#2131: note that codecs.open() always opens files in binary mode.

Doc/library/codecs.rst

@@ -206,7 +206,8 @@ utility functions:
 .. function:: open(filename, mode[, encoding[, errors[, buffering]]])
 
    Open an encoded file using the given *mode* and return a wrapped version
-   providing transparent encoding/decoding.
+   providing transparent encoding/decoding.  The default file mode is ``'r'``
+   meaning to open the file in read mode.
 
    .. note::
 
@@ -214,6 +215,13 @@ utility functions:
       i.e. Unicode objects for most built-in codecs.  Output is also codec-dependent
       and will usually be Unicode as well.
 
+   .. note::
+
+      Files are always opened in binary mode, even if no binary mode was
+      specified.  This is done to avoid data loss due to encodings using 8-bit
+      values.  This means that no automatic conversion of ``'\n'`` is done
+      on reading and writing.
+
    *encoding* specifies the encoding which is to be used for the file.
 
    *errors* may be given to define the error handling. It defaults to ``'strict'``