new method "to_bytes" and change in pynac constant

src/sage/libs/pynac/constant.pyx

@@ -16,6 +16,7 @@ Wrapper around Pynac's constants
 
 from __future__ import absolute_import, division, print_function
 
+from sage.misc.six import to_bytes
 from .pynac cimport *
 from sage.symbolic.expression cimport new_Expression_from_GEx
 from sage.symbolic.ring import SR
@@ -52,6 +53,8 @@ cdef class PynacConstant:
             raise ValueError
 
         self._name = name
+        name = to_bytes(name)
+        texname = to_bytes(texname)
 
         # For the constants explicitly defined in constant.cpp in the
         # Pynac library, we use those symbols. Otherwise, we create a

src/sage/misc/six.py

@@ -133,3 +133,34 @@ def u(x):
     if isinstance(x, bytes):
         return x.decode("utf-8")
     raise TypeError('input has no conversion to unicode')
+
+
+def to_bytes(x):
+    r"""
+    Convert `x` to bytes, assuming UTF-8 encoding.
+
+    Python2 behaviour:
+
+    If input is str, returns the input.
+
+    If input is unicode, convert to bytes using utf8-encoding.
+
+    Python3 behaviour:
+
+    If input is str, convert to bytes using utf8-encoding.
+
+    If input is bytes, returns the input.
+
+    EXAMPLES::
+
+        sage: from sage.misc.six import to_bytes
+        sage: to_bytes("500 €")
+        '500 \xe2\x82\xac'
+        sage: to_bytes(u"500 \u20ac")
+        '500 \xe2\x82\xac'
+    """
+    if isinstance(x, text_type):  # py2 unicode and py3 str
+        return x.encode("utf-8")
+    if isinstance(x, bytes):
+        return x
+    raise TypeError('input has no conversion to unicode')