diff -r 6cb93e45a3be Lib/tempfile.py
--- a/Lib/tempfile.py	Sun Dec 15 19:50:13 2013 +0100
+++ b/Lib/tempfile.py	Sun Dec 15 23:08:51 2013 +0200
@@ -27,6 +27,7 @@
 
 # Imports.
 
+import functools as _functools
 import warnings as _warnings
 import sys as _sys
 import io as _io
@@ -349,6 +350,15 @@
         # (i.e. methods are cached, closed and friends are not)
         file = self.__dict__['file']
         a = getattr(file, name)
+        if hasattr(a, '__call__'):
+            func = a
+            @_functools.wraps(func)
+            def func_wrapper(*args, **kwargs):
+                # This is to prevent self getting out of scope too early,
+                # see bug 18879
+                local_self_copy = self
+                return func(*args, **kwargs)
+            a = func_wrapper
         if not isinstance(a, int):
             setattr(self, name, a)
         return a
diff -r 6cb93e45a3be Lib/test/test_tempfile.py
--- a/Lib/test/test_tempfile.py	Sun Dec 15 19:50:13 2013 +0100
+++ b/Lib/test/test_tempfile.py	Sun Dec 15 23:08:51 2013 +0200
@@ -689,6 +689,15 @@
         self.do_create(pre="a", suf="b")
         self.do_create(pre="aa", suf=".txt")
 
+    def test_issue18879(self):
+        f = self.do_create()
+        write = f.write
+        write2 = f.write
+        del f
+        write(b'foo')
+        del write
+        write2(b'bar')
+
     def test_creates_named(self):
         # NamedTemporaryFile creates files with names
         f = tempfile.NamedTemporaryFile()