Add capnp-json serializer script. Also fix bugs in from_dict

capnp/capnp.pyx

@@ -568,7 +568,15 @@ import collections as _collections
 cdef _from_dict_helper(msg, field, d):
     d_type = type(d)
     if d_type is dict:
-        sub_msg = getattr(msg, field)
+        try:
+            sub_msg = getattr(msg, field)
+        except Exception as e:
+            str_error = str(e)
+            if 'expected isSetInUnion(field)' in str_error:
+                msg.init(field)
+                sub_msg = getattr(msg, field)
+            else:
+                raise
         for key, val in d.iteritems():
             if key != 'which':
                 _from_dict_helper(sub_msg, key, val)
@@ -584,6 +592,7 @@ cdef _from_dict_helper(msg, field, d):
     else:
         setattr(msg, field, d)
 
+
 cdef _from_dict(msg, d):
     for key, val in d.iteritems():
         if key != 'which':

scripts/capnp-json.py

@@ -0,0 +1,43 @@
+#!/usr/bin/env python
+
+import argparse
+import sys
+import json
+import capnp
+
+def parse_args():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("command")
+    parser.add_argument("schema_file")
+    parser.add_argument("struct_name")
+
+    return parser.parse_args()
+
+def encode(schema_file, struct_name):
+    schema = capnp.load(schema_file)
+
+    struct_schema = getattr(schema, struct_name)
+    
+    struct_dict = json.load(sys.stdin)
+    struct = struct_schema.from_dict(struct_dict)
+
+    struct.write(sys.stdout)
+
+def decode(schema_file, struct_name):
+    schema = capnp.load(schema_file)
+
+    struct_schema = getattr(schema, struct_name)
+    struct = struct_schema.read(sys.stdin)
+    
+    json.dump(struct.to_dict(), sys.stdout)
+
+def main():
+    args = parse_args()
+
+    command = args.command
+    kwargs = vars(args)
+    del kwargs['command']
+
+    globals()[command](**kwargs) # hacky way to get defined functions, and call function with name=command
+
+main()