SimpleITK is not available on Windows if Slicer is installed in a path that contains special characters

[lassoan]

SimpleITK is not available if Slicer is installed in a path that contains special (non-ASCII) characters.

The reason is that CPython 3.6 on Windows does not correctly compute "sys.prefix" if the path contains non-ASCII characters, therefore lib\Python\Lib\site-packages\easy-install.pth is not found (which adds ./simpleitk-2.0.0-py3.6-win-amd64.egg to the search path at startup) and so SimpleITK's egg is not found.

Steps to reproduce

• Install (or copy) Slicer into a subfolder named "Félix".
• Start Slicer
• Type import SimpleITK in the Python console => SimpleITK module is not found

Prefix folder path can be printed using these commands:

import sys
sys.prefix

Expected behavior

SimpleITK should be found.

Environment

• Slicer version: Slicer-4.11.20200930
• Operating system: Windows

[pieper]

Will this be fixed if we just update to a newer version of Python?

[lassoan]

I see related fixes in Python-3.10, so probably the definitive solution will be to upgrade to that.

A workaround is to type this into the Python console before entering Simple Filters module (or before trying to use SimpleITK anywhere):

import sys
sys.path.append(slicer.app.applicationDirPath()+'/../lib/Python/Lib/site-packages/SimpleITK-2.0.1-py3.6-win-amd64.egg')

As a workaround, we could add paths listed in lib\Python\Lib\site-packages\easy-install.pth at Slicer startup. There could be other consequences of the invalid prefix path, though, so it would not be a very reliable solution. For example, pip_install does not work with corrupted prefix path (simply changing sys.prefix does not solve the problem). Maybe a more effective workaround would be to check the path at startup and advise the user to reinstall Slicer into a location that does not contain special characters.

By the way, even Microsoft seems to run into trouble due to this (see for example https://docs.microsoft.com/en-us/office/troubleshoot/office-suite-issues/username-contains-special-character), and I expect that many other software will have problems, too.

[lassoan]

@jcfr What do you think?
@jamesobutler Have you tried to build Slicer with Python-3.9 or Python-3.10 on Windows?

[jamesobutler]

I have not tried 3.9 or the current 3.10-alpha. I would’ve tried using the PRs mentioned in #5014, but there is currently only the potential for build success with 3.8.

[jcfr]

I started working on updating python-cmake-buildsystem, we are long overdue for an update.

[lassoan]

I've tried a stock Python-3.6.7 and it determined sys.prefix correctly, even if parent folder contained special characters. So, most likely the issue is that Slicer uses UTF-8 process code page and Python does not expect this.

Maybe more recent Python versions are better prepared for this. Maybe some build options are available? In general, switching Python from codepages to UTF-8 seems to be an ongoing debate, with lots of pushback from CPython core developers (worrying about backward compatibility): https://discuss.python.org/t/pep-597-enable-utf-8-mode-by-default-on-windows/3122/58. So, I'm not sure if a good solution exist at all or we need to patch CPython.

[lassoan]

New user report: https://discourse.slicer.org/t/open3d-pip-install-issue/16377

[lassoan]

We should check this again after we switched to the pip-installed (not self-built) SimpleITK.