UE5 - Python & PySide2

Python plugin

Enable the plugins you wish to use. There might be more scripting modules under the scripting tab depending on what you have installed and what you wish to be able to modify with python.

  • Edit > Plugins > Scripting >
    • Editor Scripting Utilities
    • Python Editor Script Plugin
    • Sequencer Scripting
    • etc …

Developer Mode

The developer mode will enable extra warnings eg for deprecated code. It will also generate stub code for use to get auto completion for external IDEs. The stub file will be found here: Your Project > Intermediate > PythonStub > unreal.py. The stub file is generated every time the editor starts and is generated with your exact set of available code, plugins etc. Note that the file is quite large, about 10 mb, so your might have to increase the parsing limit of your external IDE to get the auto-complete working.

  • Edit > Editor Preferences > Plugins > Python
    • Developer Mode : Enable

You can also enable it on a project basis. Note! The tooltip of this setting informs us that “Most of the time you want to enable bDeveloperMode in the Editor Preferences instead”. But if you would like to enabled it on a project basis:

  • Edit > Project Settings > Plugins > Python
    • Developer Mode : Enable
    • Note! as stated above you probably want to enabled it with the editor prefes instead

Python Version

I am using Unreal 5.0 and it uses Python 3.9.7 by default. The path to the Python location is:

  • C:\Program Files\Epic Games\UE_5.0\Engine\Binaries\ThirdParty\Python3\Win64

You can not change the version of Python in a binary build (a build from the launcher). This is because the Python plugin needs to be recompiled. If you are using a source build, the simplest way of replacing the Pyhon version is to set an environment variable called “UE_PYTHON_DIR” and point that to the Python installation you wish to use. The downside to this is that it is not very portable since it ends up baking a path into your build to a local Python SDK. This means that if it were run on another system the user must have the same version of python in the same place. A better way to replace the Python version is to replace the Python version that are shipped with the engine. Theese are stored inside the Third Party directory and then rebuild. Because the location is inside the engine path it will be portable.

Output Log

  • Output Log
    • Window > Output Log
    • With the bottom-left dropdown you can switch between:
      • CMD
      • Python
      • Python (REPL)
    • Clear Log
      • RMB click > Clear Log
    • Filters
      • You can use the Filters dropdown on the top of the Output Log to disable warnings if you find them annoying. (Could be useful to have them on, but in certain situations it might improve the readability of the log)
      • Categories
        • If you only want to log Python print statements
          • Filters > Categories > Show All
            To disable all
          • Then enable LogPython (that you find in the lower middle of the list)

Init Script

If the Editor detects a script file called init_unreal.py in any of the paths it is configured to use, it automatically runs that script immediately.

This is a good approach for situations where you are working on a Project or a Plugin and you know that everyone working with that content needs to run the same initialization code every time the Editor starts up. You could put your initialization code inside a script with this name, and put it into the Content/Python folder within that Project or Plugin.

When you use a relative path to run a Python script, or to import another script module using the import command in one of your scripts, the script that you run or import can be in any path that is listed in the sys.path variable of your Python environment.

To list the paths that unreal looks for python scripts we can run the following in the output log:

  • init_unreal.py
    • If the Editor detects a script file called init_unreal.py in any of the paths it is configured to use it automatically runs that script immediately.
  • Add init file
    • Lets use C:/Users/user_name/Documents/UnrealEngine/Python as the directory where we will keep our init_unreal.py file.
    • If this dir does not exist, create it.
    • In this script we can add additional paths that we want to have in our sys.path, for instance site.packages for PySide2 etc
    • We can also create a menu on the main menu for our tools
    • it might look something like:
import unreal
import sys


import petfactory.menu

  • Project Settings
    • You can also specify Startup Scripts and Additional Paths that the engine will add to sys.path on a per project basis Project Settings > Python > Startup Scrpts etc


Console Variables

  • Note!
    • To be able to use the function below the Console Variables Editor plugin needs to be loaded.
    • Console Variables Editor Docs
  • Set CVar
    • unreal.ConsoleVariablesEditorFunctionLibrary.set_console_variable_by_name_int(‘r.RayTracing.Reflections.MaxBounces’, 3)
    • unreal.ConsoleVariablesEditorFunctionLibrary.set_console_variable_by_name_float(‘r.RayTracing.Reflections.MaxRoughness’, .5)
    • etc …
  • Get CVar
    • unreal.ConsoleVariablesEditorFunctionLibrary.get_console_variable_string_value(‘r.RayTracing.Reflections.MaxBounces’)
    • etc …


  • Get/Set
    • You can read/write properties with get_editor_property/set_editor_property
    • In the python api docs under the section Editor Properties you can see available properties for the class
  • Which class has the property?
    • The properties that you see in the details panel (in the UI of the editor) is not always on the actor itself, but on a component of the actor, so you might have to look up the api docs on the component itself to find the proprties you want to get/set.
  • Find the “API name” of the UI Property
    • To find the property in the api docs that matches the property that you see in the UI, hover the mouse over the property and the first row will be the name of the propery in the docs.
      • Note it will be snake cased and lower cased


  • EditorActorSubsystem
    • When using the “EditorActorSubsystem” class we need to call methods on an instance of the class
    • There are 2 ways of creating an instance that of the subsystem that we can call our methods on
      • actor_subsystem = unreal.get_editor_subsystem(unreal.EditorActorSubsystem)
      • actor_subsystem = unreal.EditorActorSubsystem()
        • Note the parenthesis meaning that we call it to get an instance
      • actors = actor_subsystem.get_all_level_actors()
    • I guess this is similar to the other subsystems…
  • EditorUtilityLibrary
    • unreal.EditorUtilityLibrary.get_selected_assets()
      Note! that get_selected_assets can be called as a static method of the unreal.EditorUtilityLibrary, but it can also be called on the instance of it like actor subsystem. Why it is designed like this is I do not know.
  • EditorAssetLibrary
    • unreal.EditorAssetLibrary.list_assets(directory_path)
      Note! that list_assets is also called as a static method, but can also be called as an instance method


def get_static_mesh_lod_data():

	# note to have generate lods for an asset we can open it up in the static mesh editor

	# and under LOD Settings set the LOD Group combobox to for instance "Large Prop"

	# to regenerate LODs based on the lod group "preset"

	static_mesh_list = get_content_browser_asset_of_class(asset_path='/Game/Pet', class_name='StaticMesh')
	for static_mesh in static_mesh_list:

		static_mesh_name = static_mesh.get_name()
		static_mesh_tri_count_list = []
		num_lods = static_mesh.get_num_lods()

		for lod_index in range(num_lods):
			# a section is if we have multiple unweleded elements in a mesh

			num_sections = static_mesh.get_num_sections(lod_index)
			lod_tri_count = 0

			for section_index in range(num_sections):
				section_data = unreal.ProceduralMeshLibrary.get_section_from_static_mesh(static_mesh, lod_index, section_index)
				# the second element of array is triangles, that we then divide by 3 sine the verts are shared by the triangles

				section_tri_count = len(section_data[1])/3
				lod_tri_count += section_tri_count


		mesh_reduction_percent_list = [int((tri_count/static_mesh_tri_count_list[0])*100) for tri_count in static_mesh_tri_count_list]
		print(f' \n{static_mesh_name} has:\n\t{static_mesh_tri_count_list} triangles which is a reduction of\n\t{mesh_reduction_percent_list} percent\n')

def get_static_mesh_data():
	static_mesh_list = get_content_browser_asset_of_class(asset_path='/Game/Pet', class_name='StaticMesh')
	for static_mesh in static_mesh_list:
		asset_import_data = static_mesh.get_editor_property('asset_import_data')

		# below is a list of the base classes of "FbxStaticMeshImportData"

		# unreal.FbxStaticMeshImportData

		# unreal.FbxMeshImportData

		# unreal.FbxAssetImportData

		# unreal.AssetImportData

		# unreal.Object

		# unreal._ObjectBase

		# unreal._WrapperBase

		# object (the python object class)

		# we will call the method "extract_filenames" of the base class unreal.AssetImportData

		file_path = asset_import_data.extract_filenames()

def get_content_browser_asset_of_class(asset_path, class_name):

	asset_path_list = unreal.EditorAssetLibrary.list_assets(asset_path)
	asset_list = []
	for asset_path in asset_path_list:
		asset_data = unreal.EditorAssetLibrary.find_asset_data(asset_path)
		# asset_name = asset_data.asset_name

		asset_class_name = asset_data.asset_class
		asset = asset_data.get_asset()
		if asset_class_name == class_name:

	return asset_list
def get_all_actors():
	actor_subsystem = unreal.get_editor_subsystem(unreal.EditorActorSubsystem)
	return actor_subsystem.get_all_level_actors()

def get_content_browser_selection():
	'''Note! does not include folders'''
	return unreal.EditorUtilityLibrary.get_selected_assets()

package_path = self.package_path_le.text()
sequence_name = self.sequence_name_le.text()
asset_path = f'{package_path}/{sequence_name}'

asset_data = unreal.EditorAssetLibrary.find_asset_data(asset_path)


If you look inside the python install dir you can se pip there. I read somewhere that you can just use pip to install PySide2 in the python dir that comes with unreal. I have not tried that yet. Instead I just created a virtual env elsewhere, installed Pyside2 in that env and pointed unreal to the site-packages of that install. See this post to setup a virtual environment

  • Add site-pacakges dir
    • Project Settings > Plugins > Python > Additional Paths: ../path/to/venv/Lib/site-packages
    • or use the init_unreal.py script to add it to sys


Here is a small snippet of how to creater a widget that is parented to the main window.

import sys

from PySide2 import QtGui
from PySide2 import QtWidgets

import unreal

class TestWidget(QtWidgets.QWidget):
	def __init__(self, parent=None):
		super(TestWidget, self).__init__(parent)

		vbox = QtWidgets.QVBoxLayout(self)
		btn = QtWidgets.QPushButton('Test')

	def btn_clicked(self):

app = None
if not QtWidgets.QApplication.instance():
	app = QtWidgets.QApplication(sys.argv)	

widget = TestWidget()

Then we can execute the script:

  • Tools > Execute Python Script : Browse to the script location

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