fix: pretend we didn't import third-party packages we usenedbat/fix-tomli-tests-1228

2 files changed, 21 insertions, 5 deletions

diff --git a/coverage/misc.py b/coverage/misc.py
index 11dad23e..d11b5ace 100644
--- a/coverage/misc.py
+++ b/coverage/misc.py
@@ -5,6 +5,7 @@
 
 import errno
 import hashlib
+import importlib
 import importlib.util
 import inspect
 import locale
@@ -43,6 +44,24 @@ def isolate_module(mod):
 os = isolate_module(os)
 
 
+def import_extra(modname):
+    """Import a third-party module we need, but might not be installed.
+
+    This also cleans out the module after the import, so that coverage won't
+    appear to have imported it.  This lets the extra package use coverage for
+    their own tests.
+    """
+    try:
+        mod = importlib.import_module(modname)
+    except ImportError:
+        mod = None
+    else:
+        for name in list(sys.modules.keys()):
+            if name.startswith(modname):
+                del sys.modules[name]
+    return mod
+
+
 def dummy_decorator_with_args(*args_unused, **kwargs_unused):
     """Dummy no-op implementation of a decorator with arguments."""
     def _decorator(func):
diff --git a/coverage/tomlconfig.py b/coverage/tomlconfig.py
index 203192c9..91bd61bd 100644
--- a/coverage/tomlconfig.py
+++ b/coverage/tomlconfig.py
@@ -8,13 +8,10 @@ import os
 import re
 
 from coverage.exceptions import CoverageException
-from coverage.misc import substitute_variables
+from coverage.misc import import_extra, substitute_variables
 
 # TOML support is an install-time extra option.
-try:
-    import tomli
-except ImportError:         # pragma: not covered
-    tomli = None
+tomli = import_extra("tomli")
 
 
 class TomlDecodeError(Exception):