New subclass payload layout by youknowone · Pull Request #6319 · RustPython/RustPython

youknowone

🧩 Analysis chain

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🌐 Web query:

CPython ssl.load_dh_params FileNotFoundError errno Windows POSIX behavior

💡 Result:

Short answer

ssl.SSLContext.load_dh_params(dhfile) opens the file with the C runtime and calls OpenSSL's PEM_read_DHparams; if fopen/_Py_fopen_obj fails Python exposes the OS error (FileNotFoundError / OSError) with errno. [1][2]
On POSIX this typically behaves as you expect (fopen fails → errno = ENOENT → FileNotFoundError). On Windows the same underlying fopen can also fail for other reasons (path encoding/mbcs issues, wide/ANSI conversion, or Windows-specific fopen behavior), so you can also get FileNotFoundError or other OSError variants even when a path that looks valid to Python strings is used. This is a known class of platform differences for ssl file-loading APIs. [2][3]

Why this happens (brief technical outline)

CPython's _ssl implementation calls _Py_fopen_obj(filepath, "rb") and then PEM_read_DHparams on the FILE*. If _Py_fopen_obj returns NULL, CPython inspects errno and raises an OSError (FileNotFoundError if errno==ENOENT). If PEM_read_DHparams returns NULL with errno==0, a different SSL error is raised. (Code and reproduction notes shown in community / source snippets). [2]
Differences in how Windows converts Python filename objects to the C fopen call (MBCS/ANSI vs wide APIs used by Python internals, and what OpenSSL or the C runtime expects) produce platform-dependent failures; similar issues have been reported and fixed historically for other ssl file-loading functions (e.g. load_verify_locations). [3][4]

What to do

Verify the file exists and is readable from Python (os.path.exists, open(...,'rb')). If that succeeds but load_dh_params still fails, the problem is likely an encoding/ABI/path-form issue on Windows (non-ASCII characters, reserved names, long paths). Use a plain ASCII path or a path exposed via a file descriptor opened in Python and re-create the file in a known-good location. [1][2]
If you need a robust workaround on Windows for non-encodable paths, open the file yourself in Python (open(path,'rb')) to confirm accessibility; report/attach a minimal reproducer to the CPython issue tracker if fopen succeeds but load_dh_params still fails. See linked reports about related Windows path/encoding behavior in the ssl module. [3][4]

Sources

CPython ssl docs (SSLContext.load_dh_params). [1]
Community excerpt showing the _ssl.c implementation calling _Py_fopen_obj and handling errno / PEM_read_DHparams. [2]
Python bug tracker / issue about Windows path/encoding differences for ssl.load_verify_locations (same class of problem). [3]
Python ssl module documentation note about platform-dependent behavior (OpenSSL/OS differences). [4]

References
[1] Python ssl docs — SSLContext.load_dh_params. (docs.python.org)
[2] Community discussion / snippet showing _ssl__SSLContext_load_dh_params implementation and errno handling. (community.letsencrypt.org)
[3] Python issue: SSLContext.load_verify_locations Windows encoding/path behavior. (bugs.python.org)
[4] Python ssl module notes on platform/OpenSSL differences. (docs.python.org)

Citations:

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Refactor to eliminate the existence check and use proper error handling. Instead of checking Path::exists() and hardcoding errno=2, try to read the file directly and match on std::io::ErrorKind to convert errors properly via into_pyexception(). This approach: