Teach this class about "highest-level syntactic breaks" but only for

headers with no charset or 'us-ascii' charsets.  Actually this is only
partially true: we know about semicolons (but not true parameters) and
we know about whitespace (but not technically folding whitespace).
Still it should be good enough for all practical purposes.

Other changes include:

__init__(): Add a continuation_ws argument, which defaults to a single
space.  Set this to change the whitespace used for continuation lines
when a header must be split.  Also, changed the way header line
lengths are calculated, so that they take into account continuation_ws
(when tabs-expanded) and any provided header_name parameter.  This
should do much better on returning split headers for which the first
and subsequent lines must fit into a specified width.

guess_maxlinelen(): Removed.  I don't think we need this method as
part of the public API.

encode_chunks() -> _encode_chunks(): I don't think we need this one as
part of the public API either.

Lib/email/Header.py

@@ -16,7 +16,9 @@ except SyntaxError:
 
 CRLFSPACE = '\r\n '
 CRLF = '\r\n'
-NLSPACE = '\n '
+NL = '\n'
+SPACE8 = ' ' * 8
+EMPTYSTRING = ''
 
 MAXLINELEN = 76
 
@@ -92,11 +94,12 @@ def decode_header(header):
 
 
 class Header:
-    def __init__(self, s, charset=None, maxlinelen=None, header_name=None):
+    def __init__(self, s, charset=None, maxlinelen=None, header_name=None,
+                 continuation_ws=' '):
         """Create a MIME-compliant header that can contain many languages.
 
         Specify the initial header value in s.  Specify its character set as a
-        Charset object in the charset argument.  If none, a default Charset
+        Charset object in the charset argument.  If None, a default Charset
         instance will be used.
 
         You can later append to the header with append(s, charset) below;
@@ -104,43 +107,41 @@ class Header:
         here.  In fact, it's optional, and if not given, defaults to the
         charset specified in the constructor.
 
-        The maximum line length can be specified explicitly via maxlinelen.
+        The maximum line length can be specified explicit via maxlinelen.  For
-        You can also pass None for maxlinelen and the name of a header field
+        splitting the first line to a shorter value (to account for the field
-        (e.g. "Subject") to let the constructor guess the best line length to
+        header which isn't included in s, e.g. `Subject') pass in the name of
-        use.  The default maxlinelen is 76.
+        the field in header_name.  The default maxlinelen is 76.
+
+        continuation_ws must be RFC 2822 compliant folding whitespace (usually
+        either a space or a hard tab) which will be prepended to continuation
+        lines.
         """
         if charset is None:
             charset = Charset()
         self._charset = charset
+        self._continuation_ws = continuation_ws
+        cws_expanded_len = len(continuation_ws.replace('\t', SPACE8))
         # BAW: I believe `chunks' and `maxlinelen' should be non-public.
         self._chunks = []
         self.append(s, charset)
         if maxlinelen is None:
-            if header_name is None:
+            maxlinelen = MAXLINELEN
-                self._maxlinelen = MAXLINELEN
+        if header_name is None:
-            else:
+            # We don't know anything about the field header so the first line
-                self.guess_maxlinelen(header_name)
+            # is the same length as subsequent lines.
+            self._firstlinelen = maxlinelen
         else:
-            self._maxlinelen = maxlinelen
+            # The first line should be shorter to take into account the field
+            # header.  Also subtract off 2 extra for the colon and space.
+            self._firstlinelen = maxlinelen - len(header_name) - 2
+        # Second and subsequent lines should subtract off the length in
+        # columns of the continuation whitespace prefix.
+        self._maxlinelen = maxlinelen - cws_expanded_len
 
     def __str__(self):
         """A synonym for self.encode()."""
         return self.encode()
 
-    def guess_maxlinelen(self, s=None):
-        """Guess the maximum length to make each header line.
-
-        Given a header name (e.g. "Subject"), set this header's maximum line
-        length to an appropriate length to avoid line wrapping.  If s is not
-        given, return the previous maximum line length and don't set it.
-
-        Returns the new maximum line length.
-        """
-        # BAW: is this semantic necessary?
-        if s is not None:
-            self._maxlinelen = MAXLINELEN - len(s) - 2
-        return self._maxlinelen
-
     def append(self, s, charset=None):
         """Append string s with Charset charset to the MIME header.
 
@@ -150,7 +151,7 @@ class Header:
             charset = self._charset
         self._chunks.append((s, charset))
 
-    def _split(self, s, charset):
+    def _split(self, s, charset, firstline=0):
         # Split up a header safely for use with encode_chunks.  BAW: this
         # appears to be a private convenience method.
         splittable = charset.to_splittable(s)
@@ -159,6 +160,20 @@ class Header:
 
         if elen <= self._maxlinelen:
             return [(encoded, charset)]
+        # BAW: I'm not sure what the right test here is.  What we're trying to
+        # do is be faithful to RFC 2822's recommendation that ($2.2.3):
+        #
+        # "Note: Though structured field bodies are defined in such a way that
+        #  folding can take place between many of the lexical tokens (and even
+        #  within some of the lexical tokens), folding SHOULD be limited to
+        #  placing the CRLF at higher-level syntactic breaks."
+        #
+        # For now, I can only imagine doing this when the charset is us-ascii,
+        # although it's possible that other charsets may also benefit from the
+        # higher-level syntactic breaks.
+        #
+        elif charset == 'us-ascii':
+            return self._ascii_split(s, charset, firstline)
         # BAW: should we use encoded?
         elif elen == len(s):
             # We can split on _maxlinelen boundaries because we know that the
@@ -166,13 +181,119 @@ class Header:
             splitpnt = self._maxlinelen
             first = charset.from_splittable(splittable[:splitpnt], 0)
             last = charset.from_splittable(splittable[splitpnt:], 0)
-            return self._split(first, charset) + self._split(last, charset)
         else:
             # Divide and conquer.
             halfway = _floordiv(len(splittable), 2)
             first = charset.from_splittable(splittable[:halfway], 0)
             last = charset.from_splittable(splittable[halfway:], 0)
-            return self._split(first, charset) + self._split(last, charset)
+        # Do the split
+        return self._split(first, charset, firstline) + \
+               self._split(last, charset)
+
+    def _ascii_split(self, s, charset, firstline):
+        # Attempt to split the line at the highest-level syntactic break
+        # possible.  Note that we don't have a lot of smarts about field
+        # syntax; we just try to break on semi-colons, then whitespace.
+        rtn = []
+        lines = s.splitlines()
+        while lines:
+            line = lines.pop(0)
+            if firstline:
+                maxlinelen = self._firstlinelen
+                firstline = 0
+            else:
+                line = line.lstrip()
+                maxlinelen = self._maxlinelen
+            # Short lines can remain unchanged
+            if len(line.replace('\t', SPACE8)) <= maxlinelen:
+                rtn.append(line)
+            else:
+                oldlen = len(line)
+                # Try to break the line on semicolons, but if that doesn't
+                # work, try to split on folding whitespace.
+                while len(line) > maxlinelen:
+                    i = line.rfind(';', 0, maxlinelen)
+                    if i < 0:
+                        break
+                    rtn.append(line[:i] + ';')
+                    line = line[i+1:]
+                # Is the remaining stuff still longer than maxlinelen?
+                if len(line) <= maxlinelen:
+                    # Splitting on semis worked
+                    rtn.append(line)
+                    continue
+                # Splitting on semis didn't finish the job.  If it did any
+                # work at all, stick the remaining junk on the front of the
+                # `lines' sequence and let the next pass do its thing.
+                if len(line) <> oldlen:
+                    lines.insert(0, line)
+                    continue
+                # Otherwise, splitting on semis didn't help at all.
+                parts = re.split(r'(\s+)', line)
+                if len(parts) == 1 or (len(parts) == 3 and
+                                       parts[0].endswith(':')):
+                    # This line can't be split on whitespace.  There's now
+                    # little we can do to get this into maxlinelen.  BAW:
+                    # We're still potentially breaking the RFC by possibly
+                    # allowing lines longer than the absolute maximum of 998
+                    # characters.  For now, let it slide.
+                    #
+                    # len(parts) will be 1 if this line has no `Field: '
+                    # prefix, otherwise it will be len(3).
+                    rtn.append(line)
+                    continue
+                # There is whitespace we can split on.
+                first = parts.pop(0)
+                sublines = [first]
+                acc = len(first)
+                while parts:
+                    len0 = len(parts[0])
+                    len1 = len(parts[1])
+                    if acc + len0 + len1 <= maxlinelen:
+                        sublines.append(parts.pop(0))
+                        sublines.append(parts.pop(0))
+                        acc += len0 + len1
+                    else:
+                        # Split it here, but don't forget to ignore the
+                        # next whitespace-only part
+                        if first <> '':
+                            rtn.append(EMPTYSTRING.join(sublines))
+                        del parts[0]
+                        first = parts.pop(0)
+                        sublines = [first]
+                        acc = len(first)
+                rtn.append(EMPTYSTRING.join(sublines))
+        return [(chunk, charset) for chunk in rtn]
+
+    def _encode_chunks(self):
+        """MIME-encode a header with many different charsets and/or encodings.
+
+        Given a list of pairs (string, charset), return a MIME-encoded string
+        suitable for use in a header field.  Each pair may have different
+        charsets and/or encodings, and the resulting header will accurately
+        reflect each setting.
+
+        Each encoding can be email.Utils.QP (quoted-printable, for ASCII-like
+        character sets like iso-8859-1), email.Utils.BASE64 (Base64, for
+        non-ASCII like character sets like KOI8-R and iso-2022-jp), or None
+        (no encoding).
+
+        Each pair will be represented on a separate line; the resulting string
+        will be in the format:
+
+        "=?charset1?q?Mar=EDa_Gonz=E1lez_Alonso?=\n
+          =?charset2?b?SvxyZ2VuIEL2aW5n?="
+        """
+        chunks = []
+        for header, charset in self._chunks:
+            if charset is None or charset.header_encoding is None:
+                # There's no encoding for this chunk's charsets
+                _max_append(chunks, header, self._maxlinelen)
+            else:
+                _max_append(chunks, charset.header_encode(header, 0),
+                            self._maxlinelen, ' ')
+        joiner = NL + self._continuation_ws
+        return joiner.join(chunks)
 
     def encode(self):
         """Encode a message header, possibly converting charset and encoding.
@@ -194,34 +315,6 @@ class Header:
         """
         newchunks = []
         for s, charset in self._chunks:
-            newchunks += self._split(s, charset)
+            newchunks += self._split(s, charset, 1)
         self._chunks = newchunks
-        return self.encode_chunks()
+        return self._encode_chunks()
-
-    def encode_chunks(self):
-        """MIME-encode a header with many different charsets and/or encodings.
-
-        Given a list of pairs (string, charset), return a MIME-encoded string
-        suitable for use in a header field.  Each pair may have different
-        charsets and/or encodings, and the resulting header will accurately
-        reflect each setting.
-
-        Each encoding can be email.Utils.QP (quoted-printable, for ASCII-like
-        character sets like iso-8859-1), email.Utils.BASE64 (Base64, for
-        non-ASCII like character sets like KOI8-R and iso-2022-jp), or None
-        (no encoding).
-
-        Each pair will be represented on a separate line; the resulting string
-        will be in the format:
-
-        "=?charset1?q?Mar=EDa_Gonz=E1lez_Alonso?=\n
-          =?charset2?b?SvxyZ2VuIEL2aW5n?="
-        """
-        chunks = []
-        for header, charset in self._chunks:
-            if charset is None:
-                _max_append(chunks, header, self._maxlinelen, ' ')
-            else:
-                _max_append(chunks, charset.header_encode(header, 0),
-                            self._maxlinelen, ' ')
-        return NLSPACE.join(chunks)