#7124: Fix `--doctest-modules` crashing when `__main__.py` is present (#8949)

* Fix ``--doctest-modules`` crashing when ``__main__.py`` is present

changelog/7124.bugfix.rst

@@ -0,0 +1 @@
+Fixed an issue where ``__main__.py`` would raise an ``ImportError`` when ``--doctest-modules`` was provided.

src/_pytest/doctest.py

@@ -125,7 +125,9 @@ def pytest_collect_file(
 ) -> Optional[Union["DoctestModule", "DoctestTextfile"]]:
     config = parent.config
     if fspath.suffix == ".py":
-        if config.option.doctestmodules and not _is_setup_py(fspath):
+        if config.option.doctestmodules and not any(
+            (_is_setup_py(fspath), _is_main_py(fspath))
+        ):
             mod: DoctestModule = DoctestModule.from_parent(parent, path=fspath)
             return mod
     elif _is_doctest(config, fspath, parent):
@@ -148,6 +150,10 @@ def _is_doctest(config: Config, path: Path, parent: Collector) -> bool:
     return any(fnmatch_ex(glob, path) for glob in globs)
 
 
+def _is_main_py(path: Path) -> bool:
+    return path.name == "__main__.py"
+
+
 class ReprFailDoctest(TerminalRepr):
     def __init__(
         self, reprlocation_lines: Sequence[Tuple[ReprFileLocation, Sequence[str]]]

testing/example_scripts/doctest/main_py/__main__.py

@@ -0,0 +1,2 @@
+def test_this_is_ignored():
+    assert True

testing/example_scripts/doctest/main_py/test_normal_module.py

@@ -0,0 +1,6 @@
+def test_doc():
+    """
+    >>> 10 > 5
+    True
+    """
+    assert False

testing/test_doctest.py

@@ -6,6 +6,7 @@ from typing import Optional
 
 import pytest
 from _pytest.doctest import _get_checker
+from _pytest.doctest import _is_main_py
 from _pytest.doctest import _is_mocked
 from _pytest.doctest import _is_setup_py
 from _pytest.doctest import _patch_unwrap_mock_aware
@@ -811,6 +812,11 @@ class TestDoctests:
         result = pytester.runpytest(p, "--doctest-modules")
         result.stdout.fnmatch_lines(["*collected 0 items*"])
 
+    def test_main_py_does_not_cause_import_errors(self, pytester: Pytester):
+        p = pytester.copy_example("doctest/main_py")
+        result = pytester.runpytest(p, "--doctest-modules")
+        result.stdout.fnmatch_lines(["*collected 2 items*", "*1 failed, 1 passed*"])
+
     def test_invalid_setup_py(self, pytester: Pytester):
         """
         Test to make sure that pytest reads setup.py files that are not used
@@ -1518,3 +1524,11 @@ def test_is_setup_py_different_encoding(tmp_path: Path, mod: str) -> None:
     )
     setup_py.write_bytes(contents.encode("cp1252"))
     assert _is_setup_py(setup_py)
+
+
+@pytest.mark.parametrize(
+    "name, expected", [("__main__.py", True), ("__init__.py", False)]
+)
+def test_is_main_py(tmp_path: Path, name: str, expected: bool) -> None:
+    dunder_main = tmp_path.joinpath(name)
+    assert _is_main_py(dunder_main) == expected