Issue #15056: imp.cache_from_source() and source_from_cache() raise

NotimplementedError when sys.implementation.cache_tag is None.

Thanks to Pranav Ravichandran for taking an initial stab at the patch.

Doc/library/imp.rst

@@ -180,14 +180,19 @@ file paths.
    source *path*.  For example, if *path* is ``/foo/bar/baz.py`` the return
    value would be ``/foo/bar/__pycache__/baz.cpython-32.pyc`` for Python 3.2.
    The ``cpython-32`` string comes from the current magic tag (see
-   :func:`get_tag`).  The returned path will end in ``.pyc`` when
+   :func:`get_tag`; if :attr:`sys.implementation.cache_tag` is not defined then
-   ``__debug__`` is True or ``.pyo`` for an optimized Python
+   :exc:`NotImplementedError` will be raised).  The returned path will end in
+   ``.pyc`` when ``__debug__`` is True or ``.pyo`` for an optimized Python
    (i.e. ``__debug__`` is False).  By passing in True or False for
    *debug_override* you can override the system's value for ``__debug__`` for
    extension selection.
 
    *path* need not exist.
 
+   .. versionchanged:: 3.3
+      If :attr:`sys.implementation.cache_tag` is ``None``, then
+      :exc:`NotImplementedError` is raised.
+
 
 .. function:: source_from_cache(path)
 
@@ -195,7 +200,13 @@ file paths.
    file path.  For example, if *path* is
    ``/foo/bar/__pycache__/baz.cpython-32.pyc`` the returned path would be
    ``/foo/bar/baz.py``.  *path* need not exist, however if it does not conform
-   to :pep:`3147` format, a ``ValueError`` is raised.
+   to :pep:`3147` format, a ``ValueError`` is raised. If
+   :attr:`sys.implementation.cache_tag` is not defined,
+   :exc:`NotImplementedError` is raised.
+
+   .. versionchanged:: 3.3
+      Raise :exc:`NotImplementedError` when
+      :attr:`sys.implementation.cache_tag` is not defined.
 
 
 .. function:: get_tag()
@@ -203,6 +214,10 @@ file paths.
    Return the :pep:`3147` magic tag string matching this version of Python's
    magic number, as returned by :func:`get_magic`.
 
+   .. note::
+      You may use :attr:`sys.implementation.cache_tag` directly starting
+      in Python 3.3.
+
 
 The following functions help interact with the import system's internal
 locking mechanism.  Locking semantics of imports are an implementation

Lib/imp.py

@@ -58,9 +58,12 @@ def source_from_cache(path):
 
     The .pyc/.pyo file does not need to exist; this simply returns the path to
     the .py file calculated to correspond to the .pyc/.pyo file.  If path does
-    not conform to PEP 3147 format, ValueError will be raised.
+    not conform to PEP 3147 format, ValueError will be raised. If
+    sys.implementation.cache_tag is None then NotImplementedError is raised.
 
     """
+    if sys.implementation.cache_tag is None:
+        raise NotImplementedError('sys.implementation.cache_tag is None')
     head, pycache_filename = os.path.split(path)
     head, pycache = os.path.split(head)
     if pycache != _bootstrap._PYCACHE:

Lib/importlib/_bootstrap.py

@@ -321,6 +321,8 @@ def cache_from_source(path, debug_override=None):
     If debug_override is not None, then it must be a boolean and is taken as
     the value of __debug__ instead.
 
+    If sys.implementation.cache_tag is None then NotImplementedError is raised.
+
     """
     debug = __debug__ if debug_override is None else debug_override
     if debug:
@@ -329,7 +331,10 @@ def cache_from_source(path, debug_override=None):
         suffixes = OPTIMIZED_BYTECODE_SUFFIXES
     head, tail = _path_split(path)
     base_filename, sep, _ = tail.partition('.')
-    filename = ''.join([base_filename, sep, _TAG, suffixes[0]])
+    tag = sys.implementation.cache_tag
+    if tag is None:
+        raise NotImplementedError('sys.implementation.cache_tag is None')
+    filename = ''.join([base_filename, sep, tag, suffixes[0]])
     return _path_join(head, _PYCACHE, filename)
 
 
@@ -649,7 +654,10 @@ class _LoaderBasics:
         code_object = self.get_code(name)
         module.__file__ = self.get_filename(name)
         if not sourceless:
-            module.__cached__ = cache_from_source(module.__file__)
+            try:
+                module.__cached__ = cache_from_source(module.__file__)
+            except NotImplementedError:
+                module.__cached__ = module.__file__
         else:
             module.__cached__ = module.__file__
         module.__package__ = name
@@ -718,9 +726,12 @@ class SourceLoader(_LoaderBasics):
 
         """
         source_path = self.get_filename(fullname)
-        bytecode_path = cache_from_source(source_path)
         source_mtime = None
-        if bytecode_path is not None:
+        try:
+            bytecode_path = cache_from_source(source_path)
+        except NotImplementedError:
+            bytecode_path = None
+        else:
             try:
                 st = self.path_stats(source_path)
             except NotImplementedError:
@@ -1417,7 +1428,6 @@ def __import__(name, globals={}, locals={}, fromlist=[], level=0):
 
 
 _MAGIC_NUMBER = None  # Set in _setup()
-_TAG = None  # Set in _setup()
 
 
 def _setup(sys_module, _imp_module):
@@ -1479,7 +1489,6 @@ def _setup(sys_module, _imp_module):
     # Constants
     setattr(self_module, '_relax_case', _make_relax_case())
     setattr(self_module, '_MAGIC_NUMBER', _imp_module.get_magic())
-    setattr(self_module, '_TAG', sys.implementation.cache_tag)
     if builtin_os == 'nt':
         SOURCE_SUFFIXES.append('.pyw')
 

Lib/test/test_imp.py

@@ -231,6 +231,8 @@ class PEP3147Tests(unittest.TestCase):
 
     tag = imp.get_tag()
 
+    @unittest.skipUnless(sys.implementation.cache_tag is not None,
+                         'requires sys.implementation.cache_tag not be None')
     def test_cache_from_source(self):
         # Given the path to a .py file, return the path to its PEP 3147
         # defined .pyc file (i.e. under __pycache__).
@@ -239,6 +241,12 @@ class PEP3147Tests(unittest.TestCase):
                               'qux.{}.pyc'.format(self.tag))
         self.assertEqual(imp.cache_from_source(path, True), expect)
 
+    def test_cache_from_source_no_cache_tag(self):
+        # Non cache tag means NotImplementedError.
+        with support.swap_attr(sys.implementation, 'cache_tag', None):
+            with self.assertRaises(NotImplementedError):
+                imp.cache_from_source('whatever.py')
+
     def test_cache_from_source_no_dot(self):
         # Directory with a dot, filename without dot.
         path = os.path.join('foo.bar', 'file')
@@ -283,6 +291,9 @@ class PEP3147Tests(unittest.TestCase):
             imp.cache_from_source('\\foo\\bar\\baz/qux.py', True),
             '\\foo\\bar\\baz\\__pycache__\\qux.{}.pyc'.format(self.tag))
 
+    @unittest.skipUnless(sys.implementation.cache_tag is not None,
+                         'requires sys.implementation.cache_tag to not be '
+                         'None')
     def test_source_from_cache(self):
         # Given the path to a PEP 3147 defined .pyc file, return the path to
         # its source.  This tests the good path.
@@ -291,6 +302,13 @@ class PEP3147Tests(unittest.TestCase):
         expect = os.path.join('foo', 'bar', 'baz', 'qux.py')
         self.assertEqual(imp.source_from_cache(path), expect)
 
+    def test_source_from_cache_no_cache_tag(self):
+        # If sys.implementation.cache_tag is None, raise NotImplementedError.
+        path = os.path.join('blah', '__pycache__', 'whatever.pyc')
+        with support.swap_attr(sys.implementation, 'cache_tag', None):
+            with self.assertRaises(NotImplementedError):
+                imp.source_from_cache(path)
+
     def test_source_from_cache_bad_path(self):
         # When the path to a pyc file is not in PEP 3147 format, a ValueError
         # is raised.

Misc/NEWS

@@ -31,6 +31,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #15056: imp.cache_from_source() and source_from_cache() raise
+  NotImplementedError when sys.implementation.cache_tag is set to None.
+
 - Issue #15256: Grammatical mistake in exception raised by imp.find_module().
 
 - Issue #5931: wsgiref environ variable SERVER_SOFTWARE will specify an