@@ -43,6 +43,14 @@ The :mod:`linecache` module defines the following functions:
    changed on disk, and you require the updated version.  If *filename* is omitted,
    it will check all the entries in the cache.
 
+.. function:: lazycache(filename, module_globals)
+
+   Capture enough detail about a non-file based module to permit getting its
+   lines later via :func:`getline` even if *module_globals* is None in the later
+   call. This avoids doing I/O until a line is actually needed, without having
+   to carry the module globals around indefinitely.
+
+   .. versionadded:: 3.5
 
 Example::
 
 
@@ -136,6 +136,130 @@ The module defines the following functions:
 
    .. versionadded:: 3.4
 
+.. function:: walk_stack(f)
+
+   Walk a stack following f.f_back from the given frame, yielding the frame and
+   line number for each frame. If f is None, the current stack is used.
+   This helper is used with *Stack.extract*.
+
+   .. versionadded:: 3.5
+
+.. function:: walk_tb(tb)
+
+   Walk a traceback following tb_next yielding the frame and line number for
+   each frame. This helper is used with *Stack.extract*.
+
+   .. versionadded:: 3.5
+
+The module also defines the following classes:
+
+:class:`TracebackException` Objects
+-----------------------------------
+
+:class:`.TracebackException` objects are created from actual exceptions to
+capture data for later printing in a lightweight fashion.
+
+.. class:: TracebackException(exc_type, exc_value, exc_traceback, limit=None, lookup_lines=True)
+
+   Capture an exception for later rendering. limit, lookup_lines are as for
+   the :class:`.StackSummary` class.
+
+   .. versionadded:: 3.5
+
+.. classmethod:: `.from_exception`(exc, limit=None, lookup_lines=True)
+
+   Capture an exception for later rendering. limit and lookup_lines
+   are as for the :class:`.StackSummary` class.
+
+   .. versionadded:: 3.5
+
+.. attribute:: `.__cause__` A TracebackException of the original *__cause__*.
+
+.. attribute:: `.__context__` A TracebackException of the original *__context__*.
+.. attribute:: `.__suppress_context__` The *__suppress_context__* value from the
+               original exception.
+.. attribute:: `.stack` A `StackSummary` representing the traceback.
+.. attribute:: `.exc_type` The class of the original traceback.
+.. attribute:: `.filename` For syntax errors - the filename where the error
+               occured.
+.. attribute:: `.lineno` For syntax errors - the linenumber where the error
+               occured.
+.. attribute:: `.text` For syntax errors - the text where the error
+               occured.
+.. attribute:: `.offset` For syntax errors - the offset into the text where the
+               error occured.
+.. attribute:: `.msg` For syntax errors - the compiler error message.
+
+.. method::  TracebackException.format(chain=True)
+
+    Format the exception.
+
+    If chain is not *True*, *__cause__* and *__context__* will not be formatted.
+
+    The return value is a generator of strings, each ending in a newline and
+    some containing internal newlines. `print_exception` is a wrapper around
+    this method which just prints the lines to a file.
+
+    The message indicating which exception occurred is always the last
+    string in the output.
+
+   .. versionadded:: 3.5
+
+.. method::  TracebackException.format_exception_only()
+
+    Format the exception part of the traceback.
+
+    The return value is a generator of strings, each ending in a newline.
+
+    Normally, the generator emits a single string; however, for
+    SyntaxError exceptions, it emites several lines that (when
+    printed) display detailed information about where the syntax
+    error occurred.
+
+    The message indicating which exception occurred is always the last
+    string in the output.
+
+   .. versionadded:: 3.5
+
+
+:class:`StackSummary` Objects
+-----------------------------
+
+:class:`.StackSummary` objects represent a call stack ready for formatting.
+
+.. classmethod:: StackSummary.extract(frame_gen, limit=None, lookup_lines=True)
+
+   Construct a StackSummary object from a frame generator (such as is returned by
+   `walk_stack` or `walk_tb`.
+
+   If limit is supplied, only this many frames are taken from frame_gen.
+   If lookup_lines is False, the returned FrameSummary objects will not have read
+   their lines in yet, making the cost of creating the StackSummary cheaper (which
+   may be valuable if it may not actually get formatted).
+
+   .. versionadded:: 3.5
+
+.. classmethod:: StackSummary.from_list(a_list)
+
+   Construct a StackSummary object from a supplied old-style list of tuples. Each
+   tuple should be a 4-tuple with filename, lineno, name, line as the elements.
+
+   .. versionadded:: 3.5
+
+:class:`FrameSummary` Objects
+-----------------------------
+
+FrameSummary objects represent a single frame in a traceback.
+
+.. class:: FrameSummary(filename, lineno, name, lookup_line=True, locals=None, line=None)
+   :noindex:
+
+   Represent a single frame in the traceback or stack that is being formatted
+   or printed. It may optionally have a stringified version of the frames
+   locals included in it. If *lookup_line* is False, the source code is not
+   looked up until the FrameSummary has the :attr:`line` attribute accessed (which
+   also happens when casting it to a tuple). Line may be directly provided, and
+   will prevent line lookups happening at all.
 
 .. _traceback-example:
 
 
@@ -5,6 +5,7 @@
 that name.
 """
 
+import functools
 import sys
 import os
 import tokenize
@@ -21,7 +22,9 @@ def getline(filename, lineno, module_globals=None):
 
 # The cache
 
-cache = {} # The cache
+# The cache. Maps filenames to either a thunk which will provide source code,
+# or a tuple (size, mtime, lines, fullname) once loaded.
+cache = {}
 
 
 def clearcache():
@@ -36,6 +39,9 @@ def getlines(filename, module_globals=None):
     Update the cache if it doesn't contain an entry for this file already."""
 
     if filename in cache:
+        entry = cache[filename]
+        if len(entry) == 1:
+            return updatecache(filename, module_globals)
         return cache[filename][2]
     else:
         return updatecache(filename, module_globals)
@@ -54,7 +60,11 @@ def checkcache(filename=None):
             return
 
     for filename in filenames:
-        size, mtime, lines, fullname = cache[filename]
+        entry = cache[filename]
+        if len(entry) == 1:
+            # lazy cache entry, leave it lazy.
+            continue
+        size, mtime, lines, fullname = entry
         if mtime is None:
             continue   # no-op for files loaded via a __loader__
         try:
@@ -72,7 +82,8 @@ def updatecache(filename, module_globals=None):
     and return an empty list."""
 
     if filename in cache:
-        del cache[filename]
+        if len(cache[filename]) != 1:
+            del cache[filename]
     if not filename or (filename.startswith('<') and filename.endswith('>')):
         return []
 
@@ -82,27 +93,23 @@ def updatecache(filename, module_globals=None):
     except OSError:
         basename = filename
 
-        # Try for a __loader__, if available
-        if module_globals and '__loader__' in module_globals:
-            name = module_globals.get('__name__')
-            loader = module_globals['__loader__']
-            get_source = getattr(loader, 'get_source', None)
-
-            if name and get_source:
-                try:
-                    data = get_source(name)
-                except (ImportError, OSError):
-                    pass
-                else:
-                    if data is None:
-                        # No luck, the PEP302 loader cannot find the source
-                        # for this module.
-                        return []
-                    cache[filename] = (
-                        len(data), None,
-                        [line+'\n' for line in data.splitlines()], fullname
-                    )
-                    return cache[filename][2]
+        # Realise a lazy loader based lookup if there is one
+        # otherwise try to lookup right now.
+        if lazycache(filename, module_globals):
+            try:
+                data = cache[filename][0]()
+            except (ImportError, OSError):
+                pass
+            else:
+                if data is None:
+                    # No luck, the PEP302 loader cannot find the source
+                    # for this module.
+                    return []
+                cache[filename] = (
+                    len(data), None,
+                    [line+'\n' for line in data.splitlines()], fullname
+                )
+                return cache[filename][2]
 
         # Try looking through the module search path, which is only useful
         # when handling a relative filename.
@@ -132,3 +139,36 @@ def updatecache(filename, module_globals=None):
     size, mtime = stat.st_size, stat.st_mtime
     cache[filename] = size, mtime, lines, fullname
     return lines
+
+
+def lazycache(filename, module_globals):
+    """Seed the cache for filename with module_globals.
+
+    The module loader will be asked for the source only when getlines is
+    called, not immediately.
+
+    If there is an entry in the cache already, it is not altered.
+
+    :return: True if a lazy load is registered in the cache,
+        otherwise False. To register such a load a module loader with a
+        get_source method must be found, the filename must be a cachable
+        filename, and the filename must not be already cached.
+    """
+    if filename in cache:
+        if len(cache[filename]) == 1:
+            return True
+        else:
+            return False
+    if not filename or (filename.startswith('<') and filename.endswith('>')):
+        return False
+    # Try for a __loader__, if available
+    if module_globals and '__loader__' in module_globals:
+        name = module_globals.get('__name__')
+        loader = module_globals['__loader__']
+        get_source = getattr(loader, 'get_source', None)
+
+        if name and get_source:
+            get_lines = functools.partial(get_source, name)
+            cache[filename] = (get_lines,)
+            return True
+    return False
@@ -7,6 +7,7 @@
 
 
 FILENAME = linecache.__file__
+NONEXISTENT_FILENAME = FILENAME + '.missing'
 INVALID_NAME = '!@$)(!@#_1'
 EMPTY = ''
 TESTS = 'inspect_fodder inspect_fodder2 mapping_tests'
@@ -126,6 +127,49 @@ def test_checkcache(self):
                 self.assertEqual(line, getline(source_name, index + 1))
                 source_list.append(line)
 
+    def test_lazycache_no_globals(self):
+        lines = linecache.getlines(FILENAME)
+        linecache.clearcache()
+        self.assertEqual(False, linecache.lazycache(FILENAME, None))
+        self.assertEqual(lines, linecache.getlines(FILENAME))
+
+    def test_lazycache_smoke(self):
+        lines = linecache.getlines(NONEXISTENT_FILENAME, globals())
+        linecache.clearcache()
+        self.assertEqual(
+            True, linecache.lazycache(NONEXISTENT_FILENAME, globals()))
+        self.assertEqual(1, len(linecache.cache[NONEXISTENT_FILENAME]))
+        # Note here that we're looking up a non existant filename with no
+        # globals: this would error if the lazy value wasn't resolved.
+        self.assertEqual(lines, linecache.getlines(NONEXISTENT_FILENAME))
+
+    def test_lazycache_provide_after_failed_lookup(self):
+        linecache.clearcache()
+        lines = linecache.getlines(NONEXISTENT_FILENAME, globals())
+        linecache.clearcache()
+        linecache.getlines(NONEXISTENT_FILENAME)
+        linecache.lazycache(NONEXISTENT_FILENAME, globals())
+        self.assertEqual(lines, linecache.updatecache(NONEXISTENT_FILENAME))
+
+    def test_lazycache_check(self):
+        linecache.clearcache()
+        linecache.lazycache(NONEXISTENT_FILENAME, globals())
+        linecache.checkcache()
+
+    def test_lazycache_bad_filename(self):
+        linecache.clearcache()
+        self.assertEqual(False, linecache.lazycache('', globals()))
+        self.assertEqual(False, linecache.lazycache('<foo>', globals()))
+
+    def test_lazycache_already_cached(self):
+        linecache.clearcache()
+        lines = linecache.getlines(NONEXISTENT_FILENAME, globals())
+        self.assertEqual(
+            False,
+            linecache.lazycache(NONEXISTENT_FILENAME, globals()))
+        self.assertEqual(4, len(linecache.cache[NONEXISTENT_FILENAME]))
+
+
 def test_main():
     support.run_unittest(LineCacheTests)