[feat] Parse convertible mapping objects with json module

The advantage is that we can automatically convert nested mappings, like for
example the `extra_resources` attribute, which can thus be set from the command
line.

reframe/utility/typecheck.py

@@ -95,6 +95,7 @@
 
 import abc
 import datetime
+import json
 import re
 
 
@@ -322,19 +323,25 @@ def __rfm_cast_str__(cls, s):
         mappping_type = cls._type
         key_type = cls._key_type
         value_type = cls._value_type
-        seq = []
-        for key_datum in s.split(','):
-            try:
-                k, v = key_datum.split(':')
-            except ValueError:
-                # Re-raise as TypeError
-                raise TypeError(
-                    f'cannot convert string {s!r} to {cls.__name__!r}'
-                ) from None
-
-            seq.append((key_type(k), value_type(v)))
-
-        return mappping_type(seq)
+
+        try:
+            d = json.loads(s)
+        except json.JSONDecodeError:
+            seq = []
+            for key_datum in s.split(','):
+                try:
+                    k, v = key_datum.split(':')
+                except ValueError:
+                    # Re-raise as TypeError
+                    raise TypeError(
+                        f'cannot convert string {s!r} to {cls.__name__!r}'
+                    ) from None
+
+                seq.append((key_type(k), value_type(v)))
+
+            return mappping_type(seq)
+        else:
+            return mappping_type(d)
 
 
 class _StrType(_SequenceType):

unittests/test_typecheck.py

@@ -7,7 +7,6 @@
 
 import reframe.utility.typecheck as typ
 
-
 def assert_type_hierarchy(builtin_type, ctype):
     assert isinstance(ctype, type)
     assert issubclass(builtin_type, ctype)
@@ -208,6 +207,14 @@ def test_mapping_type():
     # Test conversions
     assert typ.Dict[str, int]('a:1,b:2') == {'a': 1, 'b': 2}
 
+    # Conversion with JSON syntax, for nested dictionaries
+    s = '{"gpu":{"num_gpus_per_node":8}, "mpi": {"num_slots": 64}}'
+    assert (typ.Dict[str, typ.Dict[str, object]](s) ==
+            {
+                "gpu": {"num_gpus_per_node": 8},
+                "mpi": {"num_slots": 64},
+            })
+
     with pytest.raises(TypeError):
         typ.Dict[str, int]('a:1,b')