Issue #27487: Warn if submodule already imported before runpy execution

Also try to clarify the find_spec() error message.

1 files changed, 10 insertions, 1 deletions

diff --git a/Lib/runpy.py b/Lib/runpy.py
index af6205d..e290c21 100644
--- a/Lib/runpy.py
+++ b/Lib/runpy.py
@@ -114,6 +114,15 @@ def _get_module_details(mod_name, error=ImportError):
             if e.name is None or (e.name != pkg_name and
                     not pkg_name.startswith(e.name + ".")):
                 raise
+        # Warn if the module has already been imported under its normal name
+        existing = sys.modules.get(mod_name)
+        if existing is not None and not hasattr(existing, "__path__"):
+            from warnings import warn
+            msg = "{mod_name!r} found in sys.modules after import of " \
+                "package {pkg_name!r}, but prior to execution of " \
+                "{mod_name!r}; this may result in unpredictable " \
+                "behaviour".format(mod_name=mod_name, pkg_name=pkg_name)
+            warn(RuntimeWarning(msg))
 
     try:
         spec = importlib.util.find_spec(mod_name)
@@ -121,7 +130,7 @@ def _get_module_details(mod_name, error=ImportError):
         # This hack fixes an impedance mismatch between pkgutil and
         # importlib, where the latter raises other errors for cases where
         # pkgutil previously raised ImportError
-        msg = "Error while finding spec for {!r} ({}: {})"
+        msg = "Error while finding module specification for {!r} ({}: {})"
         raise error(msg.format(mod_name, type(ex).__name__, ex)) from ex
     if spec is None:
         raise error("No module named %s" % mod_name)