Plumbing for from __future__ import annotations (PEP 563)

The string form is recovered by unparsing the AST.
ambv · Jan 26, 2018 · Nov 10, 2017 · Nov 10, 2017 · Nov 10, 2017 · Dec 5, 2017
commit 863d3e986fd0beec767383c302b06dd59436938f
diff --git a/Include/ast.h b/Include/ast.h
@@ -15,6 +15,10 @@ PyAPI_FUNC(mod_ty) PyAST_FromNodeObject(
     PyCompilerFlags *flags,
     PyObject *filename,
     PyArena *arena);
+PyAPI_FUNC(PyObject *) PyAST_UnicodeFromAstExpr(
+    expr_ty e,
+    int omit_parens,
+    int omit_string_brackets);
 
 #ifdef __cplusplus
 }

diff --git a/Include/code.h b/Include/code.h
@@ -82,6 +82,7 @@ typedef struct {
 
 #define CO_FUTURE_BARRY_AS_BDFL  0x40000
 #define CO_FUTURE_GENERATOR_STOP  0x80000
+#define CO_FUTURE_ANNOTATIONS    0x100000
 
 /* This value is found in the co_cell2arg array when the associated cell
    variable does not correspond to an argument. */

diff --git a/Include/compile.h b/Include/compile.h
@@ -16,7 +16,7 @@ PyAPI_FUNC(PyCodeObject *) PyNode_Compile(struct _node *, const char *);
 #define PyCF_MASK (CO_FUTURE_DIVISION | CO_FUTURE_ABSOLUTE_IMPORT | \
                    CO_FUTURE_WITH_STATEMENT | CO_FUTURE_PRINT_FUNCTION | \
                    CO_FUTURE_UNICODE_LITERALS | CO_FUTURE_BARRY_AS_BDFL | \
-                   CO_FUTURE_GENERATOR_STOP)
+                   CO_FUTURE_GENERATOR_STOP | CO_FUTURE_ANNOTATIONS)
 #define PyCF_MASK_OBSOLETE (CO_NESTED)
 #define PyCF_SOURCE_IS_UTF8  0x0100
 #define PyCF_DONT_IMPLY_DEDENT 0x0200
@@ -45,6 +45,7 @@ typedef struct {
 #define FUTURE_UNICODE_LITERALS "unicode_literals"
 #define FUTURE_BARRY_AS_BDFL "barry_as_FLUFL"
 #define FUTURE_GENERATOR_STOP "generator_stop"
+#define FUTURE_ANNOTATIONS "annotations"
 
 struct _mod; /* Declare the existence of this type */
 #define PyAST_Compile(mod, s, f, ar) PyAST_CompileEx(mod, s, f, -1, ar)

diff --git a/Lib/__future__.py b/Lib/__future__.py
@@ -57,13 +57,14 @@
     "unicode_literals",
     "barry_as_FLUFL",
     "generator_stop",
+    "annotations",
 ]
 
 __all__ = ["all_feature_names"] + all_feature_names
 
-# The CO_xxx symbols are defined here under the same names used by
-# compile.h, so that an editor search will find them here.  However,
-# they're not exported in __all__, because they don't really belong to
+# The CO_xxx symbols are defined here under the same names defined in
+# code.h and used by compile.h, so that an editor search will find them here.
+# However, they're not exported in __all__, because they don't really belong to
 # this module.
 CO_NESTED            = 0x0010   # nested_scopes
 CO_GENERATOR_ALLOWED = 0        # generators (obsolete, was 0x1000)
@@ -74,6 +75,7 @@
 CO_FUTURE_UNICODE_LITERALS = 0x20000 # unicode string literals
 CO_FUTURE_BARRY_AS_BDFL = 0x40000
 CO_FUTURE_GENERATOR_STOP  = 0x80000 # StopIteration becomes RuntimeError in generators
+CO_FUTURE_ANNOTATIONS     = 0x100000  # annotations become strings at runtime
 
 class _Feature:
     def __init__(self, optionalRelease, mandatoryRelease, compiler_flag):
@@ -132,9 +134,13 @@ def __repr__(self):
                             CO_FUTURE_UNICODE_LITERALS)
 
 barry_as_FLUFL = _Feature((3, 1, 0, "alpha", 2),
-                         (3, 9, 0, "alpha", 0),
-                         CO_FUTURE_BARRY_AS_BDFL)
+                          (3, 9, 0, "alpha", 0),
+                          CO_FUTURE_BARRY_AS_BDFL)
 
 generator_stop = _Feature((3, 5, 0, "beta", 1),
-                         (3, 7, 0, "alpha", 0),
-                         CO_FUTURE_GENERATOR_STOP)
+                          (3, 7, 0, "alpha", 0),
+                          CO_FUTURE_GENERATOR_STOP)
+
+annotations = _Feature((3, 7, 0, "beta", 1),
+                       (4, 0, 0, "alpha", 0),
+                       CO_FUTURE_ANNOTATIONS)
diff --git a/Python/compile.c b/Python/compile.c
@@ -1658,13 +1658,27 @@ compiler_visit_kwonlydefaults(struct compiler *c, asdl_seq *kwonlyargs,
     return 0;
 }
 
+static int
+compiler_visit_annexpr(struct compiler *c, expr_ty annotation)
+{
+    static PyObject *ann_as_str;
+    ann_as_str = PyAST_UnicodeFromAstExpr(annotation, 1, 1);
+    if (!ann_as_str)
+        return 0;
+    ADDOP_N(c, LOAD_CONST, ann_as_str, consts);
+    return 1;
+}
+
 static int
 compiler_visit_argannotation(struct compiler *c, identifier id,
     expr_ty annotation, PyObject *names)
 {
     if (annotation) {
         PyObject *mangled;
-        VISIT(c, expr, annotation);
+        if (c->c_future->ff_features & CO_FUTURE_ANNOTATIONS)
+            VISIT(c, annexpr, annotation)
+        else
+            VISIT(c, expr, annotation);
         mangled = _Py_Mangle(c->u->u_private, id);
         if (!mangled)
             return 0;
@@ -4646,7 +4660,10 @@ compiler_annassign(struct compiler *c, stmt_ty s)
             if (!mangled) {
                 return 0;
             }
-            VISIT(c, expr, s->v.AnnAssign.annotation);
+            if (c->c_future->ff_features & CO_FUTURE_ANNOTATIONS)
+                VISIT(c, annexpr, s->v.AnnAssign.annotation)
+            else
+                VISIT(c, expr, s->v.AnnAssign.annotation);
             /* ADDOP_N decrefs its argument */
             ADDOP_N(c, STORE_ANNOTATION, mangled, names);
         }

diff --git a/Python/future.c b/Python/future.c
@@ -42,6 +42,8 @@ future_check_features(PyFutureFeatures *ff, stmt_ty s, PyObject *filename)
             ff->ff_features |= CO_FUTURE_BARRY_AS_BDFL;
         } else if (strcmp(feature, FUTURE_GENERATOR_STOP) == 0) {
             ff->ff_features |= CO_FUTURE_GENERATOR_STOP;
+        } else if (strcmp(feature, FUTURE_ANNOTATIONS) == 0) {
+            ff->ff_features |= CO_FUTURE_ANNOTATIONS;
         } else if (strcmp(feature, "braces") == 0) {
             PyErr_SetString(PyExc_SyntaxError,
                             "not a chance");