bpo-26789: Fix logging.FileHandler._open() at exit (GH-23053)

The logging.FileHandler class now keeps a reference to the builtin
open() function to be able to open or reopen the file during Python
finalization.

Fix errors like:

    Exception ignored in: (...)
    Traceback (most recent call last):
      (...)
      File ".../logging/__init__.py", line 1463, in error
      File ".../logging/__init__.py", line 1577, in _log
      File ".../logging/__init__.py", line 1587, in handle
      File ".../logging/__init__.py", line 1649, in callHandlers
      File ".../logging/__init__.py", line 948, in handle
      File ".../logging/__init__.py", line 1182, in emit
      File ".../logging/__init__.py", line 1171, in _open
    NameError: name 'open' is not defined

1 files changed, 7 insertions, 2 deletions

diff --git a/Lib/logging/__init__.py b/Lib/logging/__init__.py
index 265e286..badfd65 100644
--- a/Lib/logging/__init__.py
+++ b/Lib/logging/__init__.py
@@ -1148,6 +1148,10 @@ class FileHandler(StreamHandler):
         self.encoding = encoding
         self.errors = errors
         self.delay = delay
+        # bpo-26789: FileHandler keeps a reference to the builtin open()
+        # function to be able to open or reopen the file during Python
+        # finalization.
+        self._builtin_open = open
         if delay:
             #We don't open the stream, but we still need to call the
             #Handler constructor to set level, formatter, lock etc.
@@ -1183,8 +1187,9 @@ class FileHandler(StreamHandler):
         Open the current base file with the (original) mode and encoding.
         Return the resulting stream.
         """
-        return open(self.baseFilename, self.mode, encoding=self.encoding,
-                    errors=self.errors)
+        open_func = self._builtin_open
+        return open_func(self.baseFilename, self.mode,
+                         encoding=self.encoding, errors=self.errors)
 
     def emit(self, record):
         """