Include '%a' in the string formatting types that are optimised into f-strings.

CHANGES.rst

@@ -55,6 +55,9 @@ Features added
 * Declarations for the ``pylifecycle`` C-API functions were added in a new .pxd file
   ``cpython.pylifecycle``.
 
+* ``%a`` is included in the string formatting types that are optimised into f-strings.
+  In this case, it is also automatically mapped to ``%r`` in Python 2.x.
+
 * New C macro ``CYTHON_HEX_VERSION`` to access Cython's version in the same style as
   ``PY_HEX_VERSION``.
 

Cython/Compiler/Optimize.py

@@ -4311,16 +4311,16 @@ class ConstantFolding(Visitor.VisitorTransform, SkipDeclarations):
                 warning(pos, "Too few arguments for format placeholders", level=1)
                 can_be_optimised = False
                 break
-            if format_type in u'srfdoxX':
+            if format_type in u'asrfdoxX':
                 format_spec = s[1:]
                 if format_type in u'doxX' and u'.' in format_spec:
                     # Precision is not allowed for integers in format(), but ok in %-formatting.
                     can_be_optimised = False
-                elif format_type in u'rs':
+                elif format_type in u'ars':
                     format_spec = format_spec[:-1]
                 substrings.append(ExprNodes.FormattedValueNode(
                     arg.pos, value=arg,
-                    conversion_char=format_type if format_type in u'rs' else None,
+                    conversion_char=format_type if format_type in u'ars' else None,
                     format_spec=ExprNodes.UnicodeNode(
                         pos, value=EncodedString(format_spec), constant_result=format_spec)
                         if format_spec else None,

tests/run/unicode_formatting.pyx

+# mode: run
+# tag: stringformat
+
+from __future__ import unicode_literals
+
+
+def ascii_format(a, int b, list c):
+    """
+    >>> print(ascii_format('x', 2, [1]))
+    -'x'-2-[1]-
+    """
+    return '-%a-%a-%a-' % (a, b, c)
+
+
+def repr_format(a, int b, list c):
+    """
+    >>> print(repr_format('x', 2, [1]))
+    -'x'-2-[1]-
+    """
+    return '-%r-%r-%r-' % (a, b, c)
+
+
+def str_format(a, int b, list c):
+    """
+    >>> print(str_format('x', 2, [1]))
+    -x-2-[1]-
+    """
+    return '-%s-%s-%s-' % (a, b, c)
+
+
+def mix_format(a, int b, list c):
+    """
+    >>> print(mix_format('x', 2, [1]))
+    -x-2-[1]-
+    """
+    return '-%s-%r-%a-' % (a, b, c)