Houdini 20.0 Python scripting

Per-node user-defined data

You can use HOM to store and retrieve arbitrary data on individual nodes.

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Sometimes you need to store extra information on individual nodes. For example, you may want to…

  • Associate scene-level nodes with production database entries or asset tags.

  • Attach additional versioning information beyond what is already stored by Houdini.

  • Tag nodes as having been operated on by some tool to avoid unnecessary work.

Methods on the hou.OpNode object let you get/set/delete values in user data dictionaries. Houdini has two different types of user data on nodes. They differ in whether or not they are saved with the scene file, and what types of values are allowed.




Saved with scene file

String only

Not saved with scene file

Any object

The “cached” user data dictionary is useful for nodes implemented in Python to save computed values between cooks to avoid recomputing them.

User data example

>>> import time
>>> # Get a node instance
>>> n = hou.node("/obj/sphere1")
>>> # Put a name/value on the node
>>> n.setUserData("last_indexed", str(time.time()))
>>> # Get the named value back from the node
>>> n.userData("last_indexed")
>>> # Get a dictionary of all name/value pairs on the node
>>> n.userDataDict()
{'last_indexed': '1260572254.21'}
>>> # Accessing a non-existent name returns None
>>> print n.userData("foo")
>>> # Remove a value from the node
>>> n.destroyUserData("last_indexed")

The contents of the string can be structured data such as XML, JSON, compressed binary data, or pickled Python objects, however you are responsible for encoding and decoding structured data to/from simple strings.

For example, here are two functions you could use to store compressed JSON representations of Python values:

import zlib
import hutil.json

def setCompressedJSON(node, data):
        "acme_studio", zlib.compress(hutil.json.utf8Dumps(data)))

def compressedJSON(node):
    str_data = node.userData("acme_studio")
    if str_data is None:
        return None
    return hutil.json.utf8Loads(zlib.decompress(str_data))
>>> n = hou.node("/obj/geo1")
>>> setNodeData(n, {"version": 1, "database_id": 1723})
>>> print nodeData(n)["database_id"]

Showing custom fields in the node info window

  • If you prefix a user data key with nodeinfo_, the key (without the prefix) and the value will be shown as a custom field in the node info popup window.

  • If a nodeinfo_ key ends with _node, the value will be treated as a node path and become a clickable link to go to that node.

  • If a nodeinfo_ key ends with _url, the value will be treated as a URL and become a clickable link to open that URL in a browser tab.

Cached user data example

The following example code would exist in the Cook tab of a node type (e.g. a SOP) implemented using Python. It avoids unnecessarily reparsing a configuration file between subsequent cooks.

def cook(node):
    config_file_name = node.evalParm("configfile")
    if node.cachedUserData("config_file_name") != config_file_name:
        config = ConfigParser.RawConfigParser()
        node.setCachedUserData("config", config)
        node.setCachedUserData("config_file_name", config_file_name)
        config = node.cachedUserData("config")

    # Use the configuration values and perform the cook.
    #value = config.getfloat("foo")
    # ...



  • Trying to get a non-existent value returns None rather than raising a KeyError. Since the cached dictionary allows storing None as a value, you may need to check whether the key exists using key in node.cachedUserDataDict() to tell the difference.

  • To remove a value from a node, use hou.OpNode.destroyUserData or hou.OpNode.destroyCachedUserData. Doing del node.userDataDict()["foo"] will not change the values on the nodes.

  • If you're storing structured data in a user data value (for example, as a JSON string), consider adding a “version” key to the data so you can tell which version of the data structure you're working with. Experience shows this comes in handy when you need to change or extend the stored format.

See also

Python scripting

Getting started

Next steps


  • hou

    Module containing all the sub-modules, classes, and functions to access Houdini.

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