### Author:  [@user27182](https://discourse.vtk.org/u/user27182)
#### Post date: [January 27, 2026, 4:50pm UTC](https://discourse.vtk.org/t/vtkcommunicator-marshaldataobject-mutates-locale-unexpectedly/16260/1 "2026-01-27T16:50:18Z")
This seems like a bug, since I would expect `locale` not to be mutated when calling vtk code like this:
```auto
import locale
import vtk
l = locale.getlocale()
e = locale.getencoding()
print(f"Locale before: {l}, Encoding: {e}")
vtk.vtkCommunicator.MarshalDataObject(vtk.vtkPolyData(), vtk.vtkCharArray())
l = locale.getlocale()
e = locale.getencoding()
print(f"Locale after: {l}, Encoding: {e}")
```
```auto
Locale before: ('en_US', 'UTF-8'), Encoding: UTF-8
Locale after: (None, None), Encoding: US-ASCII
```
This mutation has implications for python users, since `vtk.vtkCommunicator.MarshalDataObject` is called when pickling vtk objects:
>
This is even more confusing when users aren’t even pickling objects themselves, e.g. pickling is used by `copy.deepcopy`, resulting in locale being unexpectedly mutated with this code:
```py
import copy
import vtk
poly = vtk.vtkPolyData()
poly_copy = copy.deepcopy(poly)
```
Is this normal/expected? Should this be tracked as an issue in GitLab?
---
### Author:  [@ben.boeckel](https://discourse.vtk.org/u/ben.boeckel)
#### Post date: [January 27, 2026, 6:38pm UTC](https://discourse.vtk.org/t/vtkcommunicator-marshaldataobject-mutates-locale-unexpectedly/16260/2 "2026-01-27T18:38:39Z")
@jaswantp
---
### Author:  [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [January 27, 2026, 6:44pm UTC](https://discourse.vtk.org/t/vtkcommunicator-marshaldataobject-mutates-locale-unexpectedly/16260/3 "2026-01-27T18:44:10Z")
@ben.boeckel i do not know about the `vtkCommunicator.MarshalDataObject` method. The marshalling that we worked on is different.
---
### Author:  [@user27182](https://discourse.vtk.org/u/user27182)
#### Post date: [January 27, 2026, 7:09pm UTC](https://discourse.vtk.org/t/vtkcommunicator-marshaldataobject-mutates-locale-unexpectedly/16260/4 "2026-01-27T19:09:04Z")
The marshalling is using a generic data object writer to serialize the vtk object:
>
Using this info, we can reproduce the bug by simply saving the mesh to file, e.g. with `vtkPolyDataWriter`:
```auto
import locale
import vtk
l = locale.getlocale()
e = locale.getencoding()
print(f"Locale before: {l}, Encoding: {e}")
poly = vtk.vtkPolyData()
writer = vtk.vtkPolyDataWriter()
writer.SetFileName("poly.vtk")
writer.SetInputData(poly)
writer.Write()
l = locale.getlocale()
e = locale.getencoding()
print(f"Locale after: {l}, Encoding: {e}")
```
So it’s not specific to pickling or marshalling, but is related to the IO or writer modules.
---
### Author:  [@user27182](https://discourse.vtk.org/u/user27182)
#### Post date: [January 27, 2026, 7:25pm UTC](https://discourse.vtk.org/t/vtkcommunicator-marshaldataobject-mutates-locale-unexpectedly/16260/5 "2026-01-27T19:25:13Z")
and if we comment out `writer.Write()`, then the locale is _not_ mutated. So this mutation is caused by the `Write` method somehow.
Perhaps this is the culprit?
>
---
### Author:  [@ben.boeckel](https://discourse.vtk.org/u/ben.boeckel)
#### Post date: [January 28, 2026, 1:39am UTC](https://discourse.vtk.org/t/vtkcommunicator-marshaldataobject-mutates-locale-unexpectedly/16260/6 "2026-01-28T01:39:16Z")
Yes, that looks to be the culprit. We should convert to using stream object locale APIs instead of messing with the global state.
---
### Author:  [@user27182](https://discourse.vtk.org/u/user27182)
#### Post date: [January 28, 2026, 3:22pm UTC](https://discourse.vtk.org/t/vtkcommunicator-marshaldataobject-mutates-locale-unexpectedly/16260/7 "2026-01-28T15:22:26Z")
> We should convert to using stream object locale APIs instead of messing with the global state.
Sounds like this should be tracked in GitLab then as a bug-fix / maintenance item?
Here’s the original MR: [https://gitlab.kitware.com/vtk/vtk/-/merge\_requests/5080](https://gitlab.kitware.com/vtk/vtk/-/merge_requests/5080)
The locale is _supposed_ to be restored here:
>
But it appears no tests were added with the MR to ensure this actually works.
---
### Author:  [@ben.boeckel](https://discourse.vtk.org/u/ben.boeckel)
#### Post date: [January 28, 2026, 6:17pm UTC](https://discourse.vtk.org/t/vtkcommunicator-marshaldataobject-mutates-locale-unexpectedly/16260/8 "2026-01-28T18:17:13Z")
Even with restoration, any multi-threading will see the locale switch on it suddenly. Yes, please open an issue for it.
---
### Author:  [@user27182](https://discourse.vtk.org/u/user27182)
#### Post date: [January 28, 2026, 6:26pm UTC](https://discourse.vtk.org/t/vtkcommunicator-marshaldataobject-mutates-locale-unexpectedly/16260/9 "2026-01-28T18:26:18Z")
[https://gitlab.kitware.com/vtk/vtk/-/issues/19928](https://gitlab.kitware.com/vtk/vtk/-/issues/19928)