Add new interactivity mode for ast.

Allow to display the result of the last assignment of it has a unique
target.

docs/source/whatsnew/pr/interactive_assignment.rst

@@ -0,0 +1,22 b''
+IPython can now trigger the display hook on last assignment of cells.
+Up until 6.0 the following code wouldn't show the value of the assigned
+variable::
+
+    In[1]: xyz = "something"
+    # nothing shown
+
+You would have to actually make it the last statement::
+
+    In [2]: xyz = "something else"
+    ...   : xyz
+    Out[2]: "something else"
+
+With the option ``InteractiveShell.ast_node_interactivity='last_expr_or_assign'``
+you can now do::
+
+    In [2]: xyz = "something else"
+    Out[2]: "something else"
+
+This option can be toggled at runtime with the ``%config`` magic, and will
+trigger on assignment ``a = 1``, augmented assignment ``+=``, ``-=``, ``|=`` ...
+as well as type annotated assignments: ``a:int = 2``.

IPython/core/tests/test_displayhook.py

@@ -1,55 +1,103 b''
 from IPython.testing.tools import AssertPrints, AssertNotPrints
 
 ip = get_ipython()
 
 def test_output_displayed():
     """Checking to make sure that output is displayed"""
   
     with AssertPrints('2'):
        ip.run_cell('1+1', store_history=True)
       
     with AssertPrints('2'):
         ip.run_cell('1+1 # comment with a semicolon;', store_history=True)
 
     with AssertPrints('2'):
         ip.run_cell('1+1\n#commented_out_function();', store_history=True)
 
       
 def test_output_quiet():
     """Checking to make sure that output is quiet"""
   
     with AssertNotPrints('2'):
         ip.run_cell('1+1;', store_history=True)
       
     with AssertNotPrints('2'):
         ip.run_cell('1+1; # comment with a semicolon', store_history=True)
 
     with AssertNotPrints('2'):
         ip.run_cell('1+1;\n#commented_out_function()', store_history=True)
 
 def test_underscore_no_overrite_user():
     ip.run_cell('_ = 42', store_history=True)
     ip.run_cell('1+1', store_history=True)
 
     with AssertPrints('42'):
         ip.run_cell('print(_)', store_history=True)
 
     ip.run_cell('del _', store_history=True)
     ip.run_cell('6+6', store_history=True)
     with AssertPrints('12'):
         ip.run_cell('_', store_history=True)
 
 
 def test_underscore_no_overrite_builtins():
     ip.run_cell("import gettext ; gettext.install('foo')", store_history=True)
     ip.run_cell('3+3', store_history=True)
 
     with AssertPrints('gettext'):
         ip.run_cell('print(_)', store_history=True)
 
     ip.run_cell('_ = "userset"', store_history=True)
 
     with AssertPrints('userset'):
         ip.run_cell('print(_)', store_history=True)
     ip.run_cell('import builtins; del builtins._')
 
+
+def test_interactivehooks_ast_modes():
+    """
+    Test that ast nodes can be triggerd with different modes
+    """
+    saved_mode = ip.ast_node_interactivity
+    ip.ast_node_interactivity = 'last_expr_or_assign'
+
+    try:
+        with AssertPrints('2'):
+            ip.run_cell('a = 1+1', store_history=True)
+
+        with AssertPrints('9'):
+            ip.run_cell('b = 1+8 # comment with a semicolon;', store_history=False)
+
+        with AssertPrints('7'):
+            ip.run_cell('c = 1+6\n#commented_out_function();', store_history=True)
+
+        ip.run_cell('d = 11', store_history=True)
+        with AssertPrints('12'):
+            ip.run_cell('d += 1', store_history=True)
+
+        with AssertNotPrints('42'):
+            ip.run_cell('(u,v) = (41+1, 43-1)')
+
+    finally:
+        ip.ast_node_interactivity = saved_mode
+
+def test_interactivehooks_ast_modes_semi_supress():
+    """
+    Test that ast nodes can be triggerd with different modes and supressed
+    by semicolon
+    """
+    saved_mode = ip.ast_node_interactivity
+    ip.ast_node_interactivity = 'last_expr_or_assign'
+
+    try:
+        with AssertNotPrints('2'):
+            ip.run_cell('x = 1+1;', store_history=True)
+
+        with AssertNotPrints('7'):
+            ip.run_cell('y = 1+6; # comment with a semicolon', store_history=True)
+
+        with AssertNotPrints('9'):
+            ip.run_cell('z = 1+8;\n#commented_out_function()', store_history=True)
+
+    finally:
+        ip.ast_node_interactivity = saved_mode