Index: Doc/library/xml.etree.elementtree.rst
===================================================================
--- Doc/library/xml.etree.elementtree.rst	(revision 78956)
+++ Doc/library/xml.etree.elementtree.rst	(working copy)
@@ -148,20 +148,22 @@
    arguments.  Returns an element instance.
 
 
-.. function:: tostring(element, encoding=None, method="xml")
+.. function:: tostring(element, encoding="us-ascii", method="xml")
 
    Generates a string representation of an XML element, including all
    subelements.  *element* is an :class:`Element` instance.  *encoding* [1]_ is
-   the output encoding (default is None).  *method* is either ``"xml"``,
+   the output encoding (default is US-ASCII).  Use ``encoding=False`` to
+   generate a Unicode string.  *method* is either ``"xml"``,
    ``"html"`` or ``"text"`` (default is ``"xml"``).  Returns an (optionally)
    encoded string containing the XML data.
 
 
-.. function:: tostringlist(element, encoding=None, method="xml")
+.. function:: tostringlist(element, encoding="us-ascii", method="xml")
 
    Generates a string representation of an XML element, including all
    subelements.  *element* is an :class:`Element` instance.  *encoding* [1]_ is
-   the output encoding (default is None).   *method* is either ``"xml"``,
+   the output encoding (default is US-ASCII).  Use ``encoding=False`` to
+   generate a Unicode string.  *method* is either ``"xml"``,
    ``"html"`` or ``"text"`` (default is ``"xml"``).  Returns a list of
    (optionally) encoded strings containing the XML data.  It does not guarantee
    any specific sequence, except that ``"".join(tostringlist(element)) ==
@@ -430,6 +432,7 @@
 
 
    .. method:: getroot()
+
       Returns the root element for this tree.
 
 
@@ -457,11 +460,12 @@
       root element.
 
 
-   .. method:: write(file, encoding=None, xml_declaration=None, method="xml")
+   .. method:: write(file, encoding="us-ascii", xml_declaration=None, method="xml")
 
       Writes the element tree to a file, as XML.  *file* is a file name, or a
       file object opened for writing.  *encoding* [1]_ is the output encoding
-      (default is None).  *xml_declaration* controls if an XML declaration
+      (default is US-ASCII).  Use ``encoding=False`` to write a Unicode string.
+      *xml_declaration* controls if an XML declaration
       should be added to the file.  Use False for never, True for always, None
       for only if not US-ASCII or UTF-8 (default is None).  *method* is either
       ``"xml"``, ``"html"`` or ``"text"`` (default is ``"xml"``).  Returns an
Index: Lib/test/test_xml_etree.py
===================================================================
--- Lib/test/test_xml_etree.py	(revision 78956)
+++ Lib/test/test_xml_etree.py	(working copy)
@@ -65,14 +65,14 @@
     if not hasattr(method, '__call__'):
         print(method, "not callable")
 
-def serialize(elem, to_string=True, **options):
+def serialize(elem, to_string=True, encoding=False, **options):
     import io
-    if options.get("encoding"):
+    if encoding is not False:
         file = io.BytesIO()
     else:
         file = io.StringIO()
     tree = ET.ElementTree(elem)
-    tree.write(file, **options)
+    tree.write(file, encoding=encoding, **options)
     if to_string:
         return file.getvalue()
     else:
@@ -531,7 +531,7 @@
     >>> elem.set('testa', 'testval')
     >>> elem.set('testb', 'test2')
     >>> ET.tostring(elem)
-    '<test testa="testval" testb="test2">aa</test>'
+    b'<test testa="testval" testb="test2">aa</test>'
     >>> sorted(elem.keys())
     ['testa', 'testb']
     >>> sorted(elem.items())
@@ -541,7 +541,7 @@
     >>> elem.attrib['testb'] = 'test1'
     >>> elem.attrib['testc'] = 'test2'
     >>> ET.tostring(elem)
-    '<test testa="testval" testb="test1" testc="test2">aa</test>'
+    b'<test testa="testval" testb="test1" testc="test2">aa</test>'
     """
 
 def makeelement():
@@ -581,7 +581,7 @@
 
     >>> tree = ET.parse(SIMPLE_XMLFILE)
     >>> normalize_crlf(tree)
-    >>> tree.write(sys.stdout)
+    >>> tree.write(sys.stdout, encoding=False)
     <root>
        <element key="value">text</element>
        <element>text</element>tail
@@ -589,7 +589,7 @@
     </root>
     >>> tree = ET.parse(SIMPLE_NS_XMLFILE)
     >>> normalize_crlf(tree)
-    >>> tree.write(sys.stdout)
+    >>> tree.write(sys.stdout, encoding=False)
     <ns0:root xmlns:ns0="namespace">
        <ns0:element key="value">text</ns0:element>
        <ns0:element>text</ns0:element>tail
@@ -630,17 +630,17 @@
 def parseliteral():
     """
     >>> element = ET.XML("<html><body>text</body></html>")
-    >>> ET.ElementTree(element).write(sys.stdout)
+    >>> ET.ElementTree(element).write(sys.stdout, encoding=False)
     <html><body>text</body></html>
     >>> element = ET.fromstring("<html><body>text</body></html>")
-    >>> ET.ElementTree(element).write(sys.stdout)
+    >>> ET.ElementTree(element).write(sys.stdout, encoding=False)
     <html><body>text</body></html>
     >>> sequence = ["<html><body>", "text</bo", "dy></html>"]
     >>> element = ET.fromstringlist(sequence)
     >>> print(ET.tostring(element))
-    <html><body>text</body></html>
-    >>> print("".join(ET.tostringlist(element)))
-    <html><body>text</body></html>
+    b'<html><body>text</body></html>'
+    >>> print(b"".join(ET.tostringlist(element)))
+    b'<html><body>text</body></html>'
     >>> ET.tostring(element, "ascii")
     b"<?xml version='1.0' encoding='ascii'?>\\n<html><body>text</body></html>"
     >>> _, ids = ET.XMLID("<html><body>text</body></html>")
@@ -869,10 +869,10 @@
     """
     >>> elem = ET.XML("<html><body>text</body></html>")
     >>> ET.tostring(elem)
-    '<html><body>text</body></html>'
+    b'<html><body>text</body></html>'
     >>> elem = ET.fromstring("<html><body>text</body></html>")
     >>> ET.tostring(elem)
-    '<html><body>text</body></html>'
+    b'<html><body>text</body></html>'
     """
 
 def check_encoding(encoding):
@@ -1227,14 +1227,14 @@
     Test ProcessingInstruction directly
 
     >>> ET.tostring(ET.ProcessingInstruction('test', 'instruction'))
-    '<?test instruction?>'
+    b'<?test instruction?>'
     >>> ET.tostring(ET.PI('test', 'instruction'))
-    '<?test instruction?>'
+    b'<?test instruction?>'
 
     Issue #2746
 
     >>> ET.tostring(ET.PI('test', '<testing&>'))
-    '<?test <testing&>?>'
+    b'<?test <testing&>?>'
     >>> ET.tostring(ET.PI('test', '<testing&>\xe3'), 'latin1')
     b"<?xml version='1.0' encoding='latin1'?>\\n<?test <testing&>\\xe3?>"
     """
@@ -1637,11 +1637,11 @@
 
     >>> e = ET.Element('SomeTag', text="def _f():\n  return 3\n")
     >>> ET.tostring(e)
-    '<SomeTag text="def _f():&#10;  return 3&#10;" />'
+    b'<SomeTag text="def _f():&#10;  return 3&#10;" />'
     >>> ET.XML(ET.tostring(e)).get("text")
     'def _f():\n  return 3\n'
     >>> ET.tostring(ET.XML(ET.tostring(e)))
-    '<SomeTag text="def _f():&#10;  return 3&#10;" />'
+    b'<SomeTag text="def _f():&#10;  return 3&#10;" />'
 
     """
 
@@ -1692,15 +1692,15 @@
     """
 
     >>> ET.tostring(ET.Element("{http://namespace.invalid/does/not/exist/}title"))
-    '<ns0:title xmlns:ns0="http://namespace.invalid/does/not/exist/" />'
+    b'<ns0:title xmlns:ns0="http://namespace.invalid/does/not/exist/" />'
     >>> ET.register_namespace("foo", "http://namespace.invalid/does/not/exist/")
     >>> ET.tostring(ET.Element("{http://namespace.invalid/does/not/exist/}title"))
-    '<foo:title xmlns:foo="http://namespace.invalid/does/not/exist/" />'
+    b'<foo:title xmlns:foo="http://namespace.invalid/does/not/exist/" />'
 
     And the Dublin Core namespace is in the default list:
 
     >>> ET.tostring(ET.Element("{http://purl.org/dc/elements/1.1/}title"))
-    '<dc:title xmlns:dc="http://purl.org/dc/elements/1.1/" />'
+    b'<dc:title xmlns:dc="http://purl.org/dc/elements/1.1/" />'
 
     """
 
@@ -1786,7 +1786,7 @@
     '{${stuff}}localname'
     >>> t = ET.ElementTree(e)
     >>> ET.tostring(e)
-    '<ns0:localname xmlns:ns0="${stuff}" />'
+    b'<ns0:localname xmlns:ns0="${stuff}" />'
 
     """
 
Index: Lib/xml/etree/ElementTree.py
===================================================================
--- Lib/xml/etree/ElementTree.py	(revision 78956)
+++ Lib/xml/etree/ElementTree.py	(working copy)
@@ -792,7 +792,8 @@
     # @def write(file, **options)
     # @param file A file name, or a file object opened for writing.
     # @param **options Options, given as keyword arguments.
-    # @keyparam encoding Optional output encoding (default is None).
+    # @keyparam encoding Optional output encoding (default is US-ASCII).
+    #     Use False to return a Unicode string.
     # @keyparam method Optional output method ("xml", "html", "text" or
     #     "c14n"; default is "xml").
     # @keyparam xml_declaration Controls if an XML declaration should
@@ -811,6 +812,13 @@
         elif method not in _serialize:
             # FIXME: raise an ImportError for c14n if ElementC14N is missing?
             raise ValueError("unknown method %r" % method)
+        if encoding in (None, True):
+            if method == "c14n":
+                encoding = "utf-8"
+            else:
+                encoding = "us-ascii"
+        elif encoding == str:  # lxml.etree compatibility.
+            encoding = False
         if hasattr(file_or_filename, "write"):
             file = file_or_filename
         else:
@@ -827,20 +835,14 @@
                     _raise_serialization_error(text)
         else:
             write = file.write
-        if not encoding:
-            if method == "c14n":
-                encoding = "utf-8"
-            else:
-                encoding = None
-        elif xml_declaration or (xml_declaration is None and
-                                 encoding not in ("utf-8", "us-ascii")):
-            if method == "xml":
-                encoding_ = encoding
-                if not encoding:
-                    # Retrieve the default encoding for the xml declaration
-                    import locale
-                    encoding_ = locale.getpreferredencoding()
-                write("<?xml version='1.0' encoding='%s'?>\n" % encoding_)
+        if method == "xml" and (xml_declaration or (xml_declaration is None and
+                        encoding and encoding not in ("utf-8", "us-ascii"))):
+            declared_encoding = encoding
+            if not encoding:
+                # Retrieve the default encoding for the xml declaration
+                import locale
+                declared_encoding = locale.getpreferredencoding()
+            write("<?xml version='1.0' encoding='%s'?>\n" % declared_encoding)
         if method == "text":
             _serialize_text(write, self._root)
         else:
@@ -1131,7 +1133,8 @@
 # otherwise it is a bytes array.
 #
 # @param element An Element instance.
-# @keyparam encoding Optional output encoding (default is None).
+# @keyparam encoding Optional output encoding (default is US-ASCII).
+#     Use False to return a Unicode string.
 # @keyparam method Optional output method ("xml", "html", "text" or
 #     "c14n"; default is "xml").
 # @return An (optionally) encoded string containing the XML data.
@@ -1144,6 +1147,10 @@
     file = dummy()
     file.write = data.append
     ElementTree(element).write(file, encoding, method=method)
+    if encoding is None:
+        encoding = True
+    elif encoding == str:  # lxml.etree compatibility.
+        encoding = False
     if encoding:
         return b"".join(data)
     else:
@@ -1155,6 +1162,7 @@
 #
 # @param element An Element instance.
 # @keyparam encoding Optional output encoding (default is US-ASCII).
+#     Use False to return a Unicode string.
 # @keyparam method Optional output method ("xml", "html", "text" or
 #     "c14n"; default is "xml").
 # @return A sequence object containing the XML data.
@@ -1184,7 +1192,7 @@
     # debugging
     if not isinstance(elem, ElementTree):
         elem = ElementTree(elem)
-    elem.write(sys.stdout)
+    elem.write(sys.stdout, encoding=False)
     tail = elem.getroot().tail
     if not tail or tail[-1] != "\n":
         sys.stdout.write("\n")