String annotations [PEP 563] (#4390)

* Document `from __future__ import annotations`
* Provide plumbing and tests for `from __future__ import annotations`
* Implement unparsing the AST back to string form

This is required for PEP 563 and as such only implements a part of the
unparsing process that covers expressions.

1 files changed, 13 insertions, 7 deletions

diff --git a/Lib/__future__.py b/Lib/__future__.py
index 63b2be3524..ce8bed7a64 100644
--- a/Lib/__future__.py
+++ b/Lib/__future__.py
@@ -57,13 +57,14 @@ all_feature_names = [
     "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_PRINT_FUNCTION  = 0x10000   # print function
 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 @@ unicode_literals = _Feature((2, 6, 0, "alpha", 2),
                             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)