automatically enable language level 3 if jupyter kernel uses Python 3

CHANGES.rst

@@ -53,6 +53,10 @@ Bugs fixed
 Other changes
 -------------
 
+* The "%%cython" IPython/jupyter magic now defaults to the language level of
+  the current jupyter kernel.  The language level can be set explicitly with
+  "%%cython -2" or "%%cython -3".
+
 * Usage of ``Cython.Distutils.build_ext`` is now discouraged.
 
 

Cython/Build/IpythonMagic.py

@@ -152,8 +152,12 @@ class CythonMagics(Magics):
 
     @magic_arguments.magic_arguments()
     @magic_arguments.argument(
-        '-3', dest='cython3', action='store_true', default=False,
-        help="Switch to Python 3 syntax."
+        '-3', dest='language_level', action='store_const', const=3, default=None,
+        help="Select Python 3 syntax."
+    )
+    @magic_arguments.argument(
+        '-2', dest='language_level', action='store_const', const=2, default=None,
+        help="Select Python 2 syntax."
     )
     @magic_arguments.argument(
         '-c', '--compile-args', action='append', default=[],
@@ -272,7 +276,10 @@ class CythonMagics(Magics):
                     annotate=args.annotate,
                     force=True,
                 )
-                if args.cython3:
+                if args.language_level is not None:
+                    assert args.language_level in (2, 3)
+                    opts['language_level'] = args.language_level
+                elif sys.version_info[0] > 2:
                     opts['language_level'] = 3
                 build_extension.extensions = cythonize([extension], **opts)
             except CompileError:

Cython/Build/Tests/TestIpythonMagic.py

@@ -28,7 +28,7 @@ def f(x):
 """)
 
 cython3_code = py3compat.str_to_unicode("""\
-def f(x):
+def f(int x):
     return 2 / x
 
 def call(x):
@@ -87,13 +87,19 @@ class TestIPythonMagic(CythonTest):
         self.assertEqual(ip.user_ns['g'], 20.0)
 
     def test_cython3(self):
-        # The Cython module named 'mymodule' defines the function f.
+        # The Cython cell defines the functions f() and call().
         ip.run_cell_magic('cython', '-3', cython3_code)
-        # This module can now be imported in the interactive namespace.
         ip.ex('g = f(10); h = call(10)')
         self.assertEqual(ip.user_ns['g'], 2.0 / 10.0)
         self.assertEqual(ip.user_ns['h'], 2.0 / 10.0)
 
+    def test_cython2(self):
+        # The Cython cell defines the functions f() and call().
+        ip.run_cell_magic('cython', '-2', cython3_code)
+        ip.ex('g = f(10); h = call(10)')
+        self.assertEqual(ip.user_ns['g'], 2 // 10)
+        self.assertEqual(ip.user_ns['h'], 2 // 10)
+
     @skip_win32
     def test_extlibs(self):
         code = py3compat.str_to_unicode("""