Fix Python executable detection for embedded Python

- sys.executable returns beam.smp when embedded
- Use sys.prefix to find actual Python binary
- Search for pythonX.Y, python3, python in prefix/bin

Author: benoitc

src/py.erl

@@ -777,21 +777,55 @@ venv_exists(Path) ->
 -spec create_venv(string(), list()) -> ok | {error, term()}.
 create_venv(Path, Opts) ->
     Installer = detect_installer(Opts),
-    Python = proplists:get_value(python, Opts, "python3"),
+    Python = case proplists:get_value(python, Opts, undefined) of
+        undefined -> get_python_executable();
+        P -> P
+    end,
     Cmd = case Installer of
         uv ->
-            %% uv venv is faster
-            case proplists:get_value(python, Opts, undefined) of
-                undefined ->
-                    io_lib:format("uv venv ~s", [quote(Path)]);
-                PyVer ->
-                    io_lib:format("uv venv --python ~s ~s", [quote(PyVer), quote(Path)])
-            end;
+            %% uv venv is faster, use --python to match the running interpreter
+            io_lib:format("uv venv --python ~s ~s", [quote(Python), quote(Path)]);
         pip ->
             io_lib:format("~s -m venv ~s", [quote(Python), quote(Path)])
     end,
     run_cmd(lists:flatten(Cmd)).
 
+%% @private Get the Python executable path
+%% When embedded, sys.executable returns the embedding app (beam.smp)
+%% so we reconstruct the path from sys.prefix and version info
+-spec get_python_executable() -> string().
+get_python_executable() ->
+    Code = <<"
+import sys, os
+# When embedded, sys.executable points to the embedding app
+# Use sys.prefix to find the actual Python installation
+if sys.platform == 'win32':
+    python = os.path.join(sys.prefix, 'python.exe')
+else:
+    ver = f'python{sys.version_info.major}.{sys.version_info.minor}'
+    # Try common locations
+    for path in [
+        os.path.join(sys.prefix, 'bin', ver),
+        os.path.join(sys.prefix, 'bin', 'python3'),
+        os.path.join(sys.prefix, 'bin', 'python'),
+        sys.executable  # fallback
+    ]:
+        if os.path.isfile(path) and os.access(path, os.X_OK):
+            python = path
+            break
+    else:
+        python = 'python3'
+python
+">>,
+    case exec(Code) of
+        ok ->
+            case eval(<<"python">>) of
+                {ok, Path} when is_binary(Path) -> binary_to_list(Path);
+                _ -> "python3"
+            end;
+        _ -> "python3"
+    end.
+
 %% @private Install dependencies from requirements file
 -spec install_deps(string(), string(), list()) -> ok | {error, term()}.
 install_deps(Path, RequirementsFile, Opts) ->

test/py_venv_SUITE.erl

@@ -60,11 +60,36 @@ end_per_suite(_Config) ->
     ok.
 
 init_per_group(_Group, Config) ->
-    Config.
+    %% Get Python executable path from the running interpreter
+    %% Note: sys.executable returns beam.smp when embedded, so we find the actual Python
+    Code = <<"
+import sys, os
+ver = f'python{sys.version_info.major}.{sys.version_info.minor}'
+for path in [
+    os.path.join(sys.prefix, 'bin', ver),
+    os.path.join(sys.prefix, 'bin', 'python3'),
+    os.path.join(sys.prefix, 'bin', 'python'),
+]:
+    if os.path.isfile(path) and os.access(path, os.X_OK):
+        _python_path = path
+        break
+else:
+    _python_path = 'python3'
+">>,
+    ok = py:exec(Code),
+    {ok, PythonPath} = py:eval(<<"_python_path">>),
+    [{python_path, binary_to_list(PythonPath)} | Config].
 
 end_per_group(_Group, _Config) ->
     ok.
 
+%% @private Create venv using the Python from config
+create_test_venv(VenvPath, Config) ->
+    PythonPath = ?config(python_path, Config),
+    Cmd = PythonPath ++ " -m venv " ++ VenvPath,
+    _ = os:cmd(Cmd),
+    ok.
+
 init_per_testcase(_TestCase, Config) ->
     %% Create unique temp directory for each test
     TempDir = filename:join(["/tmp", "py_venv_test_" ++ integer_to_list(erlang:unique_integer([positive]))]),
@@ -174,9 +199,8 @@ test_activate_venv(Config) ->
     TempDir = ?config(temp_dir, Config),
     VenvPath = filename:join(TempDir, "venv"),
 
-    %% Create venv manually
-    Cmd = "python3 -m venv " ++ VenvPath,
-    _ = os:cmd(Cmd),
+    %% Create venv manually using the same Python we're linked against
+    ok = create_test_venv(VenvPath, Config),
 
     %% Activate it
     ok = py:activate_venv(VenvPath),
@@ -192,9 +216,8 @@ test_deactivate_venv(Config) ->
     TempDir = ?config(temp_dir, Config),
     VenvPath = filename:join(TempDir, "venv"),
 
-    %% Create and activate venv
-    Cmd = "python3 -m venv " ++ VenvPath,
-    _ = os:cmd(Cmd),
+    %% Create and activate venv using the same Python we're linked against
+    ok = create_test_venv(VenvPath, Config),
     ok = py:activate_venv(VenvPath),
 
     %% Verify active
@@ -217,9 +240,8 @@ test_venv_info(Config) ->
     {ok, Info1} = py:venv_info(),
     false = maps:get(<<"active">>, Info1),
 
-    %% Create and activate
-    Cmd = "python3 -m venv " ++ VenvPath,
-    _ = os:cmd(Cmd),
+    %% Create and activate using the same Python we're linked against
+    ok = create_test_venv(VenvPath, Config),
     ok = py:activate_venv(VenvPath),
 
     %% After activation, should have all info