(Merge 3.4) Issue #22599: Enhance tokenize.open() to be able to call it during

Python finalization.

Before the module kept a reference to the builtins module, but the module
attributes are cleared during Python finalization. Instead, keep directly a
reference to the open() function.

This enhancement is not perfect, calling tokenize.open() can still fail if
called very late during Python finalization.  Usually, the function is called
by the linecache module which is called to display a traceback or emit a
warning.

Lib/test/test_traceback.py

@@ -6,6 +6,8 @@ import unittest
 import re
 from test.support import run_unittest, Error, captured_output
 from test.support import TESTFN, unlink, cpython_only
+from test.script_helper import assert_python_ok
+import textwrap
 
 import traceback
 
@@ -169,6 +171,37 @@ class SyntaxTracebackCases(unittest.TestCase):
         # Issue #18960: coding spec should has no effect
         do_test("0\n# coding: GBK\n", "h\xe9 ho", 'utf-8', 5)
 
+    def test_print_traceback_at_exit(self):
+        # Issue #22599: Ensure that it is possible to use the traceback module
+        # to display an exception at Python exit
+        code = textwrap.dedent("""
+            import sys
+            import traceback
+
+            class PrintExceptionAtExit(object):
+                def __init__(self):
+                    try:
+                        x = 1 / 0
+                    except Exception:
+                        self.exc_info = sys.exc_info()
+                        # self.exc_info[1] (traceback) contains frames:
+                        # explicitly clear the reference to self in the current
+                        # frame to break a reference cycle
+                        self = None
+
+                def __del__(self):
+                    traceback.print_exception(*self.exc_info)
+
+            # Keep a reference in the module namespace to call the destructor
+            # when the module is unloaded
+            obj = PrintExceptionAtExit()
+        """)
+        rc, stdout, stderr = assert_python_ok('-c', code)
+        expected = [b'Traceback (most recent call last):',
+                    b'  File "<string>", line 8, in __init__',
+                    b'ZeroDivisionError: division by zero']
+        self.assertEqual(stderr.splitlines(), expected)
+
 
 class TracebackFormatTests(unittest.TestCase):
 

Lib/tokenize.py

@@ -24,7 +24,6 @@ __author__ = 'Ka-Ping Yee <ping@lfw.org>'
 __credits__ = ('GvR, ESR, Tim Peters, Thomas Wouters, Fred Drake, '
                'Skip Montanaro, Raymond Hettinger, Trent Nelson, '
                'Michael Foord')
-import builtins
 from codecs import lookup, BOM_UTF8
 import collections
 from io import TextIOWrapper
@@ -431,11 +430,13 @@ def detect_encoding(readline):
     return default, [first, second]
 
 
+_builtin_open = open
+
 def open(filename):
     """Open a file in read only mode using the encoding detected by
     detect_encoding().
     """
-    buffer = builtins.open(filename, 'rb')
+    buffer = _builtin_open(filename, 'rb')
     encoding, lines = detect_encoding(buffer.readline)
     buffer.seek(0)
     text = TextIOWrapper(buffer, encoding, line_buffering=True)
@@ -658,7 +659,7 @@ def main():
         # Tokenize the input
         if args.filename:
             filename = args.filename
-            with builtins.open(filename, 'rb') as f:
+            with _builtin_open(filename, 'rb') as f:
                 tokens = list(tokenize(f.readline))
         else:
             filename = "<stdin>"