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).

1 files changed, 3 insertions, 7 deletions

diff --git a/Doc/reference/compound_stmts.rst b/Doc/reference/compound_stmts.rst
index 158d6a8..04a3948 100644
--- a/Doc/reference/compound_stmts.rst
+++ b/Doc/reference/compound_stmts.rst
@@ -610,13 +610,9 @@ following the parameter name.  Any parameter may have an annotation, even those
 ``*identifier`` or ``**identifier``.  Functions may have "return" annotation of
 the form "``-> expression``" after the parameter list.  These annotations can be
 any valid Python expression.  The presence of annotations does not change the
-semantics of a function.  The annotation values are available as values of
-a dictionary keyed by the parameters' names in the :attr:`__annotations__`
-attribute of the function object.  If the ``annotations`` import from
-:mod:`__future__` is used, annotations are preserved as strings at runtime which
-enables postponed evaluation.  Otherwise, they are evaluated when the function
-definition is executed.  In this case annotations may be evaluated in
-a different order than they appear in the source code.
+semantics of a function.  The annotation values are available as string values
+in a dictionary keyed by the parameters' names in the :attr:`__annotations__`
+attribute of the function object.
 
 .. index:: pair: lambda; expression