Doc that file.next() has undefined behaviour when called on a file opened with
'w'. Closes bug #1569057.
To be backported once 2.5 branch is unfrozen.
diff --git a/Doc/lib/libstdtypes.tex b/Doc/lib/libstdtypes.tex
index 3cad43b..0ad2ebc 100644
--- a/Doc/lib/libstdtypes.tex
+++ b/Doc/lib/libstdtypes.tex
@@ -1615,14 +1615,15 @@
iterator, typically in a \keyword{for} loop (for example,
\code{for line in f: print line}), the \method{next()} method is
called repeatedly. This method returns the next input line, or raises
-\exception{StopIteration} when \EOF{} is hit. In order to make a
-\keyword{for} loop the most efficient way of looping over the lines of
-a file (a very common operation), the \method{next()} method uses a
-hidden read-ahead buffer. As a consequence of using a read-ahead
-buffer, combining \method{next()} with other file methods (like
-\method{readline()}) does not work right. However, using
-\method{seek()} to reposition the file to an absolute position will
-flush the read-ahead buffer.
+\exception{StopIteration} when \EOF{} is hit when the file is open for
+reading (behavior is undefined when the file is open for writing). In
+order to make a \keyword{for} loop the most efficient way of looping
+over the lines of a file (a very common operation), the
+\method{next()} method uses a hidden read-ahead buffer. As a
+consequence of using a read-ahead buffer, combining \method{next()}
+with other file methods (like \method{readline()}) does not work
+right. However, using \method{seek()} to reposition the file to an
+absolute position will flush the read-ahead buffer.
\versionadded{2.3}
\end{methoddesc}
diff --git a/Misc/NEWS b/Misc/NEWS
index 12d4413..d15f673 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -736,6 +736,9 @@
Documentation
-------------
+- Bug #1569057: Document that calling file.next() when the file is open for
+ writing is undefined.
+
- Patch #1489771: the syntax rules in Python Reference Manual were
updated to reflect the current Python syntax.