Document import ./. threading issues. #1720705.
diff --git a/Doc/library/thread.rst b/Doc/library/thread.rst
index b0e55ea..968ebf1 100644
--- a/Doc/library/thread.rst
+++ b/Doc/library/thread.rst
@@ -150,6 +150,11 @@
exception will be received by an arbitrary thread. (When the :mod:`signal`
module is available, interrupts always go to the main thread.)
+* The import machinery is not thread safe. In general, an import may not
+ have the side effect of importing a module, and only the main thread
+ should import modules. Imports within or caused by a thread other than
+ the main thread isn't safe.
+
* Calling :func:`sys.exit` or raising the :exc:`SystemExit` exception is
equivalent to calling :func:`exit`.
@@ -170,4 +175,3 @@
* When the main thread exits, it does not do any of its usual cleanup (except
that :keyword:`try` ... :keyword:`finally` clauses are honored), and the
standard I/O files are not flushed.
-