Python Threads are Not Supported

In short: Python threads cause Blender to crash in hard to diagnose ways. For example, a crash can occur while rendering with Cycles, with Python drivers, while a background thread is used to download some file.

So far, no work has been done to make Blender’s Python integration thread safe, so until it’s properly supported, it’s best not make use of this.

Note that some modules in the Python standard library may use threads as well. An example is the multiprocessing.Queue class.

Python threading with Blender only works properly when the threads finish up before the script does, for example by using threading.join(). In other words, they can only be used while the main Blender thread is blocked from running.

Alternative Approaches

For running Python code independently of Blender, it is recommended to use the multiprocessing module.

Code Examples

Here is an example of threading supported by Blender:

import threading
import requests

urls = [
   "http://localhost:8000/file-1.blend",
   "http://localhost:8000/file-2.blend",
   "http://localhost:8000/file-3.blend",
]


def download(url: str) -> None:
   name = threading.current_thread().name
   print("{}: Starting".format(name))
   response = requests.get(url)
   print("{}: Request status code {}".format(name, response.status_code))


threads = [
   threading.Thread(
      name="thread-{}".format(index),
      target=lambda: download(url),
   )
   for index, url in enumerate(urls)
]

print("Starting threads...")
for t in threads:
   t.start()
print("Waiting for threads to finish...")
for t in threads:
   t.join()
print("Threads all done, now Blender can continue")

This an example of an unsupported case, where a timer which runs many times a second:

from threading import Timer

def my_timer():
      t = Timer(0.1, my_timer)
      t.setDaemon(True)
      t.start()
      print("Running...")

my_timer()

Use cases like the one above, which leave the thread running once the script finishes, may seem to work for a while, but end up causing random crashes or errors in Blender’s own drawing code.