Add a pythonpath setting to allow paths to be added to sys.path.

changelog/9114.feature.rst

@@ -0,0 +1 @@
+Added :ref:`pythonpath` setting that adds listed paths to `sys.path`.

src/_pytest/config/__init__.py

@@ -256,6 +256,7 @@ def directory_arg(path: str, optname: str) -> str:
     "reports",
     *(["unraisableexception", "threadexception"] if sys.version_info >= (3, 8) else []),
     "faulthandler",
+    "pythonpath",
 )
 
 builtin_plugins = set(default_plugins)

src/_pytest/pythonpath.py

@@ -0,0 +1,28 @@
+import sys
+from typing import List
+
+import pytest
+from pytest import Config
+from pytest import Parser
+
+
+added_paths: List[str]
+
+
+def pytest_addoption(parser: Parser) -> None:
+    parser.addini("pythonpath", type="paths", help="Add paths to sys.path", default=[])
+
+
+@pytest.hookimpl(tryfirst=True)
+def pytest_load_initial_conftests(early_config: Config) -> None:
+    global added_paths
+    added_paths = [str(p) for p in early_config.getini("pythonpath")]
+    # `pythonpath = a b` will set `sys.path` to `[a, b, x, y, z, ...]`
+    for path in reversed(added_paths):
+        sys.path.insert(0, path)
+
+
+def pytest_unconfigure() -> None:
+    global added_paths
+    for path in added_paths:
+        sys.path.remove(path)

testing/test_config.py

@@ -1268,7 +1268,13 @@ def pytest_load_initial_conftests(self):
     pm.register(m)
     hc = pm.hook.pytest_load_initial_conftests
     values = hc._nonwrappers + hc._wrappers
-    expected = ["_pytest.config", m.__module__, "_pytest.capture", "_pytest.warnings"]
+    expected = [
+        "_pytest.config",
+        m.__module__,
+        "_pytest.pythonpath",
+        "_pytest.capture",
+        "_pytest.warnings",
+    ]
     assert [x.function.__module__ for x in values] == expected
 
 

testing/test_pythonpath.py

@@ -0,0 +1,72 @@
+from textwrap import dedent
+
+import pytest
+from _pytest.pytester import Pytester
+
+
+@pytest.fixture()
+def file_structure(pytester: Pytester) -> None:
+    pytester.makepyfile(
+        test_foo="""
+        from foo import foo
+
+        def test_foo():
+            assert foo() == 1
+        """
+    )
+
+    pytester.makepyfile(
+        test_bar="""
+        from bar import bar
+
+        def test_bar():
+            assert bar() == 2
+        """
+    )
+
+    foo_py = pytester.mkdir("sub") / "foo.py"
+    content = dedent(
+        """
+        def foo():
+            return 1
+        """
+    )
+    foo_py.write_text(content, encoding="utf-8")
+
+    bar_py = pytester.mkdir("sub2") / "bar.py"
+    content = dedent(
+        """
+        def bar():
+            return 2
+        """
+    )
+    bar_py.write_text(content, encoding="utf-8")
+
+
+def test_one_dir(pytester: Pytester, file_structure) -> None:
+    pytester.makefile(".ini", pytest="[pytest]\npythonpath=sub\n")
+    result = pytester.runpytest("test_foo.py")
+    result.assert_outcomes(passed=1)
+
+
+def test_two_dirs(pytester: Pytester, file_structure) -> None:
+    pytester.makefile(".ini", pytest="[pytest]\npythonpath=sub sub2\n")
+    result = pytester.runpytest("test_foo.py", "test_bar.py")
+    result.assert_outcomes(passed=2)
+
+
+def test_module_not_found(pytester: Pytester, file_structure) -> None:
+    """Without the pythonpath setting, the module should not be found."""
+    pytester.makefile(".ini", pytest="[pytest]\n")
+    result = pytester.runpytest("test_foo.py")
+    result.assert_outcomes(errors=1)
+    expected_error = "E   ModuleNotFoundError: No module named 'foo'"
+    result.stdout.fnmatch_lines([expected_error])
+
+
+def test_no_ini(pytester: Pytester, file_structure) -> None:
+    """If no ini file, test should error."""
+    result = pytester.runpytest("test_foo.py")
+    result.assert_outcomes(errors=1)
+    expected_error = "E   ModuleNotFoundError: No module named 'foo'"
+    result.stdout.fnmatch_lines([expected_error])