Store timestamps for modules to autoreload

Closes gh-4127

At present, autoreload compares the timestamps on .py and .pyc files.
However, this can fail in a couple of ways (see gh-4127). With this
change, it caches the mtime of the .py file when we loaded it.

Some extra complexity is needed to store the mtime as soon as possible -
we scan sys.modules when autoreload is loaded, and register a
post_execute hook to check for newly imported modules after each cell.

IPython/extensions/autoreload.py

@@ -130,20 +130,23 b' class ModuleReloader(object):'
     enabled = False
     """Whether this reloader is enabled"""
 
-    failed = {}
-    """Modules that failed to reload: {module: mtime-on-failed-reload, ...}"""
-
-    modules = {}
-    """Modules specially marked as autoreloadable."""
-
-    skip_modules = {}
-    """Modules specially marked as not autoreloadable."""
-
     check_all = True
     """Autoreload all modules, not just those listed in 'modules'"""
 
-    old_objects = {}
-    """(module-name, name) -> weakref, for replacing old code objects"""
+    def __init__(self):
+        # Modules that failed to reload: {module: mtime-on-failed-reload, ...}
+        self.failed = {}
+        # Modules specially marked as autoreloadable.
+        self.modules = {}
+        # Modules specially marked as not autoreloadable.
+        self.skip_modules = {}
+        # (module-name, name) -> weakref, for replacing old code objects
+        self.old_objects = {}
+        # Module modification timestamps
+        self.modules_mtimes = {}
+
+        # Cache module modification times
+        self.check(check_all=True, do_reload=False)
 
     def mark_module_skipped(self, module_name):
         """Skip reloading the named module in the future"""
@@ -179,7 +182,33 b' class ModuleReloader(object):'
         top_module = sys.modules[top_name]
         return top_module, top_name
 
-    def check(self, check_all=False):
+    def filename_and_mtime(self, module):
+        if not hasattr(module, '__file__'):
+            return None, None
+
+        if module.__name__ == '__main__':
+            # we cannot reload(__main__)
+            return None, None
+
+        filename = module.__file__
+        path, ext = os.path.splitext(filename)
+
+        if ext.lower() == '.py':
+            py_filename = filename
+        else:
+            try:
+                py_filename = openpy.source_from_cache(filename)
+            except ValueError:
+                return None, None
+
+        try:
+            pymtime = os.stat(py_filename).st_mtime
+        except OSError:
+            return None, None
+
+        return py_filename, pymtime
+
+    def check(self, check_all=False, do_reload=True):
         """Check whether some modules need to be reloaded."""
 
         if not self.enabled and not check_all:
@@ -196,35 +225,24 b' class ModuleReloader(object):'
             if modname in self.skip_modules:
                 continue
 
-            if not hasattr(m, '__file__'):
+            py_filename, pymtime = self.filename_and_mtime(m)
+            if py_filename is None:
                 continue
 
-            if m.__name__ == '__main__':
-                # we cannot reload(__main__)
-                continue
-
-            filename = m.__file__
-            path, ext = os.path.splitext(filename)
-
-            if ext.lower() == '.py':
-                pyc_filename = openpy.cache_from_source(filename)
-                py_filename = filename
-            else:
-                pyc_filename = filename
                 try:
-                    py_filename = openpy.source_from_cache(filename)
-                except ValueError:
+                if pymtime <= self.modules_mtimes[modname]:
                     continue
-
-            try:
-                pymtime = os.stat(py_filename).st_mtime
-                if pymtime <= os.stat(pyc_filename).st_mtime:
+            except KeyError:
+                self.modules_mtimes[modname] = pymtime
                     continue
+            else:
                 if self.failed.get(py_filename, None) == pymtime:
                     continue
-            except OSError:
-                continue
 
+            self.modules_mtimes[modname] = pymtime
+
+            # If we've reached this point, we should try to reload the module
+            if do_reload:
             try:
                 superreload(m, reload, self.old_objects)
                 if py_filename in self.failed:
@@ -400,6 +418,7 b' class AutoreloadMagics(Magics):'
         super(AutoreloadMagics, self).__init__(*a, **kw)
         self._reloader = ModuleReloader()
         self._reloader.check_all = False
+        self.loaded_modules = set(sys.modules)
 
     @line_magic
     def autoreload(self, parameter_s=''):
@@ -497,9 +516,21 b' class AutoreloadMagics(Magics):'
             except:
                 pass
 
+    def post_execute_hook(self):
+        """Cache the modification times of any modules imported in this execution
+        """
+        newly_loaded_modules = set(sys.modules) - self.loaded_modules
+        for modname in newly_loaded_modules:
+            _, pymtime = self._reloader.filename_and_mtime(sys.modules[modname])
+            if pymtime is not None:
+                self._reloader.modules_mtimes[modname] = pymtime
+
+        self.loaded_modules.update(newly_loaded_modules)
+
 
 def load_ipython_extension(ip):
     """Load the extension in IPython."""
     auto_reload = AutoreloadMagics(ip)
     ip.register_magics(auto_reload)
     ip.events.register('pre_run_cell', auto_reload.pre_run_cell)
+    ip.register_post_execute(auto_reload.post_execute_hook)

IPython/extensions/tests/test_autoreload.py

@@ -50,6 +50,7 b' class FakeShell(object):'
     def run_code(self, code):
         self.events.trigger('pre_run_cell')
         exec(code, self.ns)
+        self.auto_magics.post_execute_hook()
 
     def push(self, items):
         self.ns.update(items)
@@ -59,6 +60,7 b' class FakeShell(object):'
 
     def magic_aimport(self, parameter, stream=None):
         self.auto_magics.aimport(parameter, stream=stream)
+        self.auto_magics.post_execute_hook()
 
 
 class Fixture(object):
@@ -168,12 +170,10 b" class Bar: # old-style class: weakref doesn't work for it on Python < 2.7"
             self.shell.magic_aimport(mod_name)
             stream = StringIO()
             self.shell.magic_aimport("", stream=stream)
-            nt.assert_true(("Modules to reload:\n%s" % mod_name) in
-                           stream.getvalue())
+            nt.assert_in(("Modules to reload:\n%s" % mod_name), stream.getvalue())
 
-            nt.assert_raises(
-                ImportError,
-                self.shell.magic_aimport, "tmpmod_as318989e89ds")
+            with nt.assert_raises(ImportError):
+                self.shell.magic_aimport("tmpmod_as318989e89ds")
         else:
             self.shell.magic_autoreload("2")
             self.shell.run_code("import %s" % mod_name)