Fix #759: autodoc: Add sphinx.ext.autodoc.preserve_defaults extension

Add a new extension `sphinx.ext.autodoc.preserve_defaults`.

It preserves the default argument values of function signatures in source code
and keep them not evaluated for readability.  This is an experimental
extension and it will be integrated into autodoc core in Sphinx-4.0.

2 files changed, 89 insertions, 0 deletions

diff --git a/sphinx/ext/autodoc/__init__.py b/sphinx/ext/autodoc/__init__.py
index 8d6781a1a..0b5709301 100644
--- a/sphinx/ext/autodoc/__init__.py
+++ b/sphinx/ext/autodoc/__init__.py
@@ -2634,6 +2634,7 @@ def setup(app: Sphinx) -> Dict[str, Any]:
 
     app.connect('config-inited', migrate_autodoc_member_order, priority=800)
 
+    app.setup_extension('sphinx.ext.autodoc.preserve_defaults')
     app.setup_extension('sphinx.ext.autodoc.type_comment')
     app.setup_extension('sphinx.ext.autodoc.typehints')
 
diff --git a/sphinx/ext/autodoc/preserve_defaults.py b/sphinx/ext/autodoc/preserve_defaults.py
new file mode 100644
index 000000000..3d859fe8e
--- /dev/null
+++ b/sphinx/ext/autodoc/preserve_defaults.py
@@ -0,0 +1,88 @@
+"""
+    sphinx.ext.autodoc.preserve_defaults
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+    Preserve the default argument values of function signatures in source code
+    and keep them not evaluated for readability.
+
+    :copyright: Copyright 2007-2021 by the Sphinx team, see AUTHORS.
+    :license: BSD, see LICENSE for details.
+"""
+
+import ast
+import inspect
+from typing import Any, Dict
+
+from sphinx.application import Sphinx
+from sphinx.locale import __
+from sphinx.pycode.ast import parse as ast_parse
+from sphinx.pycode.ast import unparse as ast_unparse
+from sphinx.util import logging
+
+logger = logging.getLogger(__name__)
+
+
+class DefaultValue:
+    def __init__(self, name: str) -> None:
+        self.name = name
+
+    def __repr__(self) -> str:
+        return self.name
+
+
+def get_function_def(obj: Any) -> ast.FunctionDef:
+    """Get FunctionDef object from living object.
+    This tries to parse original code for living object and returns
+    AST node for given *obj*.
+    """
+    try:
+        source = inspect.getsource(obj)
+        if source.startswith((' ', r'\t')):
+            # subject is placed inside class or block.  To read its docstring,
+            # this adds if-block before the declaration.
+            module = ast_parse('if True:\n' + source)
+            return module.body[0].body[0]  # type: ignore
+        else:
+            module = ast_parse(source)
+            return module.body[0]  # type: ignore
+    except (OSError, TypeError):  # failed to load source code
+        return None
+
+
+def update_defvalue(app: Sphinx, obj: Any, bound_method: bool) -> None:
+    """Update defvalue info of *obj* using type_comments."""
+    if not app.config.autodoc_preserve_defaults:
+        return
+
+    try:
+        function = get_function_def(obj)
+        if function.args.defaults or function.args.kw_defaults:
+            sig = inspect.signature(obj)
+            defaults = list(function.args.defaults)
+            kw_defaults = list(function.args.kw_defaults)
+            parameters = list(sig.parameters.values())
+            for i, param in enumerate(parameters):
+                if param.default is not param.empty:
+                    if param.kind in (param.POSITIONAL_ONLY, param.POSITIONAL_OR_KEYWORD):
+                        value = DefaultValue(ast_unparse(defaults.pop(0)))  # type: ignore
+                        parameters[i] = param.replace(default=value)
+                    else:
+                        value = DefaultValue(ast_unparse(kw_defaults.pop(0)))  # type: ignore
+                        parameters[i] = param.replace(default=value)
+            sig = sig.replace(parameters=parameters)
+            obj.__signature__ = sig
+    except (AttributeError, TypeError):
+        # failed to update signature (ex. built-in or extension types)
+        pass
+    except NotImplementedError as exc:  # failed to ast.unparse()
+        logger.warning(__("Failed to parse a default argument value for %r: %s"), obj, exc)
+
+
+def setup(app: Sphinx) -> Dict[str, Any]:
+    app.add_config_value('autodoc_preserve_defaults', False, True)
+    app.connect('autodoc-before-process-signature', update_defvalue)
+
+    return {
+        'version': '1.0',
+        'parallel_read_safe': True
+    }