Blender 5.1 compatibility fixes

[kelitraynaud]

Hi! I ran 2.6.7 on Blender 5.1.1 (Python 3.13) and hit three more issues. I've patched all of them locally with Claude, let me know if this can help you to push a new version.

TL;DR

# | Severity | File | Symptom on Blender 5.1.1 -- | -- | -- | -- 1 | Blocker | core/utilities.py, resources/extensions/instance_assets.py | AttributeError: 'Action' object has no attribute 'fcurves' 2 | Blocker | dependencies/rpc/factory.py | SyntaxError: expected 'else' after 'if' expression when calling any remote Unreal method 3 | Cosmetic | dependencies/rpc/factory.py | SyntaxWarning: invalid escape sequence '\.'

Issues 1 and 2 each prevent any export. Issue 3 is just a warning under Python 3.12+.

---

1. action.fcurves removed in Blender 5.0

Blender 5.0 finalised the slotted-action migration started in 4.4 and removed the legacy proxy properties action.fcurves, action.groups, and action.id_root. F-Curves now live on the channelbag of each action slot: https://developer.blender.org/docs/release_notes/5.0/python_api/ https://developer.blender.org/docs/release_notes/4.4/upgrading/slotted_actions/

Send2UE accesses action.fcurves directly in six places:

• core/utilities.py — get_custom_property_fcurve_data, remove_object_scale_keyframes, round_keyframes, scale_object_actions
• resources/extensions/instance_assets.py — post_import (anim sequence branch)

Fix

I added two small back-compat helpers at the top of core/utilities.py and routed every existing call through them:

python

def _iter_action_fcurves(action):
    """Yield all F-Curves of an Action; transparent on 4.x and 5.x."""
    if action is None:
        return
    if bpy.app.version[0] >= 5:
        try:
            for layer in action.layers:
                for strip in layer.strips:
                    for channelbag in strip.channelbags:
                        for fcurve in channelbag.fcurves:
                            yield fcurve
            return
        except AttributeError:
            pass
    for fcurve in action.fcurves:
        yield fcurve


def _remove_action_fcurve(action, fcurve):
    """Remove an F-Curve from an Action; transparent on 4.x and 5.x."""
    if bpy.app.version[0] >= 5:
        try:
            for layer in action.layers:
                for strip in layer.strips:
                    for channelbag in strip.channelbags:
                        if fcurve in list(channelbag.fcurves):
                            channelbag.fcurves.remove(fcurve)
                            return
            return
        except AttributeError:
            pass
    action.fcurves.remove(fcurve)

For instance, remove_object_scale_keyframes becomes:

python

for action in actions:
    scale_fcurves = [fc for fc in _iter_action_fcurves(action) if fc.data_path == 'scale']
    for fcurve in scale_fcurves:
        _remove_action_fcurve(action, fcurve)

instance_assets.py imports _iter_action_fcurves from send2ue.core.utilities and uses it in the for fcurve in action.fcurves loop inside post_import.

---

2. _get_docstring / _get_code in dependencies/rpc/factory.py

When the user clicks Send to Unreal, validation calls a remote method (e.g. UnrealRemoteCalls.is_using_legacy_fbx_importer()). On Blender 5.1.1 the call fails twice in a row:

2a) Client-side exec() in _get_docstring

python

exec('\n'.join(code))
function_instance = locals().copy().get(function_name)
return function_instance.__doc__

This re-execs the function source just to read __doc__. Python 3.13's parser raises SyntaxError: expected 'else' after 'if' expression on several UnrealRemoteCalls method sources (the methods themselves are fine — it's the round-trip through inspect.getsource → textwrap.dedent → split-and-filter → join that produces something the new parser doesn't like in edge cases involving comprehensions and conditional expressions).

The exec is unnecessary: at the only call site we already hold the function object. I changed it to:

python

@staticmethod
def _get_docstring(code, function_name, function=None):
    if function is not None and getattr(function, "__doc__", None) is not None:
        return function.__doc__
    # Fallback for the rare case where the function object isn't supplied:
    # extract the first triple-quoted block from the source without exec.
    import re
    source = '\n'.join(code)
    m = re.search(r'(?P<q>\"{3}|\'{3})(?P<body>.*?)(?P=q)', source, re.DOTALL)
    return m.group('body') if m else None

…and the single caller forwards the function:

python

doc_string = self._get_docstring(code, function.__name__, function=function)

2b) Server-side exec() chokes on the stripped docstring

After 2a was fixed, Send to Unreal then crashed on the server side:

File "send2ue/dependencies/rpc/base_server.py", line 208, in add_new_callable
    exec(code)
File "<string>", line 3
    Checks to see if a directory exist in unreal.
              ^^^^^^^^
SyntaxError: expected 'else' after 'if' expression

The culprit is the docstring-stripping logic in _get_code:

python

if doc_string:
    code = '\n'.join(code).replace(doc_string, '')
    code = [line for line in code.split('\n')
            if not all([char == '"' or char == "'" for char in line.strip()])]

function.__doc__ preserves the source's leading indentation, but inspect.getsource is fed through textwrap.dedent first, so the indentation no longer matches. str.replace silently fails to remove anything. Then the second filter strips lines composed entirely of triple-quotes — which means the opening and closing """ get removed but the body stays. The body becomes naked text in the function (Checks to see if a directory exist in unreal.) and Python 3.13 sees the if and demands an else.

The simplest robust fix is to stop trying to strip docstrings. They're valid Python and add only a handful of bytes per RPC call. I replaced the block with:

python

# Docstrings are valid Python; leaving them in is harmless and avoids a
# fragile indentation-sensitive string replace.
_ = doc_string
return code

After this, Send to Unreal completes end-to-end on Blender 5.1.1.

---

3. SyntaxWarning: invalid escape sequence '\.'

Just re.split('\.|\(| ', line.strip()) at factory.py:126 — needs an r prefix. Not blocking, but noisy in the console on Python 3.12+.

python

if key in re.split(r'\.|\(| ', line.strip()):

---

Testing

I tested on:

• Blender 5.1.1 with Python 3.13, Unreal 5.7.4

Static mesh, skeletal mesh, animation export, and instance-assets extension all work.

Kelit

[lhappyfacel]

So I'm confused is there a fix for windows 11?

[kelitraynaud]

it's for win11 yes, i can share a working zip
https://drive.google.com/file/d/1tb2U_VEl3QZLAlkZTI3MH8igz-QjPyPn/view?usp=drive_link

[kelitraynaud]

oops accidentally closed the issue

[lhappyfacel]

it's for win11 yes, i can share a working zip https://drive.google.com/file/d/1tb2U_VEl3QZLAlkZTI3MH8igz-QjPyPn/view?usp=drive_link

tyvm

[nsdoanfosan]

Thank you for sharing this.

I had originally given up on it, but I was able to confirm that it works properly in Blender 5.1.2 and Unreal Engine 5.7.4 as well.

Thanks again!

[REMvisual]

Posted by Claude (Claude Code) on behalf of @REMvisual — sharing our diagnosis in case it helps land a Blender 5.1 release.

Confirming item #2 and tying a few threads together, since it's the highest-impact one.

Root cause (item #2): It's a Python 3.13 change, not UE-specific. 3.13 strips leading indentation from __doc__, so in rpc/factory.py::_get_code the '\n'.join(code).replace(doc_string, '') no longer matches the still-indented source. The docstring text survives and gets shipped to UE's interpreter, which tries to parse prose as code — hence line 3: "Checks to see if a directory exist…" → expected 'else' after 'if'. Because every UnrealRemoteCalls method has a docstring, this breaks all 42 remote calls, not just directory_exists — i.e. nothing can be sent at all on Blender 5.x.

This is a duplicate of #192, and a fix already exists: PR #190 implements a clean AST-based docstring strip (parse the function, locate the docstring node, splice it out) plus a regression test. It was closed without review. That approach is more robust than a regex/__doc__ match (it can't be fooled by string literals that contain """), so it'd be worth reopening/merging #190 for this item rather than starting over.

Item #3 (SyntaxWarning: invalid escape sequence '\.') is factory.py line ~117: re.split('\.|\(| ', ...) — fixed by making it a raw string: re.split(r'\.|\(| ', ...).

Verified the item-#2 root cause and fix on Blender 5.1.2 (Python 3.13.9) + UE5 remote execution against send2ue 2.6.7.

[DanaFo]

I was checking out the zip file posted here, I did some work on Claude to stress test this a bit using a python parity checker.
Confirming this patch works, on a different engine version than the one you tested.

Environment

• Send2UE 2.6.7 + the Kelit_b5 zip from this thread
• Blender [5.1.2] / Python 3.13
• Unreal Engine 5.8 (you tested 5.7.4)
• Windows, legacy FBX importer forced via Interchange.FeatureFlags.Import.FBX=False

What I exercised

• Static mesh export — OK
• Skeletal mesh export (armature + skinned mesh) — OK, bones and channel-0 UVs intact
• Animation sequence export — OK: imports 1:1, scale is correct, plays correctly in PIE
• All exported pieces show correct scale and consistent pivot offsets in-engine
• Material slots transfer as expected (slots only, no textures — as designed)

Since fix (1) rewrites remove_object_scale_keyframes and scale_object_actions, the animation export is the case that actually executes the patched scale-keyframe path — mesh-only exports never touch it. That path is working on 5.8.

Extra data point on the FBX geometry path
I exported the same rigged piece through both Send2UE and Better FBX 6.3.0 (native Autodesk FBX SDK, so a fully independent implementation) and compared the files directly:

• 1184 vertices / 4814 loops / 1225 polygons — identical on both sides
• Channel-0 UVs bit-identical: max per-loop delta 0.000e+00, matching order-independent fingerprints
• Different FBX binary versions (7400 from Send2UE, 7500 from Better FBX), same result

So the Blender-4-era monkeypatches in core/io/fbx_b4.py are not perturbing geometry or UVs under Blender 5.x — consistent with those patches being armature/bindpose/animation-only.

One thing missing from the zip
Fix (3) is not applied in the distributed Kelit_b5 build — dependencies/rpc/factory.py:126 still reads:

if key in re.split('\.|\(| ', line.strip()):

without the r prefix, so the SyntaxWarning is still emitted on Python 3.12+. Cosmetic only, but worth folding in if this becomes a PR.

Thanks for the zip!