Support for MappingProxyType as a type and as default for mapping types

CHANGELOG.rst

@@ -12,6 +12,15 @@ The semantic versioning only considers the public API as described in
 paths are considered internals and can change in minor and patch releases.
 
 
+v4.32.0 (2024-07-??)
+--------------------
+
+Added
+^^^^^
+- Support for ``MappingProxyType`` as a type and as default for mapping types
+  (`#540 <https://github.com/omni-us/jsonargparse/pull/540>`__).
+
+
 v4.31.0 (2024-06-27)
 --------------------
 

DOCUMENTATION.rst

@@ -400,8 +400,9 @@ Some notes about this support are:
   :ref:`restricted-strings` and paths and URLs as explained in sections
   :ref:`parsing-paths` and :ref:`parsing-urls`.
 
-- ``Dict``, ``Mapping``, ``MutableMapping``, and ``TypedDict`` are supported but
-  only with ``str`` or ``int`` keys. For more details see :ref:`dict-items`.
+- ``Dict``, ``Mapping``, ``MutableMapping``, ``MappingProxyType``, and
+  ``TypedDict`` are supported but only with ``str`` or ``int`` keys. For more
+  details see :ref:`dict-items`.
 
 - ``Tuple``, ``Set`` and ``MutableSet`` are supported even though they can't be
   represented in json distinguishable from a list. Each ``Tuple`` element

jsonargparse/_typehints.py

@@ -10,7 +10,7 @@
 from copy import deepcopy
 from enum import Enum
 from functools import partial
-from types import FunctionType
+from types import FunctionType, MappingProxyType
 from typing import (
     Any,
     Callable,
@@ -145,7 +145,7 @@
     abc.Sequence,
     abc.MutableSequence,
 }
-mapping_origin_types = {Dict, dict, Mapping, MutableMapping, abc.Mapping, abc.MutableMapping}
+mapping_origin_types = {Dict, dict, Mapping, MappingProxyType, MutableMapping, abc.Mapping, abc.MutableMapping}
 callable_origin_types = {Callable, abc.Callable}
 
 literal_types = {Literal}
@@ -872,6 +872,8 @@ def adapt_typehints(
                 val = {**prev_val, val.key: val.val}
             else:
                 val = {val.key: val.val}
+        elif isinstance(val, MappingProxyType):
+            val = dict(val)
         elif not isinstance(val, dict):
             raise_unexpected_value(f"Expected a {typehint_origin}", val)
         if subtypehints is not None:
@@ -902,6 +904,8 @@ def adapt_typehints(
                 raise_unexpected_value(f"Unexpected keys: {extra_keys}", val)
             for k, v in val.items():
                 val[k] = adapt_typehints(v, typehint.__annotations__[k], **adapt_kwargs)
+        if typehint_origin == MappingProxyType and not serialize:
+            val = MappingProxyType(val)
 
     # Callable
     elif typehint_origin in callable_origin_types or typehint in callable_origin_types:

jsonargparse_tests/test_typehints.py

@@ -8,6 +8,7 @@
 from calendar import Calendar, TextCalendar
 from enum import Enum
 from pathlib import Path
+from types import MappingProxyType
 from typing import (
     Any,
     Callable,
@@ -545,6 +546,23 @@ def test_typeddict_with_args_ntotal(parser):
     ctx.match("Expected a <class 'dict'>")
 
 
+def test_mapping_proxy_type(parser):
+    parser.add_argument("--mapping", type=MappingProxyType)
+    cfg = parser.parse_args(['--mapping={"x":1}'])
+    assert isinstance(cfg.mapping, MappingProxyType)
+    assert cfg.mapping == {"x": 1}
+    assert parser.dump(cfg, format="json") == '{"mapping":{"x":1}}'
+
+
+def test_mapping_default_mapping_proxy_type(parser):
+    mapping_proxy = MappingProxyType({"x": 1})
+    parser.add_argument("--mapping", type=Mapping[str, int], default=mapping_proxy)
+    cfg = parser.parse_args([])
+    assert isinstance(cfg.mapping, Mapping)
+    assert mapping_proxy == cfg.mapping
+    assert parser.dump(cfg, format="json") == '{"mapping":{"x":1}}'
+
+
 # union tests