bpo-38605: Make 'from __future__ import annotations' the default (GH-20434)

The hard part was making all the tests pass; there are some subtle issues here, because apparently the future import wasn't tested very thoroughly in previous Python versions.

For example, `inspect.signature()` returned type objects normally (except for forward references), but strings with the future import. We changed it to try and return type objects by calling `typing.get_type_hints()`, but fall back on returning strings if that function fails (which it may do if there are future references in the annotations that require passing in a specific namespace to resolve).

3 files changed, 3 insertions, 37 deletions

diff --git a/Python/ast_opt.c b/Python/ast_opt.c
index 5efaac4c89..22ca6f23ae 100644
--- a/Python/ast_opt.c
+++ b/Python/ast_opt.c
@@ -392,7 +392,6 @@ static int astfold_expr(expr_ty node_, PyArena *ctx_, _PyASTOptimizeState *state
 static int astfold_arguments(arguments_ty node_, PyArena *ctx_, _PyASTOptimizeState *state);
 static int astfold_comprehension(comprehension_ty node_, PyArena *ctx_, _PyASTOptimizeState *state);
 static int astfold_keyword(keyword_ty node_, PyArena *ctx_, _PyASTOptimizeState *state);
-static int astfold_arg(arg_ty node_, PyArena *ctx_, _PyASTOptimizeState *state);
 static int astfold_withitem(withitem_ty node_, PyArena *ctx_, _PyASTOptimizeState *state);
 static int astfold_excepthandler(excepthandler_ty node_, PyArena *ctx_, _PyASTOptimizeState *state);
 #define CALL(FUNC, TYPE, ARG) \
@@ -595,26 +594,12 @@ astfold_comprehension(comprehension_ty node_, PyArena *ctx_, _PyASTOptimizeState
 static int
 astfold_arguments(arguments_ty node_, PyArena *ctx_, _PyASTOptimizeState *state)
 {
-    CALL_SEQ(astfold_arg, arg, node_->posonlyargs);
-    CALL_SEQ(astfold_arg, arg, node_->args);
-    CALL_OPT(astfold_arg, arg_ty, node_->vararg);
-    CALL_SEQ(astfold_arg, arg, node_->kwonlyargs);
     CALL_SEQ(astfold_expr, expr, node_->kw_defaults);
-    CALL_OPT(astfold_arg, arg_ty, node_->kwarg);
     CALL_SEQ(astfold_expr, expr, node_->defaults);
     return 1;
 }
 
 static int
-astfold_arg(arg_ty node_, PyArena *ctx_, _PyASTOptimizeState *state)
-{
-    if (!(state->ff_features & CO_FUTURE_ANNOTATIONS)) {
-        CALL_OPT(astfold_expr, expr_ty, node_->annotation);
-    }
-    return 1;
-}
-
-static int
 astfold_stmt(stmt_ty node_, PyArena *ctx_, _PyASTOptimizeState *state)
 {
     switch (node_->kind) {
@@ -622,17 +607,11 @@ astfold_stmt(stmt_ty node_, PyArena *ctx_, _PyASTOptimizeState *state)
         CALL(astfold_arguments, arguments_ty, node_->v.FunctionDef.args);
         CALL(astfold_body, asdl_seq, node_->v.FunctionDef.body);
         CALL_SEQ(astfold_expr, expr, node_->v.FunctionDef.decorator_list);
-        if (!(state->ff_features & CO_FUTURE_ANNOTATIONS)) {
-            CALL_OPT(astfold_expr, expr_ty, node_->v.FunctionDef.returns);
-        }
         break;
     case AsyncFunctionDef_kind:
         CALL(astfold_arguments, arguments_ty, node_->v.AsyncFunctionDef.args);
         CALL(astfold_body, asdl_seq, node_->v.AsyncFunctionDef.body);
         CALL_SEQ(astfold_expr, expr, node_->v.AsyncFunctionDef.decorator_list);
-        if (!(state->ff_features & CO_FUTURE_ANNOTATIONS)) {
-            CALL_OPT(astfold_expr, expr_ty, node_->v.AsyncFunctionDef.returns);
-        }
         break;
     case ClassDef_kind:
         CALL_SEQ(astfold_expr, expr, node_->v.ClassDef.bases);
@@ -656,9 +635,6 @@ astfold_stmt(stmt_ty node_, PyArena *ctx_, _PyASTOptimizeState *state)
         break;
     case AnnAssign_kind:
         CALL(astfold_expr, expr_ty, node_->v.AnnAssign.target);
-        if (!(state->ff_features & CO_FUTURE_ANNOTATIONS)) {
-            CALL(astfold_expr, expr_ty, node_->v.AnnAssign.annotation);
-        }
         CALL_OPT(astfold_expr, expr_ty, node_->v.AnnAssign.value);
         break;
     case For_kind:
diff --git a/Python/compile.c b/Python/compile.c
index f2563d7f7a..ddd2a04962 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -2026,12 +2026,7 @@ compiler_visit_argannotation(struct compiler *c, identifier id,
 {
     if (annotation) {
         PyObject *mangled;
-        if (c->c_future->ff_features & CO_FUTURE_ANNOTATIONS) {
-            VISIT(c, annexpr, annotation)
-        }
-        else {
-            VISIT(c, expr, annotation);
-        }
+        VISIT(c, annexpr, annotation);
         mangled = _Py_Mangle(c->u->u_private, id);
         if (!mangled)
             return 0;
@@ -5261,12 +5256,7 @@ compiler_annassign(struct compiler *c, stmt_ty s)
         if (s->v.AnnAssign.simple &&
             (c->u->u_scope_type == COMPILER_SCOPE_MODULE ||
              c->u->u_scope_type == COMPILER_SCOPE_CLASS)) {
-            if (c->c_future->ff_features & CO_FUTURE_ANNOTATIONS) {
-                VISIT(c, annexpr, s->v.AnnAssign.annotation)
-            }
-            else {
-                VISIT(c, expr, s->v.AnnAssign.annotation);
-            }
+            VISIT(c, annexpr, s->v.AnnAssign.annotation);
             ADDOP_NAME(c, LOAD_NAME, __annotations__, names);
             mangled = _Py_Mangle(c->u->u_private, targ->v.Name.id);
             ADDOP_LOAD_CONST_NEW(c, mangled);
diff --git a/Python/future.c b/Python/future.c
index 3cea4fee78..4b73eb6412 100644
--- a/Python/future.c
+++ b/Python/future.c
@@ -41,7 +41,7 @@ future_check_features(PyFutureFeatures *ff, stmt_ty s, PyObject *filename)
         } else if (strcmp(feature, FUTURE_GENERATOR_STOP) == 0) {
             continue;
         } else if (strcmp(feature, FUTURE_ANNOTATIONS) == 0) {
-            ff->ff_features |= CO_FUTURE_ANNOTATIONS;
+            continue;
         } else if (strcmp(feature, "braces") == 0) {
             PyErr_SetString(PyExc_SyntaxError,
                             "not a chance");