This document attempts to help you work with the Blender API in areas that can be troublesome and avoid practices that are known to give instability.

Using Operators

Blender’s operators are tools for users to access, that python can access them too is very useful nevertheless operators have limitations that can make them cumbersome to script.

Main limits are...

  • Can’t pass data such as objects, meshes or materials to operate on (operators use the context instead)
  • The return value from calling an operator gives the success (if it finished or was canceled), in some cases it would be more logical from an API perspective to return the result of the operation.
  • Operators poll function can fail where an API function would raise an exception giving details on exactly why.

Why does an operator’s poll fail?

When calling an operator gives an error like this:

>>> bpy.ops.action.clean(threshold=0.001)
RuntimeError: Operator bpy.ops.action.clean.poll() failed, context is incorrect

Which raises the question as to what the correct context might be?

Typically operators check for the active area type, a selection or active object they can operate on, but some operators are more picky about when they run.

In most cases you can figure out what context an operator needs simply be seeing how it’s used in Blender and thinking about what it does.

Unfortunately if you’re still stuck - the only way to really know whats going on is to read the source code for the poll function and see what its checking.

For python operators it’s not so hard to find the source since it’s included with Blender and the source file/line is included in the operator reference docs.

Downloading and searching the C code isn’t so simple, especially if you’re not familiar with the C language but by searching the operator name or description you should be able to find the poll function with no knowledge of C.


Blender does have the functionality for poll functions to describe why they fail, but its currently not used much, if you’re interested to help improve our API feel free to add calls to CTX_wm_operator_poll_msg_set where its not obvious why poll fails.

>>> bpy.ops.gpencil.draw()
RuntimeError: Operator bpy.ops.gpencil.draw.poll() Failed to find Grease Pencil data to draw into

The operator still doesn’t work!

Certain operators in Blender are only intended for use in a specific context, some operators for example are only called from the properties window where they check the current material, modifier or constraint.

Examples of this are:

Another possibility is that you are the first person to attempt to use this operator in a script and some modifications need to be made to the operator to run in a different context, if the operator should logically be able to run but fails when accessed from a script it should be reported to the bug tracker.

Stale Data

No updates after setting values

Sometimes you want to modify values from python and immediately access the updated values, eg:

Once changing the objects bpy.types.Object.location you may want to access its transformation right after from bpy.types.Object.matrix_world, but this doesn’t work as you might expect.

Consider the calculations that might go into working out the object’s final transformation, this includes:

  • animation function curves.
  • drivers and their pythons expressions.
  • constraints
  • parent objects and all of their f-curves, constraints etc.

To avoid expensive recalculations every time a property is modified, Blender defers making the actual calculations until they are needed.

However, while the script runs you may want to access the updated values.

This can be done by calling bpy.types.Scene.update after modifying values which recalculates all data that is tagged to be updated.

Can I redraw during the script?

The official answer to this is no, or... “You don’t want to do that”.

To give some background on the topic...

While a script executes Blender waits for it to finish and is effectively locked until its done, while in this state Blender won’t redraw or respond to user input. Normally this is not such a problem because scripts distributed with Blender tend not to run for an extended period of time, nevertheless scripts can take ages to execute and its nice to see whats going on in the view port.

Tools that lock Blender in a loop and redraw are highly discouraged since they conflict with Blenders ability to run multiple operators at once and update different parts of the interface as the tool runs.

So the solution here is to write a modal operator, that is - an operator which defines a modal() function, See the modal operator template in the text editor.

Modal operators execute on user input or setup their own timers to run frequently, they can handle the events or pass through to be handled by the keymap or other modal operators.

Transform, Painting, Fly-Mode and File-Select are example of a modal operators.

Writing modal operators takes more effort than a simple for loop that happens to redraw but is more flexible and integrates better with Blenders design.

Ok, Ok! I still want to draw from python

If you insist - yes its possible, but scripts that use this hack wont be considered for inclusion in Blender and any issues with using it wont be considered bugs, this is also not guaranteed to work in future releases.

bpy.ops.wm.redraw_timer(type='DRAW_WIN_SWAP', iterations=1)

Matrix multiplication is wrong

Every so often users complain that Blenders matrix math is wrong, the confusion comes from mathutils matrices being column-major to match OpenGL and the rest of Blenders matrix operations and stored matrix data.

This is different to numpy which is row-major which matches what you would expect when using conventional matrix math notation.

I can’t edit the mesh in edit-mode!

Blender’s EditMesh is an internal data structure (not saved and not exposed to python), this gives the main annoyance that you need to exit edit-mode to edit the mesh from python.

The reason we have not made much attempt to fix this yet is because we will likely move to BMesh mesh API eventually, so any work on the API now will be wasted effort.

With the BMesh API we may expose mesh data to python so we can write useful tools in python which are also fast to execute while in edit-mode.

For the time being this limitation just has to be worked around but we’re aware its frustrating needs to be addressed.

EditBones, PoseBones, Bone... Bones

Armature Bones in Blender have three distinct data structures that contain them. If you are accessing the bones through one of them, you may not have access to the properties you really need.


In the following examples bpy.context.object is assumed to be an armature object.

Edit Bones contains a editbones; to access them you must set the armature mode to edit mode first (editbones do not exist in object or pose mode). Use these to create new bones, set their head/tail or roll, change their parenting relationships to other bones, etc.

Example using bpy.types.EditBone in armature editmode:

This is only possible in edit mode.

>>>["Bone"].head = Vector((1.0, 2.0, 3.0))

This will be empty outside of editmode.

>>> mybones = bpy.context.selected_editable_bones

Returns an editbone only in edit mode.

>>> bpy.context.active_bone

Bones (Object Mode) contains bones. These live in object mode, and have various properties you can change, note that the head and tail properties are read-only.

Example using bpy.types.Bone in object or pose mode:

Returns a bone (not an editbone) outside of edit mode

>>> bpy.context.active_bone

This works, as with blender the setting can be edited in any mode

>>>["Bone"].use_deform = True

Accessible but read-only

>>> tail =["Bone"].tail

Pose Bones

bpy.context.object.pose.bones contains pose bones. This is where animation data resides, i.e. animatable transformations are applied to pose bones, as are constraints and ik-settings.

Examples using bpy.types.PoseBone in object or pose mode:

# Gets the name of the first constraint (if it exists)

# Gets the last selected pose bone (pose mode only)


Notice the pose is accessed from the object rather than the object data, this is why blender can have 2 or more objects sharing the same armature in different poses.


Strictly speaking PoseBone’s are not bones, they are just the state of the armature, stored in the bpy.types.Object rather than the bpy.types.Armature, the real bones are however accessible from the pose bones - bpy.types.PoseBone.bone

Armature Mode Switching

While writing scripts that deal with armatures you may find you have to switch between modes, when doing so take care when switching out of editmode not to keep references to the edit-bones or their head/tail vectors. Further access to these will crash blender so its important the script clearly separates sections of the code which operate in different modes.

This is mainly an issue with editmode since pose data can be manipulated without having to be in pose mode, however for operator access you may still need to enter pose mode.

Relative File Paths

Blenders relative file paths are not compatible with standard python modules such as sys and os.

Built in python functions don’t understand blenders // prefix which denotes the blend file path.

A common case where you would run into this problem is when exporting a material with assosiated image paths.

>>> bpy.path.abspath(image.filepath)

When using blender data from linked libraries there is an unfortunate complication since the path will be relative to the library rather then the open blend file. When the data block may be from an external blend file pass the library argument from the bpy.types.ID.

>>> bpy.path.abspath(image.filepath, library=image.library)

These returns the absolute path which can be used with native python modules.

Unicode Problems

Python supports many different encodings so there is nothing stopping you from writing a script in latin1 or iso-8859-15.

See pep-0263

However this complicates things for the python api because blend files themselves don’t have an encoding.

To simplify the problem for python integration and script authors we have decided all strings in blend files must be UTF-8 or ASCII compatible.

This means assigning strings with different encodings to an object names for instance will raise an error.

Paths are an exception to this rule since we cannot ignore the existane of non-utf-8 paths on peoples filesystems.

This means seemingly harmless expressions can raise errors, eg.

>>> print(
UnicodeEncodeError: 'ascii' codec can't encode characters in position 10-21: ordinal not in range(128)
>>> =
Traceback (most recent call last):
  File "<blender_console>", line 1, in <module>
TypeError: bpy_struct: item.attr= val: expected a string type, not str

Here are 2 ways around filesystem encoding issues:

>>> print(repr(
>>> import os
>>> filepath_bytes = os.fsencode(
>>> filepath_utf8 = filepath_bytes.decode('utf-8', "replace")
>>> = filepath_utf8

Unicode encoding/decoding is a big topic with comprehensive python documentation, to avoid getting stuck too deep in encoding problems - here are some suggestions:

  • Always use utf-8 encoiding or convert to utf-8 where the input is unknown.
  • Avoid manipulating filepaths as strings directly, use os.path functions instead.
  • Use os.fsencode() / os.fsdecode() rather then the built in string decoding functions when operating on paths.
  • To print paths or to include them in the user interface use repr(path) first or "%r" % path with string formatting.
  • Possibly - use bytes instead of python strings, when reading some input its less trouble to read it as binary data though you will still need to deciede how to treat any strings you want to use with Blender, some importers do this.

Strange errors using ‘threading’ module

Python threading with Blender only works properly when the threads finish up before the script does. By using threading.join() for example.

Heres an example of threading supported by Blender:

import threading
import time

def prod():
    print(threading.current_thread().name, "Starting")

    # do something vaguely useful
    import bpy
    from mathutils import Vector
    from random import random

    prod_vec = Vector((random() - 0.5, random() - 0.5, random() - 0.5))
    print("Prodding", prod_vec)["Cube"].location += prod_vec
    time.sleep(random() + 1.0)
    # finish

    print(threading.current_thread().name, "Exiting")

threads = [threading.Thread(name="Prod %d" % i, target=prod) for i in range(10)]

print("Starting threads...")

for t in threads:

print("Waiting for threads to finish...")

for t in threads:

This an example of a timer which runs many times a second and moves the default cube continuously while Blender runs (Unsupported).

def func():
    import bpy['Cube'].location.x += 0.05

def my_timer():
    from threading import Timer
    t = Timer(0.1, 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.

So far, no work has gone into making Blender’s python integration thread safe, so until its properly supported, best not make use of this.


Pythons threads only allow co-currency and won’t speed up your scripts on multi-processor systems, the subprocess and multiprocess modules can be used with blender and make use of multiple CPU’s too.

Help! My script crashes Blender

Ideally it would be impossible to crash Blender from python however there are some problems with the API where it can be made to crash.

Strictly speaking this is a bug in the API but fixing it would mean adding memory verification on every access since most crashes are caused by the python objects referencing Blenders memory directly, whenever the memory is freed, further python access to it can crash the script. But fixing this would make the scripts run very slow, or writing a very different kind of API which doesn’t reference the memory directly.

Here are some general hints to avoid running into these problems.

  • Be aware of memory limits, especially when working with large lists since Blender can crash simply by running out of memory.
  • Many hard to fix crashes end up being because of referencing freed data, when removing data be sure not to hold any references to it.
  • Modules or classes that remain active while Blender is used, should not hold references to data the user may remove, instead, fetch data from the context each time the script is activated.
  • Crashes may not happen every time, they may happen more on some configurations/operating-systems.


Undo invalidates all bpy.types.ID instances (Object, Scene, Mesh etc).

This example shows how you can tell undo changes the memory locations.

>>> hash(bpy.context.object)
>>> hash(bpy.context.object)

# ... move the active object, then undo

>>> hash(bpy.context.object)

As suggested above, simply not holding references to data when Blender is used interactively by the user is the only way to ensure the script doesn’t become unstable.

Edit Mode / Memory Access

Switching edit-mode bpy.ops.object.mode_set(mode='EDIT') / bpy.ops.object.mode_set(mode='OBJECT') will re-allocate objects data, any references to a meshes vertices/faces/uvs, armatures bones, curves points etc cannot be accessed after switching edit-mode.

Only the reference to the data its self can be re-accessed, the following example will crash.

mesh =
faces = mesh.faces

# this will crash

So after switching edit-mode you need to re-access any object data variables, the following example shows how to avoid the crash above.

mesh =
faces = mesh.faces

# faces have been re-allocated
faces = mesh.faces

These kinds of problems can happen for any functions which re-allocate the object data but are most common when switching edit-mode.

Array Re-Allocation

When adding new points to a curve or vertices’s/edges/faces to a mesh, internally the array which stores this data is re-allocated.

point =[0].bezier_points[0][0].bezier_points.add()

# this will crash! = 1.0, 2.0, 3.0

This can be avoided by re-assigning the point variables after adding the new one or by storing indices’s to the points rather then the points themselves.

The best way is to sidestep the problem altogether add all the points to the curve at once. This means you don’t have to worry about array re-allocation and its faster too since reallocating the entire array for every point added is inefficient.

Removing Data

Any data that you remove shouldn’t be modified or accessed afterwards, this includes f-curves, drivers, render layers, timeline markers, modifiers, constraints along with objects, scenes, groups, bones.. etc.

This is a problem in the API at the moment that we should eventually solve.