Python for artists

Introduction

These day's nobody doubts, that the ability to write even simple scripts it's a huge benefit for a CG artist. Why then it's not that common for artists to be able to code? Probably there is only one main reason: people thinking that coding is hard and requires a special gift. But this is not the case, anybody can build programs that will make their life way easier.

Follow this tutorial and you will get the knowledge on how to build your own working tools!

Python in Houdini basics | Creating a first custom tool | Build a Houdini wrapper

Python integration basics

When dealing with a programming we should decide which programming language we would use to achieve our goals first. And choosing the proper language sometimes is a really sophisticated task required a huge experience in computer science. Fortunately for CG artist, this choice has been already made by the developer of your favorite software. Lucky users of Maya, Nuke, Houdini and a lot of other major DCC (hello, Adobe!) support Python. And this is super awesome great because Python is very easy to learn and use.

You don't need any extra step to start writing Python code in Houdini, it is already there.

Python integrated into Houdini in several ways:

• Windows > Python Shell - Python shell for small scripts/commands
• Windows > Python Source Editor - Python editor which saves the code with *.hip file. Each time you open the file Python code in Source Editor will be executed. Best option to develop basic chanks of code and test their functionality. To use Python Scripts in all the scenes you need to save them to the Shelf and run from there.
• In the parameters fields of any node (switch to Python from HScript).
• Python node in SOP context - This is how you can use Python in your nodes graph to create and modify geometry.
• In assets - create asset, right-click > Type properties > Scripts tab.
• Python Panel - to create PySide UI

For the most types of tasks here, we will use Python Source Editor. It allows you to write and execute Python code in the current Houdini scene. If you are completely new to programming I recommend you to read Programming Basic tutorial first.

Launch a Python Source Editor and type a code in the top window:

print 'Hello, World!'

Press the Apply button, Houdini Console window should appear with a printed message 'Hello, World'. It was not that hard, right?

To be able to write more valuable scripts for Houdini, besides Python integration, we would need the ability to access Houdini scene components (to query and modify necessary data). This ability is provided by Application Programm Interface (API). Houdini Python API exists as Houdini Object Model (HOM). To use this module in Houdini we need to import it to our python code:

import hou

Let's extend our first Hello World program a bit by using UI built in Houdini Python API:

import hou
hou.ui.displayMessage('Hello, World!')

This code will rise a window with a "Hello, World!" message.

Creating a first custom tool

Here we will create from scratch a custom tool (let's name it Geo Creator) for Houdini with a User Interface. Geo Creator will generate an empty geometry node with a certain name ("MY_GEO") in the root of the current scene.

Not an outstanding functionality but it's just a basic example which will give you a clue of how to solve tasks in Houdini with Python. Besides, a huge amount of video tutorials starts from creating a geometry node, diving inside, deleting "File" node and creating a grid, while you can do the same by CTR + click on "Grid" shelf tool. So might be useful in some cases.

We will begin with building a functional part of the code (create geometry node) in the Python Source Editor, then we will add and setup UI and finally, we will save the code as a Python file and make it launch from the Shelf.

Develop functionality

Let's design a Python program which will solve the task of creating a geometry node with a specific name.

To create a node in Houdini with Python use a createNode() command. To run this command successfully we need two main things:

• An argument (string), which will define the type of node which will be created.
• A parent object, where this node will be created: parentObject.createNode().

The most obvious way to get the name of a required node type is to create this node with UI and take the name from Info Box window. Run Houdini, CTR + click on the Grid Tool in the Create Shelf tab and open Info Box for grid_object1 node.

The type of node is written in brackets under the object name: Geometry Object (geo), so the node type we need to create is geo.

The node created with parentObject.createNode() command will be placed inside the parent object. In the case of scene root, the parent object will be the scene (obj) context. To create a Python object from the object in the scene use hou.node() command and give it a path to object in the scene as an argument. In case of scene root, it would be hou.node('/obj/').

Delete the "grid_object1", open a Python Source Editor and type the code:

import hou

# Get scene root node
sceneRoot = hou.node('/obj/')
# Create Geometry node in scene root
sceneRoot.createNode('geo')

Run the code and you will get the Geometry node named 'geo1' in the scene root with file node inside.

To give a certain name to the newly created node we have 2 options:

• pass the desired name as a string argument after the node type argument in createNode() command. We will use this option first.
• rename node after it has been created with setName('<objectName>') command.

To get rid of the file node we would set to False run_init_scripts flag for createNode() command.

import hou

# Get scene root node
sceneRoot = hou.node('/obj/')
# Create empty "MY_GEO" geometry node in scene root
sceneRoot.createNode('geo', 'MY_GEO', run_init_scripts=False)

Run the code and get empty geometry node named MY_GEO. Great, the code is working and produce the result we aimed to achieve! If it was a real production case we probably could stop at this point and just use this code when we need to create "MY_GEO" geometry node. But since we are building a Geo Creator Shelf Tool with UI we will need some more work to make it done.

Let's use another naming option, rename geometry node after creation. To create a Python object we just need to store the result of createNode() command as a variable. Then you can use this object to apply setName() command:

import hou

# Get scene root node
sceneRoot = hou.node('/obj/')
# Create empty geometry node in scene root
geometry = sceneRoot.createNode('geo', run_init_scripts=False)
# Set geometry node name
geometry.setName('MY_GEO')

Delete the existing "MY_GEO" node and run the code. We have the same result again, "MY_GEO" node in the root of the scene. Try to run this code again and you will get an error. One more geometry node ("geo1") will be created in the scene but after creation, a script would not be able to rename the node to "MY_GEO" because the node with such a name already exists in our scene.

If we run the script several times using the first naming option (pass name as an argument to a createNode() command) we would not get an error, Houdini just add a number to an existing name (MY_GEO1, MY_GEO2, etc) which is not that bad as an error but not what we need to get as well.

Either way, we have to extend the functionality of the GeoCreator tool to be able to behave differently if the object with a required name already exists. One option to achieve this is:

• Create an object from "MY_GEO" node with hou.node() command (passing the path to "MY_GEO" as an argument).
• Then check if this object does not have a value of None.

import hou

# Get scene root node
sceneRoot = hou.node('/obj/')
# Check if "MY_GEO" exists
if hou.node('/obj/MY_GEO') == None:
    # Create empty geometry node in scene root
    geometry = sceneRoot.createNode('geo', run_init_scripts=False)
    # Set geometry node name
    geometry.setName('MY_GEO')

Now the script will create "MY_GEO" node only if this name is unique the for current scene. If the "MY_GEO" node exists, Geo Creator will do nothing and it could be frustrating for the end users. Imagine, you run a script and nothing happens, so you don't know what is going on...

Scripts always should inform users about program execution.

Usually, it could be print statements (seeing in Houdini Console) indicated particular results. For our example we will raise the window with the appropriate message in both cases, if the "MY_GEO" node exists or if it has been created:

import hou

# Get scene root node
sceneRoot = hou.node('/obj/')
# Check if "MY_GEO" exists
if hou.node('/obj/MY_GEO') == None:
    # Create empty geometry node in scene root
    geometry = sceneRoot.createNode('geo', run_init_scripts=False)
    # Set geometry node name
    geometry.setName('MY_GEO')
    # Display creation message
    hou.ui.displayMessage('MY_GEO node created!')
else:
    # Display fail message
    hou.ui.displayMessage('MY_GEO already exists in the scene')

Now let's organize our code a bit better. It may seem redundant to organize several lines of code but we do this for the learning purposes, so you will understand how to build more complex tools in the future and it becomes necessary later when we will start building UI.

What issue should be obvious to improve is that we use the same mane "MY_GEO" several times in our code. Despite it is a static name we are not going to change it (according to the initial brief) it is always better to build a code which is easy to edit and read. Common practice for such cases is to create a variable with a required name once and then use this variable in the code as many time as you need later:

import hou

# Define geometry node name
geometryName = 'MY_GEO'

# Get scene root node
sceneRoot = hou.node('/obj/')
# Check if "MY_GEO" exists
if hou.node('/obj/{}'.format(geometryName)) == None:
    # Create empty geometry node in scene root
    geometry = sceneRoot.createNode('geo', run_init_scripts=False)
    # Set geometry node name
    geometry.setName(geometryName)
    # Display creation message
    hou.ui.displayMessage('{} node created!'.format(geometryName))
else:
    # Display fail message
    hou.ui.displayMessage('{} already exists in the scene'.format(geometryName))

Here we use Pyhon string formatting to replace { } with value from variable:

print 'The fruits are {0}, {2}, {1}!'.format('Apple', 'Banana', 'Kiwi')
>>>> The fruits are Apple, Kiwi, Banana!

Now lets wrap a part of our program to a function and run this function:

import hou

# Define geometry node name
geometryName = 'MY_GEO'

def createGeoNode():
    # Get scene root node
    sceneRoot = hou.node('/obj/')
    # Check if "MY_GEO" exists
    if hou.node('/obj/{}'.format(geometryName)) == None:
        # Create empty geometry node in scene root
        geometry = sceneRoot.createNode('geo', run_init_scripts=False)
        # Set geometry node name
        geometry.setName(geometryName)
        # Display creation message
        hou.ui.displayMessage('{} node created!'.format(geometryName))
    else:
        # Display fail message
        hou.ui.displayMessage('{} already exists in the scene'.format(geometryName))

# Execute node creation 
createGeoNode()

And pass the name of geometry to our function explicitly:

import hou

# Define geometry node name
name = 'MY_GEO'

def createGeoNode(geometryName):
    # Get scene root node
    sceneRoot = hou.node('/obj/')
    # Check if "MY_GEO" exists
    if hou.node('/obj/{}'.format(geometryName)) == None:
        # Create empty geometry node in scene root
        geometry = sceneRoot.createNode('geo', run_init_scripts=False)
        # Set geometry node name
        geometry.setName(geometryName)
        # Display creation message
        hou.ui.displayMessage('{} node created!'.format(geometryName))
    else:
        # Display fail message
        hou.ui.displayMessage('{} already exists in the scene'.format(geometryName))

# Execute node creation 
createGeoNode(name)

Here we change the initial name of the variable geometryName to name to avoid confusion on how this variable is being passed through the code. If you remain the old name 'geometryName' the script will work the same.

The last step in organizing is to separate two logical parts of our script, geometry node creation and checking if the node exists:

import hou

# Define geometry node name
name = 'MY_GEO'

def checkExisting(geometryName):
    # Check if "MY_GEO" exists
    if hou.node('/obj/{}'.format(geometryName)):
        # Display fail message
        hou.ui.displayMessage('{} already exists in the scene'.format(geometryName))
        return True
    

def createGeoNode(geometryName):
    # Get scene root node
    sceneRoot = hou.node('/obj/')
    # Create empty geometry node in scene root
    geometry = sceneRoot.createNode('geo', run_init_scripts=False)
    # Set geometry node name
    geometry.setName(geometryName)
    # Display creation message
    hou.ui.displayMessage('{} node created!'.format(geometryName))

# Execute node creation 
if checkExisting(name) != True:
    createGeoNode(name)

Here we have created checkExisting() function, it verifies if a node with input name (name = "MY_GEO") exists in the scene. If so, it raises the message window and returns zero (if not, it returns None). Later we run this function and check if it does not return zero (geometry not exists), we create a geometry node (createGeoNode(name)).

Same functionality but a bit more sophisticated structure which is currently may seem over complicated but its more flexible for the future extensions. For example, if you will need to check more conditions later in addition to an existing name (if the user has select anything we need, if the selection is correct, etc) it will be more easy to implement and the code would be cleaner if we would try to fit everything in one createGeoNode() function.

Create and setup UI

One of the possible workflows for building tools with UI in Houdini is creating PySide interfaces in QT Designer (shipped with Python27) and importing *.ui files into your Python code where you will set up the functionality of UI elements.

PySide is a module which contains all possible UI elements you may need as well as functionality how these elements should work in your scripts. It is so powerful that now PySide is a standard for building interfaces in the majority of applications, including Houdini, Maya, Nuke etc. It could be hard for a complete noob to develop PySide UI but we will keep thing simple and give examples which could help to complete a lot of miscellaneous tasks.

In PySide, any UI element is called a widget. A window is a widget, a button is a widget too. If you have a button in the window this means that button is parented to a window. There are a lot of widgets in PySide to solve a variety of tasks, you can examine the whole list in Widget Box panel in QT Designer. For our GeoCreator tool will use 3 widgets:

• QWidget to create the GeoCreator window (a window will contain all UI we need for the tool),
• QPushButton to create a button (it will run the program that we created earlier),
• QLineEdit to get a text from the user (to set the name of created geometry).

To get QT Designer you need to install Python 2.7 and PySide on your system. QT designer will be located at C:/Python27/Lib/site-packages/PySide/designer.exe

To build our UI:

• Run QT Designer and create a new widget: File > New > Widget. You will have an empty window widget. You can resize it and modify some properties in Object Inspector (vertical panel located on the right side). I change ObjectName to "GeoCreator" and Window Title to "Create Geometry"
• From Widget Box (vertical panel with a list of QT widgets, located on the left side) drag into our window:
  • Line Edit (Input Widgets section), change "ObjectName" parameter to lin_name, "text" to MY_GEO,
  • Push Button (Buttons section), change "ObjectName" parameter to btn_create, "text" to Create Geometry, "minimumSize > height" = 35
• Select Window widget (either click on empty space in the window widget or click on GeoCreator in object inspector) and press "Lay Out Vertically" button on the toolbar.
• Save the file somewhere, for example 'C:/temp/uiGeoCreator.ui'. Here you can download uiGeoCreator.ui

Now clean all code in Python Source editor, go to Run PySide UI section of Python Snippets page, copy paste the code to Python Source editor and change the path to UI file:

import hou
from PySide2 import QtCore, QtUiTools, QtWidgets

class GeoCreator(QtWidgets.QWidget):
    def __init__(self):
        super(GeoCreator,self).__init__()
        ui_file = 'C:/temp/uiGeoCreator.ui'
        self.ui = QtUiTools.QUiLoader().load(ui_file, parentWidget=self)
        self.setParent(hou.ui.mainQtWindow(), QtCore.Qt.Window)

win = GeoCreator()
win.show()

Run this code and you should get your UI in Houdini!

This is a minimum amount of code to run UI file in Houdini, you can always use it as a starting point while developing your tools. Don't worry it contains class (if you have no idea what is it) or looks to scarry it would be not that hard to work with. You may take some Python tutorials about Object-Oriented Programming and classes but even without understanding those concepts you will be able to move forward and even build your own basic stuff.

Classes are the small programs, just like functions. Imagine classes as containers for basic functions. Our class "GeoCreator" contains one function __init__(), where we import UI file in code and setup how all widgets will work.

Any function declaration inside the class should contain self argument. When you define a function without a class:

def helloWorld():
    print 'Hello, World!'

When you define a function inside the class:

def helloWorld(self):
    print 'Hello, World!'

If you have a class named "GeoCreator" you will run this program same way as a usual function: GeoCreator(). If you assign the result of class execution to a variable (win = GeoCreator()) you will create an instance of a class — object win. Same thing we did when create a Houdini root scene object (sceneRoot = hou.node('/obj/')). Then you can apply a specific command to this objects, e.g. to display UI in Houdini we use show() command: win.show().

And you need PySide libraries in your program to make UI widgets works, so we import them at the beginning: from PySide2 import QtCore, QtUiTools, QtWidgets. This basic template will remain the same with any other tool you will want to create, you will need just to build actual functionality on top of that.

Next, we will bring back the functional part of our code we develop previously and link everything with UI elements.

Let's discover how to work with UI widgets we have. We want something happened when the user presses the button, so we need to create a function in the class which will be executed on button click event. Let this function print "Hello, World!" for now:

import hou
from PySide2 import QtCore, QtUiTools, QtWidgets

class GeoCreator(QtWidgets.QWidget):
    def __init__(self):
        super(GeoCreator,self).__init__()
        ui_file = 'C:/temp/uiGeoCreator.ui'
        self.ui = QtUiTools.QUiLoader().load(ui_file, parentWidget=self)
        self.setParent(hou.ui.mainQtWindow(), QtCore.Qt.Window)
        
    def buttonClicked(self):
        print 'Hello, World'

win = GeoCreator()
win.show()

If you run it now the button widget would not work, we need to link the event of particular button click with a corresponding function. We will setup all widgets in __init__ function:

import hou
from PySide2 import QtCore, QtUiTools, QtWidgets

class GeoCreator(QtWidgets.QWidget):
    def __init__(self):
        super(GeoCreator,self).__init__()
        ui_file = 'C:/temp/uiGeoCreator.ui'
        self.ui = QtUiTools.QUiLoader().load(ui_file, parentWidget=self)
        self.setParent(hou.ui.mainQtWindow(), QtCore.Qt.Window)
        
        # Setup "Create Geometry" button
        self.ui.btn_create.clicked.connect(self.buttonClicked)
        
    def buttonClicked(self):
        print 'Hello, World'

win = GeoCreator()
win.show()

Run the code, press "Create Geometry" button and you will get "Hello, World!" printed in Houdini Console.

We link buttonClicked() function to our button widget with this line:
self.ui.btn_create.clicked.connect(self.buttonClicked),

where self.ui is a UI object we created in QT Designer and imported into the code (window with button and text field),

btn_create is a button widget which launches buttonClicked() function (connect(self.buttonClicked)),

and the event which will run this function is a button click (clicked).

Now let's grab the text from the text filed (QLineEdit widget named lin_name) and print it instead of "Hello, World!". You can do it in __init__() function via variable and send this variable value to buttonClicked() function:

import hou
from PySide2 import QtCore, QtUiTools, QtWidgets

class GeoCreator(QtWidgets.QWidget):
    def __init__(self):
        super(GeoCreator,self).__init__()
        ui_file = 'C:/temp/uiGeoCreator.ui'
        self.ui = QtUiTools.QUiLoader().load(ui_file, parentWidget=self)
        self.setParent(hou.ui.mainQtWindow(), QtCore.Qt.Window)
        
        # Get name from user
        customName = self.ui.lin_name.text()
        # Setup "Create Geometry" button
        self.ui.btn_create.clicked.connect(lambda: self.buttonClicked(customName))
        
    def buttonClicked(self, name):
        print name

win = GeoCreator()
win.show()

Where we take text from ui (lin_name widget) and put it in customName variable with self.ui.lin_name.text() and send this text to buttonClicked() function lambda: self.buttonClicked(customName). Alternatively, you can create a variable which will be avaliable in buttonClicked() function (add self before variable name, othervise it would be visible only inside __init__() function) and use it there:

import hou
from PySide2 import QtCore, QtUiTools, QtWidgets

class GeoCreator(QtWidgets.QWidget):
    def __init__(self):
        super(GeoCreator,self).__init__()
        ui_file = 'C:/temp/uiGeoCreator.ui'
        self.ui = QtUiTools.QUiLoader().load(ui_file, parentWidget=self)
        self.setParent(hou.ui.mainQtWindow(), QtCore.Qt.Window)
        
        # Get name from user
        self.customName = self.ui.lin_name.text()
        # Setup "Create Geometry" button
        self.ui.btn_create.clicked.connect(self.buttonClicked)
        
    def buttonClicked(self):
        print self.customName

win = GeoCreator()
win.show()

Or we can grab user text from UI directly in buttonClicked() function:

import hou
from PySide2 import QtCore, QtUiTools, QtWidgets

class GeoCreator(QtWidgets.QWidget):
    def __init__(self):
        super(GeoCreator,self).__init__()
        ui_file = 'C:/temp/uiGeoCreator.ui'
        self.ui = QtUiTools.QUiLoader().load(ui_file, parentWidget=self)
        self.setParent(hou.ui.mainQtWindow(), QtCore.Qt.Window)
        
        # Setup "Create Geometry" button
        self.ui.btn_create.clicked.connect(self.buttonClicked)
        
    def buttonClicked(self):
        # Get name from user
        customName = self.ui.lin_name.text()
        print customName

win = GeoCreator()
win.show()

So now you have several examples of how to deal with data using classes so you can build your own stuff without understanding classes concepts (but I would recommend studying this topic anyway!). This should be enough to build a lot of tools: you know how to take string data from the user with a QlineEdit widget and set up buttons to execute your functions with a QPushButton widget. I build a complete pipeline toolset using only these two widgets.

It's time to bring a functional part of the code we develop before to our tool:

import hou
from PySide2 import QtCore, QtUiTools, QtWidgets

def checkExisting(geometryName):
    # Check if "MY_GEO" exists
    if hou.node('/obj/{}'.format(geometryName)):
        # Display fail message
        hou.ui.displayMessage('{} already exists in the scene'.format(geometryName))
        return True


def createGeoNode(geometryName):
    # Get scene root node
    sceneRoot = hou.node('/obj/')
    # Create empty geometry node in scene root
    geometry = sceneRoot.createNode('geo', run_init_scripts=False)
    # Set geometry node name
    geometry.setName(geometryName)
    # Display creation message
    hou.ui.displayMessage('{} node created!'.format(geometryName))


class GeoCreator(QtWidgets.QWidget):
    def __init__(self):
        super(GeoCreator,self).__init__()
        ui_file = 'C:/temp/uiGeoCreator.ui'
        self.ui = QtUiTools.QUiLoader().load(ui_file, parentWidget=self)
        self.setParent(hou.ui.mainQtWindow(), QtCore.Qt.Window)
         
        # Setup "Create Geometry" button
        self.ui.btn_create.clicked.connect(self.buttonClicked)
        
    def buttonClicked(self):
        customName = self.ui.lin_name.text()
        # Execute node creation 
        if checkExisting(customName) != True:
            createGeoNode(customName)
    

win = GeoCreator()
win.show()

I just copy-paste code which creates geometry node we develop before into existing code with UI, change a hardcoded name variable with a name from UI (customName = self.ui.lin_name.text()), and place code which runs checking and node creation into buttonClicked() function.

You can place all your functions inside the GeoCreator() class:

import hou
from PySide2 import QtCore, QtUiTools, QtWidgets

class GeoCreator(QtWidgets.QWidget):
    def __init__(self):
        super(GeoCreator,self).__init__()
        ui_file = 'C:/temp/uiGeoCreator.ui'
        self.ui = QtUiTools.QUiLoader().load(ui_file, parentWidget=self)
        self.setParent(hou.ui.mainQtWindow(), QtCore.Qt.Window)
        
        # Setup "Create Geometry" button
        self.ui.btn_create.clicked.connect(self.buttonClicked)
        
    def buttonClicked(self):
        customName = self.ui.lin_name.text()
        # Execute node creation 
        if checkExisting(customName) != True:
            createGeoNode(customName)
    
    def checkExisting(self, geometryName):
        # Check if "MY_GEO" exists
        if hou.node('/obj/{}'.format(geometryName)):
            # Display fail message
            hou.ui.displayMessage('{} already exists in the scene'.format(geometryName))
            return True    

    def createGeoNode(self, geometryName):
        # Get scene root node
        sceneRoot = hou.node('/obj/')
        # Create empty geometry node in scene root
        geometry = sceneRoot.createNode('geo', run_init_scripts=False)
        # Set geometry node name
        geometry.setName(geometryName)
        # Display creation message
        hou.ui.displayMessage('{} node created!'.format(geometryName))

win = GeoCreator()
win.show()

Here we added a self method to checkExisting() and createGeoNode() functions to allow them working inside the class.

Run tool from the Shell

Finally, we will create a shelf tool from our Python script. First lets put the last two lines (which launches the tool with UI) inside the run() function:

import hou
from PySide2 import QtCore, QtUiTools, QtWidgets


class GeoCreator(QtWidgets.QWidget):
    def __init__(self):
        super(GeoCreator,self).__init__()
        ui_file = 'C:/temp/uiGeoCreator.ui'
        self.ui = QtUiTools.QUiLoader().load(ui_file, parentWidget=self)
        self.setParent(hou.ui.mainQtWindow(), QtCore.Qt.Window)
        
        # Setup "Create Geometry" button
        self.ui.btn_create.clicked.connect(self.buttonClicked)
        
    def buttonClicked(self):
        customName = self.ui.lin_name.text()
        # Execute node creation 
        if checkExisting(customName) != True:
            createGeoNode(customName)
    
    def checkExisting(self, geometryName):
        # Check if "MY_GEO" exists
        if hou.node('/obj/{}'.format(geometryName)):
            # Display fail message
            hou.ui.displayMessage('{} already exists in the scene'.format(geometryName))
            return True    

    def createGeoNode(self, geometryName):
        # Get scene root node
        sceneRoot = hou.node('/obj/')
        # Create empty geometry node in scene root
        geometry = sceneRoot.createNode('geo', run_init_scripts=False)
        # Set geometry node name
        geometry.setName(geometryName)
        # Display creation message
        hou.ui.displayMessage('{} node created!'.format(geometryName))

def run():
    win = GeoCreator()
    win.show()

Save this code somewhere as a python file, for example in C:/temp/GeoCreator.py Right-click on empty space in "Create" shelf and select "New Tool...", change the label to "Geo Creator", switch to "Script" tab, paste the code and hit "Apply":

import sys
path = 'C:/temp/'
sys.path.append(path)

import GeoCreator
reload(GeoCreator)
GeoCreator.run()

The "Geo Creator" tool will appear on the "Create" shelf which will run our GeoCreator tool. Congratulations, you just finish building a first custom tool for Houdini!

One thing we need to notice is that we used a hardcoded path to our UI and Python file C:/temp which is working fine but it's not the best way to deal with such things. If you create your custom tools you probably would like to make them flexible, reliable, and easy to use in all the projects you might have. So next, and it would be a first step for building your own pipeline, you can create a Houdini Wrapper.

Wrapper

The wrapper is a Python file that defines the software environment and launch Houdini within this environment. It allows controlling a lot of things related to Houdini such as setting up $JOB variable (Houdini project root), defining paths to custom python scripts and HDAs, modifying native Houdini UIs, etc.

Before we can move forward we should define our project folder structure. It's a tricky task and a lot of options could be considered (here is a folder structure example I used to build a pipeline for Maya) but we will keep things simple here.

Say we have all our projects in a "PROJECTS" folder somewhere on HDD: C:/temp/PROJECTS, each in a separate folder "projectName_A", "projectName_B" etc. Lets each project folder contain two folders: 3D and PIPELINE.

The 3D folder is a root of the Houdini project, where we will store all Houdini data. You can use the default Houdini project structure which may be created with the File > New Project tool:

You don't need to set up each Houdini project with the New Project tool, just save this empty folder structure and copy-paste it in all next projects (or write a script to build a project).

The PIPELINE folder will contain our wrapper and all our custom tools.

Save this code as C:/temp/PROJECTS/projectName_A/PIPELINE/runHoudini.py:

# Wrapper to run Houdini with custom environment
import subprocess

# Set path to Houdini executable
houdini = 'C:/Program Files/Side Effects Software/Houdini 16.5.536/bin/houdinifx.exe'

# Run Houdini
subprocess.Popen(houdini)

Save this code as C:/temp/PROJECTS/projectName_A/PIPELINE/runHoudini.bat:

"C:\Python27\python.exe" %cd%\runHoudini.py

The runHoudini.bat runs runHoudini.by via Python located in C:/Python27 (you should have Python installed) and runHoudini.by is the actual wrapper which is doing nothing but launching Houdini right now. Double click on runHoudini.bat and you should have you Houdini launched. Check all paths you have (to Houdini and to Python) if it's not working.

Now, let's add some useful environment stuff to our wrapper:

# Wrapper to run Houdini with custom environment
import os, subprocess

# Set path to Houdini executable
houdini = 'C:/Program Files/Side Effects Software/Houdini 16.5.536/bin/houdinifx.exe'

# SETUP PROJECT ENVIRONMENT
# Get the path to PIPELINE folder
rootPipeline = os.path.dirname(__file__).replace('\\','/')
# Add path to custom python tools
os.environ['PYTHONPATH'] = '{}/tools;&'.format(rootPipeline)
# Create environment variable pointing to a pipeline folder
os.environ['ROOT_PIPELINE'] = rootPipeline
# Set root of houdini project
os.environ['JOB'] = '{}/3D/'.format(os.path.dirname(os.path.dirname(__file__)))

# Run Houdini
subprocess.Popen(houdini)

Discover what we have here:

• rootPipeline variable will contain string path to a pipeline folder, for this particular case it would be C:\temp\PROJECTS\projectName_A\PIPELINE.
• The PYTHONPATH variable allows Python to "see" all files in that location, so we can import them to our code. This variable already exists in our OS, so we add our path to existing paths (using simpols ;& at the end of a string) instead of replacing existing with our.
• The ROOT_PIPELINE is our custom variable, we will use it in our scripts to define relativa paths. For example, we will setup path to UI file via this variable.
• JOB is a core Houdini variable which define a root of Houdini project.

Copy GeoCreator.py and uiGeoCreator.ui to C:/temp/PROJECTS/projectName_A/PIPELINE/tools/ Modify GeoCreator.py, we need to update a path to UI file (don't forget import os module):

import hou
import os
from PySide2 import QtCore, QtUiTools, QtWidgets

class GeoCreator(QtWidgets.QWidget):
    def __init__(self):
        super(GeoCreator,self).__init__()
        ui_file = '{}/tools/uiGeoCreator.ui'.format(os.environ['ROOT_PIPELINE'])
        self.ui = QtUiTools.QUiLoader().load(ui_file, parentWidget=self)
        self.setParent(hou.ui.mainQtWindow(), QtCore.Qt.Window)
        
        # Setup "Create Geometry" button
        self.ui.btn_create.clicked.connect(self.buttonClicked)
        
    def buttonClicked(self):
        customName = self.ui.lin_name.text()
        # Execute node creation 
        if checkExisting(customName) != True:
            createGeoNode(customName)
    
    def checkExisting(self, geometryName):
        # Check if "MY_GEO" exists
        if hou.node('/obj/{}'.format(geometryName)):
            # Display fail message
            hou.ui.displayMessage('{} already exists in the scene'.format(geometryName))
            return True    

    def createGeoNode(self, geometryName):
        # Get scene root node
        sceneRoot = hou.node('/obj/')
        # Create empty geometry node in scene root
        geometry = sceneRoot.createNode('geo', run_init_scripts=False)
        # Set geometry node name
        geometry.setName(geometryName)
        # Display creation message
        hou.ui.displayMessage('{} node created!'.format(geometryName))

def run():
    win = GeoCreator()
    win.show()

Double click runHoudini.bat to launch Houdini. Right-click on "Geo Creator" tool icon in the shelf and select "Edit Tool.." and edit code in the "Script" tab:

import GeoCreator
reload(GeoCreator)
GeoCreator.run()

Hit "Accept" and launch our GeoCreator tool. Now we don't have any hardcoded path in our scripts, so if you will clone the projectName_A folder with content as projectName_B (or even place it in another location) everything will work as expected.

Congratulations, you made the first step to building your own pipeline!