Restore sys.path after importing test modules

src/_pytest/python.py

@@ -515,6 +515,7 @@ def _importtestmodule(self) -> ModuleType:
         # we assume we are only called once per module
         importmode = self.config.getoption("--import-mode")
         fspath = self.fspath
+        old_sys_path = sys.path[:] if importmode != "importlib" else None
         try:
             mod = fspath.pyimport(ensuresyspath=importmode)
         except SyntaxError:
@@ -554,6 +555,9 @@ def _importtestmodule(self) -> ModuleType:
                 "or @pytest.mark.skipif decorators instead, and to skip a "
                 "module use `pytestmark = pytest.mark.{skip,skipif}."
             )
+        finally:
+            if old_sys_path:
+                sys.path[:] = old_sys_path
         self.config.pluginmanager.consider_module(mod)
         return mod
 

testing/python/collect.py

@@ -56,6 +56,21 @@ def test():
             reprec = testdir.inline_run()
             reprec.assertoutcome(passed=1)
 
+    @pytest.mark.parametrize("import_mode", ("append", "prepend", "importlib"))
+    def test__importtestmodule_restores_sys_path(self, import_mode, testdir):
+        p1 = testdir.makepyfile(
+            """
+            import sys
+
+            def test():
+                assert sys.path == {!r}
+            """.format(
+                sys.path
+            )
+        )
+        reprec = testdir.inline_run("--import-mode={}".format(import_mode), str(p1))
+        reprec.assertoutcome(passed=1)
+
     def test_syntax_error_in_module(self, testdir):
         modcol = testdir.getmodulecol("this is a syntax error")
         pytest.raises(modcol.CollectError, modcol.collect)

testing/test_assertion.py

@@ -1409,6 +1409,7 @@ def test_multitask_job():
             multitask_job()
     """
     )
+    testdir.syspathinsert()
     result = testdir.runpytest(p1, "--tb=long")
     result.stdout.fnmatch_lines(
         [

testing/test_assertrewrite.py

@@ -1070,6 +1070,7 @@ def test_loader():
                 assert file.reloaded()
             """,
         )
+        testdir.syspathinsert()
         result = testdir.runpytest()
         result.stdout.fnmatch_lines(["* 1 passed*"])
 

testing/test_monkeypatch.py

@@ -5,6 +5,7 @@
 
 import pytest
 from _pytest.monkeypatch import MonkeyPatch
+from _pytest.pytester import Testdir
 
 
 @pytest.fixture
@@ -318,7 +319,8 @@ def f():
     )
 
 
-def test_importerror(testdir):
+def test_importerror(testdir: Testdir) -> None:
+    testdir.syspathinsert()
     p = testdir.mkpydir("package")
     p.join("a.py").write(
         textwrap.dedent(

testing/test_reports.py

@@ -362,6 +362,7 @@ def test_a():
             """
             )
 
+        testdir.syspathinsert()
         reprec = testdir.inline_run()
 
         reports = reprec.getreports("pytest_runtest_logreport")