fix: pretend we didn't import third-party packages we use. #1228

tomli couldn't use coverage themselves because we imported it early.
Cleaning sys.modules means their own imports will actually execute after
coverage has started, so their files will be properly measured.

1 files changed, 19 insertions, 1 deletions

diff --git a/tests/test_misc.py b/tests/test_misc.py
index 3858c4f8..077c2434 100644
--- a/tests/test_misc.py
+++ b/tests/test_misc.py
@@ -3,11 +3,13 @@
 
 """Tests of miscellaneous stuff."""
 
+import sys
+
 import pytest
 
 from coverage.exceptions import CoverageException
 from coverage.misc import contract, dummy_decorator_with_args, file_be_gone
-from coverage.misc import Hasher, one_of, substitute_variables
+from coverage.misc import Hasher, one_of, substitute_variables, import_third_party
 from coverage.misc import USE_CONTRACTS
 
 from tests.coveragetest import CoverageTest
@@ -155,3 +157,19 @@ def test_substitute_variables_errors(text):
         substitute_variables(text, VARS)
     assert text in str(exc_info.value)
     assert "Variable NOTHING is undefined" in str(exc_info.value)
+
+
+class ImportThirdPartyTest(CoverageTest):
+    """Test import_third_party."""
+
+    run_in_temp_dir = False
+
+    def test_success(self):
+        mod = import_third_party("pytest")
+        assert mod.__name__ == "pytest"
+        assert "pytest" not in sys.modules
+
+    def test_failure(self):
+        mod = import_third_party("xyzzy")
+        assert mod is None
+        assert "xyzzy" not in sys.modules