Add new string conversion utilities.

src/sage/cpython/string.pxd

@@ -0,0 +1,90 @@
+# -*- encoding: utf-8 -*-
+#*****************************************************************************
+#       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
+
+from libc.string cimport strlen
+
+from cpython.bytes cimport (PyBytes_AsString as PyString_AsString,
+                            PyBytes_FromString as PyString_FromString)
+from cpython.unicode cimport PyUnicode_Decode, PyUnicode_AsEncodedString
+
+IF PY_MAJOR_VERSION >= 3:
+    cdef extern from "Python.h":
+        # Missing from cpython.unicode
+        char* PyUnicode_AsUTF8(object unicode)
+        object PyUnicode_DecodeLocale(const char* str, const char* errors)
+        object PyUnicode_EncodeLocale(object unicode, const char* errors)
+
+
+cdef inline str char_to_str(char* c, encoding=None):
+    IF PY_MAJOR_VERSION <= 2:
+        return <str>PyString_FromString(c)
+    ELSE:
+        if encoding is None:
+            return PyUnicode_DecodeLocale(c, "surrogateescape")
+
+        return PyUnicode_Decode(c, strlen(c), PyUnicode_AsUTF8(encoding),
+                                "surrogateescape")
+
+
+cpdef inline str bytes_to_str(bytes b, encoding=None):
+    """
+    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: import six
+        sage: from sage.cpython.string import bytes_to_str
+        sage: s = bytes_to_str(b'\xe2\x98\x83')
+        sage: if six.PY2:
+        ....:     s == b'\xe2\x98\x83'
+        ....: else:
+        ....:     s == u'☃'
+        True
+    """
+
+    IF PY_MAJOR_VERSION <= 2:
+        return <str>b
+    ELSE:
+        return char_to_str(PyString_AsString(b), encoding=encoding)
+
+
+cpdef inline bytes str_to_bytes(str s, encoding=None):
+    """
+    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: import six
+        sage: from sage.cpython.string import str_to_bytes
+        sage: if six.PY2:
+        ....:     b = str_to_bytes('\xe2\x98\x83')
+        ....: else:
+        ....:     b = str_to_bytes(u'☃')
+        sage: b == b'\xe2\x98\x83'
+        True
+    """
+    IF PY_MAJOR_VERSION <= 2:
+        return <bytes>s
+    ELSE:
+        if encoding is None:
+            return PyUnicode_EncodeLocale(s, "surrogateescape")
+
+        return PyUnicode_AsEncodedString(s, PyUnicode_AsUTF8(encoding),
+                                         "surrogateescape")

src/sage/cpython/string.pyx

@@ -0,0 +1,20 @@
+# -*- encoding: utf-8 -*-
+#*****************************************************************************
+#       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 sys
+
+
+# Provide this as a shortcut to calling sys.getfilesystemencoding(), which
+# after interpeter initialization is constant.
+FS_ENCODING = sys.getfilesystemencoding()
+
+# Functions in this module are implemented in the .pxd file for inlining.