support "%%cython -3" cell magic in IPython

Cython/Build/IpythonMagic.py

@@ -151,6 +151,10 @@ class CythonMagics(Magics):
         self._import_all(module)
 
     @magic_arguments.magic_arguments()
+    @magic_arguments.argument(
+        '-3', dest='cython3', action='store_true', default=False,
+        help="Switch to Python 3 syntax."
+    )
     @magic_arguments.argument(
         '-c', '--compile-args', action='append', default=[],
         help="Extra flags to pass to compiler via the `extra_compile_args` "
@@ -265,9 +269,11 @@ class CythonMagics(Magics):
             try:
                 opts = dict(
                     quiet=quiet,
-                    annotate = args.annotate,
-                    force = True,
+                    annotate=args.annotate,
+                    force=True,
                 )
+                if args.cython3:
+                    opts['language_level'] = 3
                 build_extension.extensions = cythonize([extension], **opts)
             except CompileError:
                 return

Cython/Build/Tests/TestIpythonMagic.py

@@ -22,10 +22,19 @@ except ImportError:
 from Cython.TestUtils import CythonTest
 
 ip = get_ipython()
-code = py3compat.str_to_unicode("""def f(x):
+code = py3compat.str_to_unicode("""\
+def f(x):
     return 2*x
 """)
 
+cython3_code = py3compat.str_to_unicode("""\
+def f(x):
+    return 2 / x
+
+def call(x):
+    return f(*(x,))
+""")
+
 
 if sys.platform == 'win32':
     # not using IPython's decorators here because they depend on "nose"
@@ -77,6 +86,14 @@ class TestIPythonMagic(CythonTest):
         ip.ex('import mymodule; g = mymodule.f(10)')
         self.assertEqual(ip.user_ns['g'], 20.0)
 
+    def test_cython3(self):
+        # The Cython module named 'mymodule' defines the function f.
+        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)
+
     @skip_win32
     def test_extlibs(self):
         code = py3compat.str_to_unicode("""