[Bug Fix] Pytester.syspathinsert() has no effect when using runpytest_subprocess() . closes #10651

AUTHORS

@@ -320,6 +320,7 @@ Oliver Bestwalter
 Omar Kohl
 Omer Hadari
 Ondřej Súkup
+Orel Damari
 Oscar Benjamin
 Parth Patel
 Patrick Hayes

changelog/10651.bugfix.rst

@@ -0,0 +1 @@
+Fixed bug where the `Pytester.syspathinsert` has no effect when using subprocess.

src/_pytest/pytester.py

@@ -900,7 +900,14 @@
         if path is None:
             path = self.path
 
-        self._monkeypatch.syspath_prepend(str(path))
+        path_str = str(path)
+        self._monkeypatch.syspath_prepend(path_str)
+        self._syspath_prepended = path_str
+
+        # Store the prepended path in an attribute that persists across method calls
+        if not hasattr(self, "_prepended_syspaths"):
+            self._prepended_syspaths = []
+        self._prepended_syspaths.append(path_str)
 
     def mkdir(self, name: str | os.PathLike[str]) -> Path:
         """Create a new (sub)directory.
@@ -1337,10 +1344,16 @@
 
         You probably want to use :py:meth:`run` instead.
         """
-        env = os.environ.copy()
-        env["PYTHONPATH"] = os.pathsep.join(
-            filter(None, [os.getcwd(), env.get("PYTHONPATH", "")])
-        )
+        env = kw.pop("env", None) or os.environ.copy()
+        pythonpath = env.get("PYTHONPATH", "")
+
+        paths_to_add = [os.getcwd()]
+        if hasattr(self, "_syspath_prepended"):
+            paths_to_add.insert(0, self._syspath_prepended)
+
+        pythonpath = os.pathsep.join(filter(None, [*paths_to_add, pythonpath]))
+
+        env["PYTHONPATH"] = pythonpath
         kw["env"] = env
 
         if stdin is self.CLOSE_STDIN:
@@ -1365,6 +1378,7 @@
         *cmdargs: str | os.PathLike[str],
         timeout: float | None = None,
         stdin: NotSetType | bytes | IO[Any] | int = CLOSE_STDIN,
+        env: dict[str, str] | None = None,
     ) -> RunResult:
         """Run a command with arguments.
 
@@ -1413,6 +1427,7 @@
                 stdout=f1,
                 stderr=f2,
                 close_fds=(sys.platform != "win32"),
+                env=env,
             )
             if popen.stdin is not None:
                 popen.stdin.close()
@@ -1488,8 +1503,33 @@
         plugins = [x for x in self.plugins if isinstance(x, str)]
         if plugins:
             args = ("-p", plugins[0], *args)
-        args = self._getpytestargs() + args
-        return self.run(*args, timeout=timeout)
+
+        env = os.environ.copy()
+        pythonpath = env.get("PYTHONPATH", "")
+
+        if hasattr(self, "_syspath_prepended"):
+            pythonpath = os.pathsep.join(
+                filter(None, [self._syspath_prepended, pythonpath])
+            )
+
+        env["PYTHONPATH"] = pythonpath
+
+        python_executable = sys.executable
+        pytest_command = [python_executable, "-m", "pytest"]
+
+        if hasattr(self, "_syspath_prepended"):
+            prepend_command = (
+                f"import sys; sys.path.insert(0, {self._syspath_prepended!r});"
+            )
+            pytest_command = [
+                python_executable,
+                "-c",
+                f"{prepend_command} import pytest; pytest.main({list(args)!r})",
+            ]
+        else:
+            pytest_command.extend(str(arg) for arg in args)
+
+        return self.run(*pytest_command, timeout=timeout, env=env)
 
     def spawn_pytest(self, string: str, expect_timeout: float = 10.0) -> pexpect.spawn:
         """Run pytest using pexpect.

testing/test_pytester.py

@@ -835,3 +835,37 @@ def test_two():
     result.assert_outcomes(passed=1, deselected=1)
     # If deselected is not passed, it is not checked at all.
     result.assert_outcomes(passed=1)
+
+
+def test_syspathinsert__in_process__path_exists(pytester: Pytester):
+    some_dir = "abcd"
+    pytester.syspathinsert(some_dir)
+    pytester.makepyfile(
+        f"""
+        import sys
+
+        def test_foo():
+            assert "{some_dir}" in sys.path
+        """
+    )
+
+    result = pytester.runpytest_inprocess()
+
+    result.assert_outcomes(passed=1)
+
+
+def test_syspathinsert__sub_process__path_exists(pytester: Pytester):
+    some_dir = "abcd"
+    pytester.syspathinsert(some_dir)
+    pytester.makepyfile(
+        f"""
+        import sys
+
+        def test_foo():
+            assert "{some_dir}" in sys.path
+        """
+    )
+
+    result = pytester.runpytest_subprocess(timeout=1)
+
+    result.assert_outcomes(passed=1)