Python Resources

Updated on July 16  2014


Deborah R. Fowler

Python in Houdini

Updated Oct 6  2013
Update April 16  2015

Python Resources

Here I will outline the where (getting the code into Houdini) and the how (hou syntax):

There are many excellent references for using python in Houdini.
One is the MasterClass on SideFX website here.
There is also information in the sidefx documentation but here is an overview to get you started:

In Houdini there are many ways to run python code and Houdini is very python friendly. One of the easiest is to create a python sop. Two other common ways are  using the python shell and python source editor (for those of you coming from a mel coding background this one will be most familiar):

The python module within HDA is like Autodesk's Maya Embedded Language.
Examples exist below as well as on the Examples link at the top of this page.

Python Sop

You can create your own nodes that use python code. Click on this link for specific instructions.

Python Shell

This is similar to a hscript textport shell. Under Windows select Python Shell - this will open a python shell window. Another way is to simply select Windows->Desktop->Technical. This is an excellent default layout which includes a python shell at the bottom middle of the screen. You can call functions you have created in the python source editor (using the hou.session prefix) or import .py file (see below).

Python Source Editor

The code you type into the python source editor is saved with your hip file.

For example, bring up the Python Source Editor and create a function to print hello world:

def printHello():
    print("Hello World")

If you add the call printHello() to the code it will print "Hello World" each time you hit Apply. (see hip file).

You can also call this function in the python shell by typing hou.session.printHello()

If you want to use houdini specific calls, you would have to import hou.
So for example, to create a sphere in the python source editor you would type

import hou

def createSphereGeo():
    aGeoNode = hou.node('obj').createNode('geo')
    aSphereNode = aGeoNode.createNode('sphere')      


In this case, each time you hit apply a new sphere would be added. (see hip file.) Below I hit apply 5 times.

Importing py files

You can also create files outside of houdini using any text editor. Notepad++ or gedit are two commonly used editors. For example, I have used the phyllotactic pattern example written in python. I have used calls to houdini to create the geometry.

Open up Houdini and set the desktop to technical (or simply open up a python shell under the Window menu). Download the py file and hip file. To run the code in the python shell (make sure you save the files appropriately, and on Windows, launch by double clicking the hipnc). NOTE: if you run houdini from the command line in the directory where the files are the imports will work perfectly.

import circlehouPhyllo

Note when you import this, it creates a compiled python file (.pyc) in your folder).

(There is a sample file here with the code in the python source editor instead.) An excellent exercise would be to modify this code so the spheres were in one geo node. Secondly, try setting this up with instances instead.

Shelf tool

You can add a tool to an existing shell by simply right clicking on the shelf icon area and selecting "New Tool". Probably best to make it in its own shelf tab.

Left click on the + shelf icon and select "New Shelf". Give it a name and label and click accept. Now under this new shelf tab, right click and select "New Tool". Click on the "New Tool" and select the Script tab. You can now enter your code here in Python (alternatively in hscript). This is now accessible from the shelf.

Using Python in expressions

Hscript and Python can co-exist in your parameters. The calls to the hou module sometimes make the expressions look longer.
In Hscript $F becomes in Python hou.frame()
or for local parameters: Hscript $CY becomes in Python lvar("CY")
(Note that it will put the non-default type in purple and the others in green but they are not broken. They can co-exist as long as you have the type that you intend to input selected before typing in the expression)

So far the where has been explained where, but what about how - syntax.

Creating nodes

Suppose you want to create some geometry, here is a quick example demonstrating some useful commands:

n = hou.node( 'obj' ).createNode( 'geo ')
mergethem = n.createNode('merge')
aSphere = n.createNode('sphere')
tx = aSphere.parm('tx')

Note that n, mergethem and aSphere are simply variables. In the first line, the hou module, node is the function (method) and 'obj' is the argument to that function. n.path() would give us obj/geo for example. We are then setting a sphere 10 units on the screen. This will give you a sphere node connected to a merge node with the display flag on the merge - so on the scene you will see a sphere at 10 units on the x-axis. For more functions/commands refer to the documentation.

If you wanted to get rid of the file node that comes with this geo node by default, the command for this would be:

someVariableName = hou.node('obj/geo1/file1')

For example:

import hou

geo = hou.node('/obj').createNode('geo')

Note below are step by step examples - if you are in a hurry for the final result at the bottom of this section labeled Final Result.

As an exercise, go back to the phyllotaxis demo above and get rid of the file nodes. Put them all in one geo, and merge them given what you have just learned above. (Answer here) - note these are a step by step examples - if you are in a hurry for the final results see below).

Next step would be to set up the network much the same way as you would interactively - with a copy node. (Answer here).
Finally, a user interface should be implemented - here is an example using a vopsop (Answer here).

Final Result

The final version with top level controls is here. This uses the addSpareParmTuple command which was made clearer thanks to an odforce posting.  Hip file final result here or if you'd prefer to import and run, final .py file here.

Simple GUI in Houdini

See the example file above (final .py file). Odforce posting clarifying addSpareParmTuple command is here
An example of adding a parameter to a node would be (angle is a parameter to my function and geo is set to a node by using hou.node):

angleTemplate = hou.FloatParmTemplate( "angle", "angle", 1, default_value=([angle]), min=0, max=360, min_is_strict=False, max_is_strict=False, naming_scheme=hou.parmNamingScheme.Base1, disable_when="", tags={})

geo.addSpareParmTuple( angleTemplate, in_folder=(["Phyllotaxis Pattern"]), create_missing_folders=True)