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User Manual: Contents | Guidelines | Blender Version 2.31

Python Scripting

Blender has a very powerful yet often overlooked feature. It exhibits an internal fully fledged Python interpreter. This allows any user to add functionalities by writing a Python script. Python is an interpreted, interactive, object-oriented programming language. It incorporates modules, exceptions, dynamic typing, very high level dynamic data types, and classes. Python combines remarkable power with very clear syntax. It was expressly designed to be usable as an extension language for applications that need a programmable interface, and this is why Blender uses it. Of the two main ways of extending Blender, the other one being binary plugins, Python scripting is more powerful, versatile yet easier to comprehend and robust. It is generally preferred to use Python scripting than writing a plugin. Actually Python scripting had somewhat limited functionalities up to Blender 2.25, the last of NaN releases. When Open Sourcing Blender many of the new developers gathered around the Foundation elected to work on it and, together with UI change, Python API is probably the single part of Blender which got the greatest development. A full reorganization of what existed was carried out and many new modules added. This evolution is still ongoing and even better integration is expected in forthcoming Blender versions.

The Text Editor

Blender has a Text Window among its windows types accessible via the button of the Window Type menu or via SHIFT-F11. The newly opened Text window is grey and empty, with a very simple toolbar (Text Toolbar.). From left to right there are the standard Window type selection button and the Window menu. Then the full screen button, followed by two toggle buttons: one which shows/hides the line numbers for the text and one which shows/hides the syntax highlighting. At the end, the regular Menu Button and two settings: the font of the editor, and the number of spaces in a tabulation.

Text Toolbar.

The Menu Button () allows you to select which Text buffer is to be displayed, as well as allowing you to create a new buffer or load a text file. If you choose to load a file the Text Window temporarily becomes a File Selection Window, with the usual functions. Once a text buffer is in the Text window, this behaves as a very simple text editor. Typing on the keyboard produces text in the text buffer. As usual pressing, LMB dragging and releasing LMB selects text. The following keyboard commands apply:

• Alt C or Ctrl C - Copy the marked text into the text clipboard;
• Alt X or Ctrl X - Cut out the marked text into the text clipboard;
• Alt V or Ctrl V - Paste the text from the clipboard to the cursor in the Text Window;

Blender's cut/copy/paste clipboard is separate from the operating system (OS) clipboard. So normally you cannot cut/paste/copy out from/into Blender. To access your OS clipboard:

• Shift Ctrl C - To copy text to the OS buffer (e.g. if you want to paste that text into another application);
• Shift Ctrl X - To cut and copy text to the OS buffer;
• Shift Ctrl V - To paste text from the OS buffer (e.g. you copied some text from your web browser or document editor);
• Alt S - Saves the text as a text file, a File Selection Window appears;
• Alt O - Loads a text, a File Selection Window appears;
• Alt F - Pops up the Find toolbox;
• Shift Alt F or RMB - Pops up the File Menu for the Text Window;
• Alt J - Pops up a Num Button where you can specify a linenumber the cursor will jump to;
• Alt P - Executes the text as a Python script;
• Alt U - Undo;
• Alt R - Redo;
• Ctrl R - Reopen (reloads) the current buffer;
• Alt M - Converts the content of the text window into 3D text (max 100 chars).

To delete a text buffer just press the 'X' button next to the buffer's name, just as you do for materials, etc. The most notable keystroke is ALT-P which makes the content of the buffer being parsed by the internal Python interpreter built into Blender. The next section will present an example of Python scripting. Before going on it is worth noticing that Blender comes with only the bare Python interpreter built in, and with a few Blender-specific modules, those described in **REF** .

Accessing Python

To have access to the standard Python modules you need a complete working Python install. You can download this from http://www.python.org

Setting the PYTHONPATH environment variable

Be sure to check on http://www.blender.org which is the exact Python version which was built into Blender to prevent compatibility issues. Blender must also be made aware of where this full Python installation is. This is done by defining a PYTHONPATH environment variable.

Setting PYTHONPATH on Win95,98,Me

Once you have installed Python in, say, C:\PYTHON22you must open the file C:\AUTOEXEC.BATwith your favourite text editor, add a line:

SET PYTHONPATH=C:\PYTHON22;C:\PYTHON22\DLLS;C:\PYTHON22\LIB;C:\PYTHON22\LIB\LIB-TK

and reboot the system.

Setting PYTHONPATH on WinNT,2000,XP

Once you have installed Python in, say, C:\PYTHON22 Go on the "My Computer" Icon on the desktop, RMB and select Properties. Select the Advanced tab and press the Environment Variables button. Below the System Variables box, (the second box), hit New. If you are not an administrator you might be unable to do that. In this case hit New in the upper box. Now, in the Variable Name box, type PYTHONPATH, in the Variable Value box,type:

C:\PYTHON22;C:\PYTHON22\DLLS;C:\PYTHON22\LIB;C:\PYTHON22\LIB\LIB-TK

Hit OK repeatedly to exit from all dialogs. You may or may not have to reboot, depending on the OS.

Setting PYTHONPATH on Linux and other UNIXes

Normally you will have Python already there. if not, install it. You will have to discover where it is. This is easy, just start a Python interactive shell by opening a shell and by typing python in there. Type the following commands:

>>> import sys
>>> print sys.path 	

and note down the output, it should look like

['', '/usr/local/lib/python2.2', '/usr/local/lib/python2.2 /plat-linux2',
'/usr/local/lib/python2.0/lib-tk', '/usr/local/lib/python2.0/lib-dynload',
'/usr/local/lib/python2.0/site-packages'] 	

Add this to your favourite rc file as an environment variable setting. For example, add in your .bashrc the line

export PYTHONPATH=/usr/local/lib/python2.2:/usr/local/lib/python2.2/plat-linux2:
                  /usr/local/lib/python2.2/lib-tk:/usr/local/lib/python2.2/lib-dynload:
                  /usr/local/lib/python2.0/site-packages

all on a single line. Open a new login shell, or logoff and login again.

A working Python example

Now that you've seen that Blender is extensible via Python scripting and that you've got the basics of script handling and how to run a script, before smashing your brain with the full python API reference let's have a look at a quick working example. We will present a tiny script to produce polygons. This indeed duplicates somewhat the SPACEAdd>>Mesh>>Circle toolbox option, but will create 'filled' polygons, not just the outline. To make the script simple yet complete it will exhibit a Graphical User Interface (GUI) completely written via Blender's API.

Headers, importing modules and globals.

The first 32 lines of code are listed below

Script header

After the necessary comments with the description of what the script does there is (lines 016-022) the importing of Python modules. Blender is the main Blender Python API module. NMesh is the module providing access to Blender's meshes, while BGL and Draw give access to the OpenGL constants and functions and to Blender's windowing interface, respectively. The math module is Python's mathematical module, but since both the 'math' and the 'os' modules are built into Blender you don't need a full Python install for this! The polygons are defined via the number of sides they have and their radius. These parameters have values which must be defined by the user via the GUI hence lines (025-026) create two 'generic button' objects, with their default starting value. Finally, the GUI objects works with, and generates, events. Events identifier are integers left to the coder to define. It is usually a good practice to define mnemonic names for events, as is done here in lines (029-031).

Drawing the GUI.

The code responsible for drawing the GUI should reside in a draw function (GUI drawing).

GUI drawing

Lines (037-039) merely grant access to global data. The real interesting stuff starts from lines (042-044). The OpenGL window is initialised, and the current position set to x=8, y=103. The origin of this reference is the lower left corner of the script window. Then the title Demo Polygon Script is printed. A further string is written (lines 047-048), then the input buttons for the parameters are created. The first (lines 049-050) is a Num Button, exactly like those in the various Blender Button Windows. For the meaning of all the parameters please refer to the API reference. Basically there is the button label, the event generated by the button, its location (x,y) and its dimensions (width, height), its value, which is a data belonging to the Button object itself, the minimum and maximum allowable values and a text string which will appear as a help while hovering on the button, as a tooltip. Lines (051-052) defines a Num Button with a slider, with a very similar syntax. Lines (055-056) finally create a Draw button which will create the polygon and an Exit button.

Managing Events.

The GUI is not drawn, and will not work, until a proper event handler is written and registered (Handling events).

Handling events

Lines (058-060) define the keyboard event handler, here responding to the Q with a plain Exit() call. More interesting are lines (062-072), in charge of managing the GUI events. Every time a GUI button is used this function is called, with the event number defined within the button as a parameter. The core of this function is hence a "select" structure executing different codes according to the event number. As a last call, the Register function is invoked. This effectively draws the GUI and starts the event capturing cycle.

Mesh handling

Finally, Main function shows the main function, the one creating the polygon. It is a rather simple mesh editing, but shows many important points of the Blender's internal data structure.

Main function

The first important line here is number (082). Here a new mesh object, poly is created. The mesh object is constituted of a list of vertices and a list of faces, plus some other interesting stuff. For our purposes the vertices and faces lists are what we need. Of course the newly created mesh is empty. The first cycle (lines 085-092) computes the x,y,z location of the NumberOfSides vertices needed to define the polygon. Being a flat figure it is z=0 for all. Line (091) calls the NMesh method Vert to create a new vertex object of co-ordinates (x,y,z). Such an object is then appended (line 092) in the poly Mesh verts list. Finally (lines 095-096) a last vertex is added in the centre. Lines (099-104) now connects these vertices to make faces. It is not required to create all vertices beforehand and then faces. You can safely create a new face as soon as all its vertices are there. Line (100) creates a new face object. A face object has its own list of vertices v (up to 4) defining it. Lines (101-103) appends three vertices to the originally empty f.v list. The vertices are two subsequent vertices of the polygon and the central vertex. These vertices must be taken from the Mesh verts list. Finally line (104) appends the newly created face to the faces list of our poly mesh.

Conclusions

If you create a polygon.py file containing the above described code and load it into a Blender text window, as you learned in the previous section, and press ALT-P in that window to run it, you will see the script disappearing and the window turn grey. In the lower left corner the GUI will be drawn (The GUI of our example.).

The GUI of our example.

By selecting, for example, 5 vertices and a radius 0.5, and by pressing the Draw button a pentagon will appear on the xy plane of the 3D window (The result of our example script.).

The result of our example script.

Python Reference

The Full Python Application Programmer Interface of Blender has a reference documentation which is a book by itself. For space reason it is not included here. Here it is :

• The Blender Python API Reference (Blender 2.46)
• The Blender Python API Reference (Blender 2.45) out-dated
• The Blender Python API Reference (Blender 2.44) out-dated
• The Blender Python API Reference (Blender 2.43) out-dated
• The Blender Python API Reference (Blender 2.42) out-dated

Python Scripts

There are more than one hundred different scripts for Blender available on the net.

As with plugins, scripts are very dynamic, changing interface, functionalities and web location fairly quickly, so for an updated list and for a live link to them please refer to one of the two main Blender sites, www.blender.org or www.blenderartists.org, and the part of this wiki which is dedicated to them.