golang/gcompat: New _Python_ package that provides Go-compatibility layer for Python

Let's start with substitute for %q that is lacking in python.
The implementation is copied from zodbtools from here:

https://lab.nexedi.com/nexedi/zodbtools/blob/2801fae9/zodbtools/util.py#L39

It is not good for golang to depend on zodbtools and the function is
minor. In the future maybe zodbtools in turn will change to import qq
from golang.

golang/gcompat.py

+# -*- coding: utf-8 -*-
+# Copyright (C) 2018  Nexedi SA and Contributors.
+#                     Kirill Smelkov <kirr@nexedi.com>
+#
+# This program is free software: you can Use, Study, Modify and Redistribute
+# it under the terms of the GNU General Public License version 3, or (at your
+# option) any later version, as published by the Free Software Foundation.
+#
+# You can also Link and Combine this program with other software covered by
+# the terms of any of the Free Software licenses or any of the Open Source
+# Initiative approved licenses and Convey the resulting work. Corresponding
+# source of such a combination shall include the source code for all other
+# software used.
+#
+# This program is distributed WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+#
+# See COPYING file for full licensing terms.
+# See https://www.nexedi.com/licensing for rationale and options.
+"""Package gcompat provides Go-compatibility layer for Python"""
+
+# qq is substitute for %q, which is missing in python.
+#
+# (python's automatic escape uses smartquotes quoting with either ' or ").
+def qq(obj):
+    # go: like %s, %q automatically converts to string
+    if not isinstance(obj, basestring):
+        obj = str(obj)
+    return _quote(obj)
+
+# _quote quotes string into valid "..." string always quoted with ".
+def _quote(s):
+    # TODO also accept unicode as input.
+    # TODO output printable UTF-8 characters as-is, but escape non-printable UTF-8 and invalid UTF-8 bytes.
+    outv = []
+    # we don't want ' to be escaped
+    for _ in s.split("'"):
+        # this escape almost everything except " character
+        # NOTE string_escape does not do smartquotes and always uses ' for quoting
+        # (repr(str) is the same except it does smartquoting picking ' or " automatically)
+        q = _.encode("string_escape")
+        q = q.replace('"', r'\"')
+        outv.append(q)
+    return '"' + "'".join(outv) + '"'

golang/gcompat_test.py

+# -*- coding: utf-8 -*-
+# Copyright (C) 2018  Nexedi SA and Contributors.
+#                     Kirill Smelkov <kirr@nexedi.com>
+#
+# This program is free software: you can Use, Study, Modify and Redistribute
+# it under the terms of the GNU General Public License version 3, or (at your
+# option) any later version, as published by the Free Software Foundation.
+#
+# You can also Link and Combine this program with other software covered by
+# the terms of any of the Free Software licenses or any of the Open Source
+# Initiative approved licenses and Convey the resulting work. Corresponding
+# source of such a combination shall include the source code for all other
+# software used.
+#
+# This program is distributed WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+#
+# See COPYING file for full licensing terms.
+# See https://www.nexedi.com/licensing for rationale and options.
+
+import gopath
+gcompat = gopath.gimport('lab.nexedi.com/kirr/go123/golang/gcompat')
+qq  = gcompat.qq
+
+def test_qq():
+    testv = (
+        # in            want without leading/trailing "
+        ('',            r""),
+        ('\'',          r"'"),
+        ('"',           r"\""),
+        ('abc\ndef',    r"abc\ndef"),
+        ('a\'c\ndef',   r"a'c\ndef"),
+        ('a\"c\ndef',   r"a\"c\ndef"),
+        # ('привет',      r"привет"),       TODO
+    )
+
+    for tin, twant in testv:
+        twant = '"' + twant + '"'   # add lead/trail "
+        assert qq(tin) == twant