Fix embedded Python executable on Windows 3.10 (#820)

* Reproduce embedded Python executable selection on Windows

Co-authored-by: Miles Cranmer <miles.cranmer@gmail.com>

* Set the embedded Python executable on Windows 3.10

Co-authored-by: Miles Cranmer <miles.cranmer@gmail.com>

* Record executable initialization fix and remove diagnostic workflow

Co-authored-by: Miles Cranmer <miles.cranmer@gmail.com>

* Verify PySR worker behavior across interpreter fixes

Co-authored-by: Miles Cranmer <miles.cranmer@gmail.com>

* Install PySR test dependencies in worker comparison

* Remove completed Windows comparison workflow

Co-authored-by: Miles Cranmer <miles.cranmer@gmail.com>

---------

Co-authored-by: Miles Cranmer <miles.cranmer@gmail.com>

Author: MilesCranmerBot

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Changelog
 
 ## Unreleased
+* Bug fix: preserve the selected Python executable when embedding Python 3.10 on Windows.
 * Bug fix: avoid precompilation cache miss when `check_bounds` option set.
 * Bug fix: `juliacall` set the environment variable `JULIA_PYTHONCALL_EXECUTABLE`
   instead of `JULIA_PYTHONCALL_EXE`, so child Julia processes resolved a new

pytest/test_all.py

@@ -5,6 +5,37 @@ def test_import():
     import juliacall
 
 
+def test_pythoncall_executable_runs_python():
+    import subprocess
+    import sys
+
+    from juliacall import Main as jl
+
+    code = '''\
+using PythonCall
+sys = pyimport("sys")
+@assert pyconvert(String, sys.executable) == only(ARGS)
+subprocess = pyimport("subprocess")
+output = subprocess.check_output([sys.executable, "-c", "print(6, end='')"])
+print(pyconvert(String, output.decode()))
+'''
+    result = subprocess.run(
+        [
+            str(jl.seval("first(Base.julia_cmd().exec)")),
+            "--project=" + str(jl.seval("dirname(Base.active_project())")),
+            "--startup-file=no",
+            "-e",
+            code,
+            sys.executable,
+        ],
+        capture_output=True,
+        text=True,
+        timeout=180,
+    )
+    assert result.returncode == 0, result.stderr
+    assert result.stdout == "6"
+
+
 def test_newmodule():
     import juliacall
 

src/C/context.jl

@@ -278,6 +278,16 @@ function init_context()
             CTX.pyprogname_w = Base.cconvert(Cwstring, CTX.pyprogname)
             Py_SetProgramName(pointer(CTX.pyprogname_w))
 
+            # Python 3.10 on Windows ignores program_name when resolving the executable.
+            if Sys.iswindows() && startswith(Base.unsafe_string(Py_GetVersion()), "3.10.")
+                ccall(
+                    dlsym(CTX.lib_ptr, :_Py_SetProgramFullPath),
+                    Cvoid,
+                    (Ptr{Cwchar_t},),
+                    pointer(CTX.pyprogname_w),
+                )
+            end
+
             # Start the interpreter and register exit hooks
             Py_InitializeEx(0)
             atexit() do