Implement --force by choosing a unique module name (using the current timestamp).

Otherwise it breaks in Windows since the existing .pyd file cannot be
overwritten, and is otherwise ineffective since the module won't be able
to be imported again.

IPython/extensions/cythonmagic.py

@@ -21,6 +21,7 b' import imp'
 import io
 import os
 import sys
+import time
 
 try:
     import hashlib
@@ -133,13 +134,12 b' class CythonMagics(Magics):'
     )
     @magic_arguments.argument(
         '-f', '--force', action='store_true', default=False,
-        help="Force the compilation of the pyx module even if it hasn't "
-             "changed"
+        help="Force the compilation of a new module, even if the source has been "
+             "previously compiled."
     )
     @magic_arguments.argument(
         '-a', '--annotate', action='store_true', default=False,
         help="Produce a colorized HTML version of the source. "
-             "(Implies --force)."
     )
     @cell_magic
     def cython(self, line, cell):
@@ -161,16 +161,27 b' class CythonMagics(Magics):'
         lib_dir = os.path.join(self.shell.ipython_dir, 'cython')
         quiet = True
         key = code, sys.version_info, sys.executable, Cython.__version__
+
+        if not os.path.exists(lib_dir):
+            os.makedirs(lib_dir)
+
+        if args.force:
+            # Force a new module name by adding the current time to the
+            # key which is hashed to determine the module name.
+            key += time.time(),
+
         module_name = "_cython_magic_" + hashlib.md5(str(key).encode('utf-8')).hexdigest()
         module_path = os.path.join(lib_dir, module_name+self.so_ext)
 
-        if args.annotate:
-            args.force = True
+        have_module = os.path.isfile(module_path)
+        need_cythonize = not have_module
 
-        if not os.path.exists(lib_dir):
-            os.makedirs(lib_dir)
+        if args.annotate:
+            html_file = os.path.join(lib_dir, module_name + '.html')
+            if not os.path.isfile(html_file):
+                need_cythonize = True
 
-        if args.force or not os.path.isfile(module_path):
+        if need_cythonize:
             c_include_dirs = args.include
             if 'numpy' in code:
                 import numpy
@@ -193,11 +204,13 b' class CythonMagics(Magics):'
                 opts = dict(
                     quiet=quiet,
                     annotate = args.annotate,
-                    force = args.force,
+                    force = True,
                     )
                 build_extension.extensions = cythonize([extension], **opts)
             except CompileError:
                 return
+
+        if not have_module:
             build_extension.build_temp = os.path.dirname(pyx_file)
             build_extension.build_lib  = lib_dir
             build_extension.run()
@@ -207,7 +220,6 b' class CythonMagics(Magics):'
         self._import_all(module)
 
         if args.annotate:
-            html_file = os.path.join(lib_dir, module_name + '.html')
             try:
                 with io.open(html_file, encoding='utf-8') as f:
                     annotated_html = f.read()