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.
-
diff --git a/Doc/library/threading.rst b/Doc/library/threading.rst
index 077ae01..73ede7b 100644
--- a/Doc/library/threading.rst
+++ b/Doc/library/threading.rst
@@ -562,6 +562,13 @@
There is a "main thread" object; this corresponds to the initial thread of
control in the Python program. It is not a daemon thread.
+.. warning::
+
+ 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.
+
There is the possibility that "dummy thread objects" are created. These are
thread objects corresponding to "alien threads", which are threads of control
started outside the threading module, such as directly from C code. Dummy