Windows适配：关于pythonnet 的默认逻辑netfx

[GGzili]

pywebview / pythonnet runtime auto-adaptation issue report

Summary

On Windows, pythonnet currently defaults to netfx when PYTHONNET_RUNTIME is not explicitly set. In environments where netfx fails but coreclr works, this leads to startup failures for applications that rely on pythonnet through pywebview.

I suggest that pythonnet should automatically detect whether netfx or coreclr is available and switch to the working runtime instead of relying on the current Windows default.

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Environment

• OS: Windows 10/11
• Python: 3.12.9
• pythonnet: 3.0.5
• clr_loader: 0.2.10
• pywebview: 6.2.1

Observed behavior:

• pythonnet.load() without PYTHONNET_RUNTIME fails
• PYTHONNET_RUNTIME=coreclr works successfully

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Problem description

When PYTHONNET_RUNTIME is not set, pythonnet uses the following default logic:

def set_runtime_from_env() -> None:
    from os import environ
    spec = environ.get("PYTHONNET_RUNTIME", "default")
    runtime = _create_runtime_from_spec(spec)
    set_runtime(runtime)

And inside _create_runtime_from_spec():

if spec == "default":
    was_default = True
    if sys.platform == "win32":
        spec = "netfx"
    else:
        spec = "mono"

So on Windows, pythonnet always tries netfx first.

In my case, that default fails with:

RuntimeError: Failed to resolve Python.Runtime.Loader.Initialize
from ...\pythonnet\runtime\Python.Runtime.dll

But if I explicitly set:

os.environ['PYTHONNET_RUNTIME'] = 'coreclr'

then pythonnet.load() succeeds.

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Why this is a problem

netfx is the legacy .NET Framework runtime, while coreclr is the modern .NET runtime. On Windows, many systems now have coreclr available and working, while netfx may fail due to runtime/assembly compatibility issues.

For applications that use pywebview on Windows, the startup path often triggers pythonnet indirectly. If pythonnet chooses the wrong runtime by default, the entire GUI startup fails.

This makes the default behavior fragile for modern Windows environments.

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What netfx and coreclr mean

netfx

netfx refers to .NET Framework, the older Windows .NET runtime.

Characteristics:

• Primarily for Windows
• Depends on the system-installed .NET Framework
• Common in older projects, WinForms, WPF, and legacy desktop assemblies
• Stable for legacy compatibility, but older in design

coreclr

coreclr refers to .NET Core / modern .NET runtime.

Characteristics:

• Microsoft’s newer cross-platform runtime
• Works on Windows, Linux, and macOS
• Modern .NET libraries and assemblies typically use this path
• Better aligned with current .NET ecosystem direction

---

Suggested improvement

I recommend that pythonnet add automatic runtime detection on Windows, such as:

1. Try netfx
2. If netfx fails to load or initialize, automatically fall back to coreclr
3. Optionally allow explicit override through PYTHONNET_RUNTIME

A smarter auto-detection flow could look like:

• If PYTHONNET_RUNTIME is set, respect it
• Otherwise, on Windows:
  • try netfx
  • if initialization fails, try coreclr
• On non-Windows:
  • continue with the current platform-specific default

This would make the library much more robust in real-world applications.

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Expected result

With auto-adaptation in place:

• Users would not need to manually set PYTHONNET_RUNTIME
• Startup would succeed more often on Windows
• Applications using pywebview would avoid unnecessary crashes caused by runtime selection
• Compatibility would be improved for both legacy and modern Windows environments

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Actual result

Without manual override:

• pythonnet defaults to netfx on Windows
• netfx may fail to initialize
• Applications crash during startup

With manual override:

• PYTHONNET_RUNTIME=coreclr
• startup succeeds

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Reproduction steps

Failing case

C:\D\Github-Project\GJ_GenericAgent\python\python.exe -c "import pythonnet; pythonnet.load()"

Result:

• fails with Failed to resolve Python.Runtime.Loader.Initialize

Working case

C:\D\Github-Project\GJ_GenericAgent\python\python.exe -c "import os; os.environ['PYTHONNET_RUNTIME']='coreclr'; import pythonnet; pythonnet.load(); print('ok')"

Result:

• prints ok

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Conclusion

The current Windows default of netfx is too rigid for modern setups.

I suggest pythonnet implement automatic runtime detection and fallback between netfx and coreclr so that applications can start reliably without manual environment configuration.

[Durgeshkarna]

How is this issue coming can you give more details in it?