Operator):
    bl_idname =  "object.operator" # skip the bpy.ops part, blender does that for you
    bl_label = "execute an operator
    def execute(self,context):
      #python commands here
      return {'FINISHED}
    def draw(self,context):
        row = self.layout.row
        row.prop("object.shade_smooth", shade smooth)

after that you need to register your class, look at an addon,scroll to the bottom and copy everything from the register function till the bottom, changing the class names to the ones in your file.

wynk · January 3, 2012, 12:24pm

The API references simply briefly says what Operators or Panels can be used for and then lists all of it’s attributes. I’ve seen them and I still don’t get why they are needed (that is, why there are separate and not part of other things), or how they can be used to create a file browser. I wouldn’t complain it should, it’s an API reference (list of everything), not a manual. Even though I’m pretty frustrated that they made the API more complex from 2.4 , previously it was just a function call and I had no difficulties using it. But I know very little, so I’ll assume it’s for the better. Still, I’m not getting the whole deal.
I’ve had a look at the example codes, they are poorly documented to be used as tutorials (though enough for real code).
I’ve had a look at the API reference like I said, not much help…
And I’ve also have seen the cookbook before (the pdf version from the blenderartists topic).

the basics is that you have a class for example…

OK, thanks for telling what is basically needed.

But I still don’t get what is exactly needed to put in the execute() and draw() functions.

I don’t even understand the difference between Panel inherited classes and Operator inherited classes.

(I would also assume that as file browsers are used so often, there would be a helper function or template class somewhere so people wouldn’t reinvent the wheel, but what do I know)

So yeah, feel like a total noob even though I’ve used many other APIs before. Am I the only one getting confused with the new API?

ldo · January 3, 2012, 9:43pm

I added a section on Python scripting to the “Blender 3D: Noob to Pro” WikiBook back in the 2.5x days. Some of the details need updating, but it does take you step-by-step through defining a custom operator and then creating a simple UI for it.

flokkievids · January 4, 2012, 5:01am

the execution of a couple of functions like invoke, execute, poll , draw, modal, init are done automatically by either python or blender.

a ‘panel’ class creates a pannel in the blender gui, for instance in the tool bar. an ‘operator’ class creates an operator. everything you do in blender is done with operators. if you click the shade smooth button, the operator bpy.ops.object.shade_smooth() is called.

wynk · January 4, 2012, 5:38am

Nice tutorial. I’m afraid to finally say that I got it all, but I’ll say what I got.

Operator is basically a class which is registered and can be used by any part of Blender like the default Blender stuff, which are also implemented as operators.
And Panels are the building block of GUI widgets. You specify where your panel will appear inside the class, by passing the appropriate strings to some bl_* variables.

However, I’m still a bit confused on how properties become GUI elements. Last time I checked properties were used in scripts to export custom stuff to 3d files and used by the game engine for the game logic.
A little more explanation on these properties would be nice.

But yeah thanks a lot. I finally don’t feel like a total noob.

wynk · January 5, 2012, 3:13am

Hey, thanks.

before I’ll read them, your file browser code returns this:


Traceback (most recent call last):
  File "&lt;blender_console&gt;", line 1, in &lt;module&gt;
  File "C:\Program Files (x86)\Blender Foundation\blender-2.60a-release-windows32\2.60\scripts\modules\bpy\ops.py", line 180, in __call__
    ret = op_call(self.idname_py(), None, kw)
RuntimeError: Error: Can't read file: "", Unable to open the file.

Seems like “” doesnt mean default dir and I can’t supply windows paths.

EDIT: Nope, those arent my issues. I don’t get the point behind Operators/ Panels/ Properties used by Panels. I think I got the first two now, but I still dont get why/how Properties work in your Panel class.

Also, I can’t get any bpy.ops to work. I want to do bpy.ops.mesh.remove_doubles(), but it doesnt work as my mouse cursor is not in 3d view when I run the script. Simply terrible impression of the new API…

blower05 · January 5, 2012, 5:43pm

wynk:

Hey, thanks.

before I’ll read them, your file browser code returns this:
Traceback (most recent call last):
  File "&lt;blender_console&gt;", line 1, in &lt;module&gt;
  File "C:\Program Files (x86)\Blender Foundation\blender-2.60a-release-windows32\2.60\scripts\modules\bpy\ops.py", line 180, in __call__
    ret = op_call(self.idname_py(), None, kw)
RuntimeError: Error: Can't read file: "", Unable to open the file.
Seems like “” doesnt mean default dir and I can’t supply windows paths.

EDIT: Nope, those arent my issues. I don’t get the point behind Operators/ Panels/ Properties used by Panels. I think I got the first two now, but I still dont get why/how Properties work in your Panel class.

Also, I can’t get any bpy.ops to work. I want to do bpy.ops.mesh.remove_doubles(), but it doesnt work as my mouse cursor is not in 3d view when I run the script. Simply terrible impression of the new API…

This should work if you give a correct path. Note this operator is to open the actual file but not popping out the file browser window.