Issue #18193: Add importlib.reload(), documenting (but not

implementing in code) the deprecation of imp.reload().

Thanks to Berker Peksag for the patch.

Doc/library/imp.rst

@@ -171,6 +171,9 @@ This module provides an interface to the mechanisms used to implement the
    the class does not affect the method definitions of the instances --- they
    continue to use the old class definition.  The same is true for derived classes.
 
+   .. deprecated:: 3.4
+      Use :func:`importlib.reload` instead.
+
 
 The following functions are conveniences for handling :pep:`3147` byte-compiled
 file paths.

Doc/library/importlib.rst

@@ -115,6 +115,73 @@ Functions
 
    .. versionadded:: 3.3
 
+.. function:: reload(module)
+
+   Reload a previously imported *module*.  The argument must be a module object,
+   so it must have been successfully imported before.  This is useful if you
+   have edited the module source file using an external editor and want to try
+   out the new version without leaving the Python interpreter.  The return value
+   is the module object (the same as the *module* argument).
+
+   When :func:`.reload` is executed:
+
+   * Python modules' code is recompiled and the module-level code re-executed,
+     defining a new set of objects which are bound to names in the module's
+     dictionary by reusing the :term:`loader` which originally loaded the
+     module.  The ``init`` function of extension modules is not called a second
+     time.
+
+   * As with all other objects in Python the old objects are only reclaimed
+     after their reference counts drop to zero.
+
+   * The names in the module namespace are updated to point to any new or
+     changed objects.
+
+   * Other references to the old objects (such as names external to the module) are
+     not rebound to refer to the new objects and must be updated in each namespace
+     where they occur if that is desired.
+
+   There are a number of other caveats:
+
+   If a module is syntactically correct but its initialization fails, the first
+   :keyword:`import` statement for it does not bind its name locally, but does
+   store a (partially initialized) module object in ``sys.modules``.  To reload
+   the module you must first :keyword:`import` it again (this will bind the name
+   to the partially initialized module object) before you can :func:`reload` it.
+
+   When a module is reloaded, its dictionary (containing the module's global
+   variables) is retained.  Redefinitions of names will override the old
+   definitions, so this is generally not a problem.  If the new version of a
+   module does not define a name that was defined by the old version, the old
+   definition remains.  This feature can be used to the module's advantage if it
+   maintains a global table or cache of objects --- with a :keyword:`try`
+   statement it can test for the table's presence and skip its initialization if
+   desired::
+
+      try:
+          cache
+      except NameError:
+          cache = {}
+
+   It is legal though generally not very useful to reload built-in or
+   dynamically loaded modules (this is not true for e.g. :mod:`sys`,
+   :mod:`__main__`, :mod:`__builtin__` and other key modules where reloading is
+   frowned upon). In many cases, however, extension modules are not designed to
+   be initialized more than once, and may fail in arbitrary ways when reloaded.
+
+   If a module imports objects from another module using :keyword:`from` ...
+   :keyword:`import` ..., calling :func:`reload` for the other module does not
+   redefine the objects imported from it --- one way around this is to
+   re-execute the :keyword:`from` statement, another is to use :keyword:`import`
+   and qualified names (*module.name*) instead.
+
+   If a module instantiates instances of a class, reloading the module that
+   defines the class does not affect the method definitions of the instances ---
+   they continue to use the old class definition.  The same is true for derived
+   classes.
+
+   .. versionadded:: 3.4
+
 
 :mod:`importlib.abc` -- Abstract base classes related to import
 ---------------------------------------------------------------

Lib/imp.py

@@ -23,6 +23,7 @@ from importlib._bootstrap import cache_from_source, source_from_cache
 
 from importlib import _bootstrap
 from importlib import machinery
+import importlib
 import os
 import sys
 import tokenize
@@ -246,31 +247,12 @@ def find_module(name, path=None):
     return file, file_path, (suffix, mode, type_)
 
 
-_RELOADING = {}
-
 def reload(module):
-    """Reload the module and return it.
+    """**DEPRECATED**
+
+    Reload the module and return it.
 
     The module must have been successfully imported before.
 
     """
-    if not module or type(module) != type(sys):
+    return importlib.reload(module)
-        raise TypeError("reload() argument must be module")
-    name = module.__name__
-    if name not in sys.modules:
-        msg = "module {} not in sys.modules"
-        raise ImportError(msg.format(name), name=name)
-    if name in _RELOADING:
-        return _RELOADING[name]
-    _RELOADING[name] = module
-    try:
-        parent_name = name.rpartition('.')[0]
-        if parent_name and parent_name not in sys.modules:
-            msg = "parent {!r} not in sys.modules"
-            raise ImportError(msg.format(parentname), name=parent_name)
-        return module.__loader__.load_module(name)
-    finally:
-        try:
-            del _RELOADING[name]
-        except KeyError:
-            pass

Lib/importlib/__init__.py

@@ -1,5 +1,5 @@
 """A pure Python implementation of import."""
-__all__ = ['__import__', 'import_module', 'invalidate_caches']
+__all__ = ['__import__', 'import_module', 'invalidate_caches', 'reload']
 
 # Bootstrap help #####################################################
 
@@ -11,6 +11,7 @@ __all__ = ['__import__', 'import_module', 'invalidate_caches']
 # initialised below if the frozen one is not available).
 import _imp  # Just the builtin component, NOT the full Python module
 import sys
+import types
 
 try:
     import _frozen_importlib as _bootstrap
@@ -90,3 +91,34 @@ def import_module(name, package=None):
                 break
             level += 1
     return _bootstrap._gcd_import(name[level:], package, level)
+
+
+_RELOADING = {}
+
+
+def reload(module):
+    """Reload the module and return it.
+
+    The module must have been successfully imported before.
+
+    """
+    if not module or not isinstance(module, types.ModuleType):
+        raise TypeError("reload() argument must be module")
+    name = module.__name__
+    if name not in sys.modules:
+        msg = "module {} not in sys.modules"
+        raise ImportError(msg.format(name), name=name)
+    if name in _RELOADING:
+        return _RELOADING[name]
+    _RELOADING[name] = module
+    try:
+        parent_name = name.rpartition('.')[0]
+        if parent_name and parent_name not in sys.modules:
+            msg = "parent {!r} not in sys.modules"
+            raise ImportError(msg.format(parentname), name=parent_name)
+        return module.__loader__.load_module(name)
+    finally:
+        try:
+            del _RELOADING[name]
+        except KeyError:
+            pass

Lib/test/test_importlib/test_api.py

@@ -151,6 +151,18 @@ class FindLoaderTests(unittest.TestCase):
         self.assertIsNone(importlib.find_loader('nevergoingtofindthismodule'))
 
 
+class ReloadTests(unittest.TestCase):
+
+    """Test module reloading for builtin and extension modules."""
+
+    def test_reload_modules(self):
+        for mod in ('tokenize', 'time', 'marshal'):
+            with self.subTest(module=mod):
+                with support.CleanImport(mod):
+                    module = importlib.import_module(mod)
+                    importlib.reload(module)
+
+
 class InvalidateCacheTests(unittest.TestCase):
 
     def test_method_called(self):

Misc/NEWS

@@ -123,6 +123,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #18193: Add importlib.reload().
+
 - Issue #18157: Stop using imp.load_module() in pydoc.
 
 - Issue #16102: Make uuid._netbios_getnode() work again on Python 3.