Error raising error in calc_packaging

[jbhopkins]

I'm trying to use comtypes to interface with a vendor provided COM interface. When I run it, I get the following error message:

(wyatt) PS C:\wyatt-control> python .\test_api.py
Traceback (most recent call last):
  File "C:\wyatt-control\test_api.py", line 11, in <module>
    astra_com = comtypes.client.CreateObject("WTC.Astra8.Application.1")
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\client\_create.py", line 122, in CreateObject
    return _manage(obj, clsid, interface=interface)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\client\_managing.py", line 113, in _manage
    obj = GetBestInterface(obj)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\client\_managing.py", line 87, in GetBestInterface
    mod = GetModule(tlib)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\client\_generate.py", line 127, in GetModule
    return ModuleGenerator(tlib, pathname).generate()
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\client\_generate.py", line 232, in generate
    wrp_code = codegen.generate_wrapper_code(items, filename=self.pathname)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\tools\codegenerator\codegenerator.py", line 202, in generate_wrapper_code
    self.generate_all(items)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\tools\codegenerator\codegenerator.py", line 110, in generate_all
    self.generate(item)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\tools\codegenerator\codegenerator.py", line 106, in generate
    mth(item)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\tools\codegenerator\codegenerator.py", line 370, in Structure
    self.generate(struct.get_body())
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\tools\codegenerator\codegenerator.py", line 106, in generate
    mth(item)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\tools\codegenerator\codegenerator.py", line 396, in StructureBody
    pack = packing.calc_packing(body.struct, fields)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\tools\codegenerator\packing.py", line 58, in calc_packing
    raise PackingError(f"PACKING FAILED: {details}")
UnboundLocalError: local variable 'details' referenced before assignment

Looking at the calc_packing code, the error arises because details is being used to catch the error, and in python 3 such variables are deleted after the except block finishes. I think it could be fixed by changing the except as:

        except PackingError as e:
            details = e
            continue

[junkmd]

Thank you for the report.

There appear to be at least two separate issues involved here:

1. The original packing/layout detection failure (PackingError)
2. A secondary UnboundLocalError caused while handling that exception (related to the behavior of variable assignment in except E as N)

The latter is a Python exception scoping issue, and I would first like to fix that so we can properly observe the original packing error.

If possible, could you please rerun the script with the proposed change applied and share:

• the warnings / traceback that are produced
• the generated files under comtypes/gen

Fundamentally, we still need to address the underlying typelib struct/ABI/packing incompatibility.
However, for now, I would like to prioritize releasing a version of comtypes that at least fixes the UnboundLocalError and allows your script to run.
PRs fixing the UnboundLocalError are welcome.

Even if a structure definition is generated without _pack_, it should generally still be safe as long as the structure instance/pointer is treated as an opaque pointer within the Python/comtypes world, without performing field access, sizeof, or memory copy operations.

At this point, we suspect that the error occurs during Structure/Union code generation, but the additional information from your environment would likely help identify the root cause more precisely.

[jbhopkins]

I was able to resolve the original packing error by switching from a 32 bit python to a 64 bit python, so I assume it was a mismatch between the python and COM in that regard. If you want the error message that produced I could go back and generate it again.

[junkmd]

If you want the error message that produced I could go back and generate it again.

That would be much appreciated.
Please also include the files generated in comtypes/gen as well.

[jbhopkins]

Here's the error:

2026-05-11 10:17:31,344 - comtypes.client._generate - MainThread - INFO - Could not import comtypes.gen._368D43B2_3A78_4EB0_93D1_5339084555E2_0_1_0: 40
2026-05-11 10:17:31,347 - comtypes.client._generate - MainThread - INFO - # Generating comtypes.gen._368D43B2_3A78_4EB0_93D1_5339084555E2_0_1_0
C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\tools\codegenerator\codegenerator.py:403: UserWarning: Structure UvDeviceDetails: PACKING FAILED: field deviceModel offset (32/64)
  warnings.warn(message, UserWarning)
C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\tools\codegenerator\codegenerator.py:403: UserWarning: Structure BaselineDetails: PACKING FAILED: field type offset (32/64)
  warnings.warn(message, UserWarning)
C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\tools\codegenerator\codegenerator.py:403: UserWarning: Structure LogonResult: PACKING FAILED: field errorMessage offset (32/64)
  warnings.warn(message, UserWarning)
C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\tools\codegenerator\codegenerator.py:403: UserWarning: Structure SampleInfo: PACKING FAILED: field description offset (32/64)
  warnings.warn(message, UserWarning)
2026-05-11 10:17:31,424 - comtypes.client._generate - MainThread - INFO - # Generating comtypes.gen.AstraLib
Traceback (most recent call last):
  File "C:\wyatt-control\wyatt_control_com.py", line 138, in <module>
    wc = WyattControl()
  File "C:\wyatt-control\wyatt_control_com.py", line 79, in __init__
    self.astra_com = comtypes.client.CreateObject("WTC.ASTRA8.Application.1")
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\client\_create.py", line 122, in CreateObject
    return _manage(obj, clsid, interface=interface)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\client\_managing.py", line 113, in _manage
    obj = GetBestInterface(obj)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\client\_managing.py", line 87, in GetBestInterface
    mod = GetModule(tlib)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\client\_generate.py", line 127, in GetModule
    return ModuleGenerator(tlib, pathname).generate()
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\client\_generate.py", line 240, in generate
    return [_create_module(name, code) for (name, code) in codebases][-1]
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\client\_generate.py", line 240, in <listcomp>
    return [_create_module(name, code) for (name, code) in codebases][-1]
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\client\_generate.py", line 212, in _create_module
    return _my_import(modulename)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\client\_generate.py", line 26, in _my_import
    return importlib.import_module(fullname)
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\importlib\__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "C:\Users\biocat\miniconda3\envs\wyatt\lib\site-packages\comtypes\gen\_368D43B2_3A78_4EB0_93D1_5339084555E2_0_1_0.py", line 159, in <module>
    assert sizeof(UvDeviceDetails) == 72, sizeof(UvDeviceDetails)
AssertionError: 44

Files:
_368D43B2_3A78_4EB0_93D1_5339084555E2_0_1_0.py
_420B2830_E718_11CF_893D_00A0C9054228_0_1_0.py
_00020430_0000_0000_C000_000000000046_0_2_0.py

[junkmd]

@jbhopkins

Thanks for the reproducer.
Either way, it turns out the script won't run because of the sizeof assertion failure.
This definitely requires the bitness of the application and Python to match, which is outside the scope of what comtypes can fix.

That said, resolving the unbound error you encountered would still be useful.
Please feel free to open a pull request with your fix.