[3.7] bpo-30840: Document relative imports (GH-12831)

Doc/reference/import.rst

@@ -921,6 +921,46 @@ it is sufficient to raise :exc:`ModuleNotFoundError` directly from
 ``None``. The latter indicates that the meta path search should continue,
 while raising an exception terminates it immediately.
 
+.. _relativeimports:
+
+Package Relative Imports
+========================
+
+Relative imports use leading dots. A single leading dot indicates a relative
+import, starting with the current package. Two or more leading dots indicate a
+relative import to the parent(s) of the current package, one level per dot
+after the first. For example, given the following package layout::
+
+    package/
+        __init__.py
+        subpackage1/
+            __init__.py
+            moduleX.py
+            moduleY.py
+        subpackage2/
+            __init__.py
+            moduleZ.py
+        moduleA.py
+
+In either ``subpackage1/moduleX.py`` or ``subpackage1/__init__.py``,
+the following are valid relative imports::
+
+    from .moduleY import spam
+    from .moduleY import spam as ham
+    from . import moduleY
+    from ..subpackage1 import moduleY
+    from ..subpackage2.moduleZ import eggs
+    from ..moduleA import foo
+
+Absolute imports may use either the ``import <>`` or ``from <> import <>``
+syntax, but relative imports may only use the second form; the reason
+for this is that::
+
+    import XXX.YYY.ZZZ
+
+should expose ``XXX.YYY.ZZZ`` as a usable expression, but .moduleY is
+not a valid expression.
+
 
 Special considerations for __main__
 ===================================

Doc/reference/simple_stmts.rst

@@ -829,7 +829,8 @@ exists. Two dots means up one package level. Three dots is up two levels, etc.
 So if you execute ``from . import mod`` from a module in the ``pkg`` package
 then you will end up importing ``pkg.mod``. If you execute ``from ..subpkg2
 import mod`` from within ``pkg.subpkg1`` you will import ``pkg.subpkg2.mod``.
-The specification for relative imports is contained within :pep:`328`.
+The specification for relative imports is contained in
+the :ref:`relativeimports` section.
 
 :func:`importlib.import_module` is provided to support applications that
 determine dynamically the modules to be loaded.

Misc/NEWS.d/next/Documentation/2019-04-14-19-46-21.bpo-30840.R-JFzw.rst

@@ -0,0 +1 @@
+Document relative imports