strconv: Fix b & friends on macos/windows

On macos and windows, Python2 is built with --enable-unicode=ucs2, which
makes it to use UTF-16 encoding for unicode characters, and so for
characters higher than U+10000 it uses surrogate encoding with _2_
unicode points, for example:

        >>> import sys
        >>> sys.maxunicode
        65535                       <-- NOTE indicates UCS2 build
        >>> s = u'\U00012345'
        >>> s
        u'\U00012345'
        >>> s.encode('utf-8')
        '\xf0\x92\x8d\x85'
        >>> len(s)
        2                           <-- NOTE _not_ 1
        >>> s[0]
        u'\ud808'
        >>> s[1]
        u'\udf45'

This leads to e.g. b tests failing for

    # tbytes                        tunicode
    (b"\xf0\x90\x8c\xbc",           u'\U0001033c'),     # Valid 4 Octet Sequence '𐌼'

    >           assert b(tunicode) == tbytes
    E           AssertionError: assert '\xed\xa0\x80\xed\xbc\xbc' == '\xf0\x90\x8c\xbc'
    E             - \xed\xa0\x80\xed\xbc\xbc
    E             + \xf0\x90\x8c\xbc

because on UCS2 python build u'\U0001033c' is represented as 2 unicode
points:

    >>> s = u'\U0001033c'
    >>> len(s)
    2
    >>> s[0]
    u'\ud800'
    >>> s[1]
    u'\udf3c'
    >>> s[0].encode('utf-8')
    '\xed\xa0\x80'
    >>> s[1].encode('utf-8')
    '\xed\xbc\xbc'

-> Fix it by detecting UCS2 build and working around by manually
combining such surrogate unicode pairs appropriately.

A reference on the subject:

https://matthew-brett.github.io/pydagogue/python_unicode.html#utf-16-ucs2-builds-of-python-and-32-bit-unicode-code-points

golang/golang_test.py

@@ -1601,6 +1601,10 @@ def test_strings():
         # non-printable utf-8
         (b'\x7f\xc2\x80\xc2\x81\xc2\x82\xc2\x83\xc2\x84\xc2\x85\xc2\x86\xc2\x87',
                                         u"\u007f\u0080\u0081\u0082\u0083\u0084\u0085\u0086\u0087"),
+
+        # some characters with U >= 0x10000
+        (b'\xf0\x9f\x99\x8f',           u'\U0001f64f'),    # 🙏
+        (b'\xf0\x9f\x9a\x80',           u'\U0001f680'),    # 🚀
     )
 
     for tbytes, tunicode in testv:

golang/strconv.py

@@ -21,6 +21,7 @@
 
 from __future__ import print_function, absolute_import
 
+import sys
 import six, unicodedata, codecs
 from six import text_type as unicode        # py2: unicode      py3: str
 from six import unichr                      # py2: unichr       py3: chr
@@ -120,7 +121,7 @@ def _quote(s):
                 emit(br'\x%02x' % ord(c))
 
             # printable utf-8 characters go as is
-            elif unicodedata.category(unichr(r))[0] in _printable_cat0:
+            elif unicodedata.category(_xunichr(r))[0] in _printable_cat0:
                 emit(s[i:isize])
 
             # everything else goes in numeric byte escapes
@@ -224,6 +225,9 @@ _printable_cat0 = frozenset(['L', 'N', 'P', 'S'])   # letters, numbers, punctuat
 
 _rune_error = 0xFFFD # unicode replacement character
 
+_ucs2_build        = (sys.maxunicode ==     0xffff)     #    ucs2
+assert _ucs2_build or sys.maxunicode >= 0x0010ffff      # or ucs4
+
 # _utf8_decode_rune decodes next UTF8-character from byte string s.
 #
 # _utf8_decode_rune(s) -> (r, size)
@@ -244,6 +248,15 @@ def _utf8_decode_rune(s):
         if len(r) == 1:
             return ord(r), l
 
+        # see comment in _utf8_encode_surrogateescape
+        if _ucs2_build and len(r) == 2:
+            try:
+                return _xuniord(r), l
+            # e.g. TypeError: ord() expected a character, but string of length 2 found
+            except TypeError:
+                l -= 1
+                continue
+
         l -= 1
         continue
 
@@ -277,7 +290,7 @@ def _utf8_decode_surrogateescape(s): # -> unicode
                     emit(unichr(b))
 
         else:
-            emit(unichr(r))
+            emit(_xunichr(r))
 
         s = s[width:]
 
@@ -290,12 +303,74 @@ def _utf8_encode_surrogateescape(s): # -> bytes
     outv = []
     emit = outv.append
 
-    for uc in s:
+    while len(s) > 0:
+        uc = s[0]; s = s[1:]
         c = ord(uc)
+
         if 0xdc80 <= c <= 0xdcff:
             # surrogate - emit unescaped byte
             emit(bchr(c & 0xff))
-        else:
+            continue
+
+        # in builds with --enable-unicode=ucs2 (default for py2 on macos and windows)
+        # python represents unicode points > 0xffff as _two_ unicode characters:
+        #
+        #   uh = u - 0x10000
+        #   c1 = 0xd800 + (uh >> 10)      ; [d800, dbff]
+        #   c2 = 0xdc00 + (uh & 0x3ff)    ; [dc00, dfff]
+        #
+        # if detected - merge those two unicode characters for .encode('utf-8') below
+        #
+        # this should be only relevant for python2, as python3 switched to "flexible"
+        # internal unicode representation: https://www.python.org/dev/peps/pep-0393
+        if _ucs2_build and (0xd800 <= c <= 0xdbff):
+            if len(s) > 0:
+                uc2 = s[0]
+                c2 = ord(uc2)
+                if 0xdc00 <= c2 <= 0xdfff:
+                    uc = uc + uc2
+                    s = s[1:]
+
         emit(uc.encode('utf-8', 'strict'))
 
     return b''.join(outv)
+
+
+# _xuniord returns ordinal for a unicode character u.
+#
+# it works correctly even if u is represented as 2 unicode surrogate points on
+# ucs2 python build.
+if not _ucs2_build:
+    _xuniord = ord
+else:
+    def _xuniord(u):
+        assert isinstance(u, unicode)
+        if len(u) == 1:
+            return ord(u)
+
+        # see _utf8_encode_surrogateescape for details
+        if len(u) == 2:
+            c1 = ord(u[0])
+            c2 = ord(u[1])
+            if (0xd800 <= c1 <= 0xdbff) and (0xdc00 <= c2 <= 0xdfff):
+                return 0x10000 | ((c1 - 0xd800) << 10) | (c2 - 0xdc00)
+
+        # let it crash
+        return ord(u)
+
+
+# _xunichr returns unicode character for an ordinal i.
+#
+# it works correctly even on ucs2 python builds, where ordinals >= 0x10000 are
+# represented as 2 unicode pointe.
+if not _ucs2_build:
+    _xunichr = unichr
+else:
+    def _xunichr(i):
+        if i < 0x10000:
+            return unichr(i)
+
+        # see _utf8_encode_surrogateescape for details
+        uh = i - 0x10000
+        return unichr(0xd800 + (uh >> 10)) + \
+               unichr(0xdc00 + (uh & 0x3ff))