Add new string conversion utilities.

src/sage/misc/string.pxd

@@ -0,0 +1,3 @@
+cdef str char_to_str(char* c, encoding=*)
+cpdef str bytes_to_str(bytes b, encoding=*)
+cpdef bytes str_to_bytes(str s, encoding=*)

src/sage/misc/string.pyx

@@ -0,0 +1,108 @@
+# -*- encoding: utf-8 -*-
+"""
+String conversion and encoding/decoding utilities, in particular for Python 2/3
+compatibility.
+"""
+
+#*****************************************************************************
+#       Copyright (C) 2017 Erik M. Bray <erik.bray@lri.fr>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 2 of the License, or
+# (at your option) any later version.
+#                  http://www.gnu.org/licenses/
+#*****************************************************************************
+
+from __future__ import absolute_import
+
+import locale
+import six
+import sys
+
+from libc.string cimport strlen
+
+from cpython.bytes cimport PyBytes_AsString as PyString_AsString
+if six.PY2:
+    from cpython.string cimport PyString_FromString
+else:
+    from cpython.bytes cimport PyBytes_FromString as PyString_FromString
+from cpython.unicode cimport (PyUnicode_Decode, PyUnicode_AsEncodedString)
+from cpython.version cimport PY_MAJOR_VERSION
+
+cdef extern from "Python.h":
+    # Missing from cpython.unicode
+    char* PyUnicode_AsUTF8(object unicode)
+
+
+DEFAULT_ENCODING = locale.getpreferredencoding()
+FS_ENCODING = sys.getfilesystemencoding()
+
+
+cdef inline str char_to_str(char* c, encoding=DEFAULT_ENCODING):
+    """
+    Converts a C ``char`` array to a Python `str` object.
+
+    On Python 3 this requires an encoding to be specified with which to
+    decode the bytes in the ``char`` array.
+    """
+
+    if PY_MAJOR_VERSION <= 2:
+        # <str> is needed here to "trick" Cython into thinking we expect
+        # this to return a str (on Python 2 it has no problem with this,
+        # but on Python 3 it balks)
+        # It doesn't matter that this cast doesn't make sense since it
+        # will never happen on Python 3
+        return <str>PyString_FromString(c)
+    else:
+        return PyUnicode_Decode(c, strlen(c), PyUnicode_AsUTF8(encoding),
+                                "surrogateescape")
+
+
+cpdef inline str bytes_to_str(bytes b, encoding=DEFAULT_ENCODING):
+    """
+    Convertes `bytes` to `str`.
+
+    On Python 2 this is a no-op since ``bytes is str``.  On Python 3
+    this decodes the given `bytes` to a Python 3 unicode `str` using the
+    specified encoding.
+
+    EXAMPLES::
+
+        sage: from six import PY2; from sage.misc import six
+        sage: six.DEFAULT_ENCODING = 'utf-8'
+        sage: s = six.bytes_to_str(b'\xe2\x98\x83')
+        sage: if PY2:
+        ....:     s == b'\xe2\x98\x83'
+        ....: else:
+        ....:     s == u'☃'
+        True
+    """
+
+    return char_to_str(PyString_AsString(b), encoding=encoding)
+
+
+cpdef inline bytes str_to_bytes(str s, encoding=DEFAULT_ENCODING):
+    """
+    Convertes `str` to `bytes`.
+
+    On Python 2 this is a no-op since ``str is bytes``.  On Python 3
+    this encodes the given `str` to a Python 3 `bytes` using the
+    specified encoding.
+
+    EXAMPLES::
+
+        sage: from six import PY2; from sage.misc import six
+        sage: six.DEFAULT_ENCODING = 'utf-8'
+        sage: if PY2:
+        ....:     b = six.str_to_bytes('\xe2\x98\x83')
+        ....: else:
+        ....:     b = six.str_to_bytes(u'☃')
+        sage: b == b'\xe2\x98\x83'
+        True
+    """
+    if PY_MAJOR_VERSION <= 2:
+        return <bytes>s
+    else:
+        return PyUnicode_AsEncodedString(s, PyUnicode_AsUTF8(encoding),
+                                         "surrogateescape")