Fixing 'Poll Failed' Errors When Calling Blender Operators From Python

Understanding the Cause of “Poll Failed” Errors

When calling Blender operators from Python scripts, you may occasionally see “Poll Failed” errors. These errors occur when the Python script attempts to access the Blender interface while Blender is still processing a previous operation.

For example, imagine you have a script that performs the following steps:

Adds a subdivision surface modifier to the active object
Immediately accesses the updated mesh data

In this case, Blender is still busy adding the subdivision surface when the script attempts to access the mesh data in step 2. This leads to a race condition where the Python script runs ahead of Blender’s internal queue of tasks. When this happens, you get a “Poll Failed” error indicating the previous operator is still executing.

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Calling operators from Python can lead to timeouts

Blender uses a queuing system and runs operators like modifiers and boolean operations asynchronously from Python script execution. This helps keep Blender responsive during long-running tasks.

But it also means Python scripts can continue executing even though Blender is still busy. Accessing data that Blender is still processing leads to the “Poll Failed” errors.

Essentially, Blender is telling the Python script: “Hang on, I’m still working on what you previously told me to do!”

Techniques to Avoid Timeout Errors

There are a few best practices you can follow to avoid “Poll Failed” errors when scripting in Blender:

Using smaller meshes

Large and complex meshes take longer to modify and process. Using simpler assets with fewer vertices can help avoid timeouts between Blender and your Python scripts.

For testing, reduce subdivision levels, use lower polygon modeling techniques, and avoid highly detailed meshes until necessary.

Increasing timeout values

You can increase the timeout threshold for operators using the bpy.context.window_manager.operator_preset_add() function in Python.

This won’t prevent the timeouts, but it does give Blender more time to process before hitting errors. Use this carefully though – long timeouts can still freeze Blender even if it avoids console errors.

Avoiding unnecessary complexity

Try to design scripts that minimize back and forth between Blender data and Python code. Every switch of context has potential for timeouts as you wait for Blender to catch up.

Plan ahead so your scripts perform all the Blender operations first, before accessing the results. This workflow avoids polling for data in mid-process.

Handling “Poll Failed” Errors Gracefully

Despite best efforts, “Poll Failed” errors still occur. Here are ways to handle these gracefully in production scripts:

Checking if operators finished executing

Wrap operator calls in a try/except block and use bpy.ops.wm.operators() to check if the previous operator is still running:

  
    import bpy
    try:
      bpy.ops.object.modifier_add(type='SUBSURF')
      if len(bpy.ops.wm.operators()) > 0:
        print("Previous operator still running")
        # Pause or abort
    except Exception as e:
      print(f"Unexpected error: {e}")

This avoids directly accessing data that may not be ready yet. The script can choose to pause, abort, or retry instead.

Retrying operators if they failed

For non-interactive scripts that don’t require user input, automatically retrying failed operators can help get past transient “Poll Failed” cases.

Use an incrementing counter and sleep() calls to add pauses between retries:

  
    import time
    
    retries = 0
    while retries < 3:
      try:
        # Call operators here
        break # Succeeded, so break out of loop
      except Exception as e:
        retries += 1
        print(f"Retrying operator, retry {retries}/3")
        time.sleep(1) # Pause before retrying

Make sure to cap the retries to avoid endless loops!

Warning users if retries are unsuccessful

Even with retries, some errors will still occur. Make sure to notify users if operators ultimately fail after x retries:

    
    import sys
    
    retries = 3
    for attempt in range(retries):
      try: 
        # Call operators 
        break
      except Exception as e:
        if attempt == retries-1: # Last attempt          
          print("Operator failed after %i retries, aborting" % retries)
          sys.exit(1) # Abort script
        else:
          print("Transient error, retrying operator...")

Quitting gracefully avoids hangs and lets users troubleshoot the larger workflow.

Example Code for Robust Blender Scripting

Here are some concrete examples of using the above techniques for real Blender scripts:

Example using try/except for operator calls

  
    import bpy
    
    try:
      # Add subsurf modifier
      mod = bpy.ops.object.modifier_add(type='SUBSURF')  
    except Exception as e:
      print(f"Adding modifier failed: {e}")
      
    try:  
      # Check if operator is done
      if len(bpy.ops.wm.operators()) > 0:
        print("Previous operator not finished, skipping mesh access")
      else:
        # Now safe to access mesh data
        obj = bpy.context.object  
        print(f"Mesh has {len(obj.data.vertices)} vertices") 
    except Exception as e:
      print(f"Getting mesh failed: {e}")

Here any failures are handled gracefully without halting script execution.

Example for retrying operators

  
    import bpy
    import time
    import sys
    
    attempts = 0   
    max_attempts = 3  
    while attempts < max_attempts:   
      try:
        bpy.ops.object.modifier_apply(modifier="MyModifier")
        print("Modifier applied successfully!")
        break
      except Exception as e:
        attempts += 1
        print(f"Applying modifier failed, retrying (Attempt {attempts}/{max_attempts})")
        time.sleep(2) # Wait 2 seconds
        
    if attempts >= max_attempts:
      print("Failed to apply modifier after %i tries, aborting" % max_attempts)  
      sys.exit(1)

This shows the full workflow - trying the operator, retrying on failure, aborting if too many failures accrue.

Expand and adapt these examples to make your scripts more robust!

Fixing ‘Poll Failed’ Errors When Calling Blender Operators From Python