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).
diff --git a/Lib/test/test_dis.py b/Lib/test/test_dis.py
index 4533a01..bbaddd5 100644
--- a/Lib/test/test_dis.py
+++ b/Lib/test/test_dis.py
@@ -227,28 +227,26 @@
2 0 SETUP_ANNOTATIONS
2 LOAD_CONST 0 (1)
4 STORE_NAME 0 (x)
- 6 LOAD_NAME 1 (int)
- 8 LOAD_NAME 2 (__annotations__)
- 10 LOAD_CONST 1 ('x')
+ 6 LOAD_CONST 1 ('int')
+ 8 LOAD_NAME 1 (__annotations__)
+ 10 LOAD_CONST 2 ('x')
12 STORE_SUBSCR
- 3 14 LOAD_NAME 3 (fun)
- 16 LOAD_CONST 0 (1)
- 18 CALL_FUNCTION 1
- 20 LOAD_NAME 2 (__annotations__)
- 22 LOAD_CONST 2 ('y')
- 24 STORE_SUBSCR
+ 3 14 LOAD_CONST 3 ('fun(1)')
+ 16 LOAD_NAME 1 (__annotations__)
+ 18 LOAD_CONST 4 ('y')
+ 20 STORE_SUBSCR
- 4 26 LOAD_CONST 0 (1)
- 28 LOAD_NAME 4 (lst)
- 30 LOAD_NAME 3 (fun)
- 32 LOAD_CONST 3 (0)
- 34 CALL_FUNCTION 1
- 36 STORE_SUBSCR
- 38 LOAD_NAME 1 (int)
- 40 POP_TOP
- 42 LOAD_CONST 4 (None)
- 44 RETURN_VALUE
+ 4 22 LOAD_CONST 0 (1)
+ 24 LOAD_NAME 2 (lst)
+ 26 LOAD_NAME 3 (fun)
+ 28 LOAD_CONST 5 (0)
+ 30 CALL_FUNCTION 1
+ 32 STORE_SUBSCR
+ 34 LOAD_NAME 4 (int)
+ 36 POP_TOP
+ 38 LOAD_CONST 6 (None)
+ 40 RETURN_VALUE
"""
compound_stmt_str = """\