Write out node values immutably

This updates the node value serializer to write out immutable
collections. This formatter is used for protocol tests and trait
values. In the former case it doesn't matter either way, and in the
latter we explicitly want immutable values.

Author: JordonPhillips

codegen/core/src/main/java/software/amazon/smithy/python/codegen/writer/PythonWriter.java

@@ -67,7 +67,7 @@ public PythonWriter(PythonSettings settings, String fullPackageName, boolean add
         trimBlankLines();
         trimTrailingSpaces();
         putFormatter('T', new PythonSymbolFormatter());
-        putFormatter('N', new PythonNodeFormatter());
+        putFormatter('N', new PythonNodeFormatter(this));
         this.addCodegenWarningHeader = addCodegenWarningHeader;
     }
 
@@ -325,6 +325,12 @@ private Boolean isOperationSymbol(Symbol typeSymbol) {
     }
 
     private final class PythonNodeFormatter implements BiFunction<Object, String, String> {
+        private final PythonWriter writer;
+
+        PythonNodeFormatter(PythonWriter writer) {
+            this.writer = writer;
+        }
+
         @Override
         public String apply(Object node, String indent) {
             if (node instanceof Optional<?>) {
@@ -334,16 +340,18 @@ public String apply(Object node, String indent) {
                 throw new CodegenException(
                         "Invalid type provided to $D. Expected a Node, but found `" + node + "`");
             }
-            return ((Node) node).accept(new PythonNodeFormatVisitor(indent));
+            return ((Node) node).accept(new PythonNodeFormatVisitor(indent, writer));
         }
     }
 
     private final class PythonNodeFormatVisitor implements NodeVisitor<String> {
 
         private String indent;
+        private final PythonWriter writer;
 
-        PythonNodeFormatVisitor(String indent) {
+        PythonNodeFormatVisitor(String indent, PythonWriter writer) {
             this.indent = indent;
+            this.writer = writer;
         }
 
         @Override
@@ -379,10 +387,10 @@ public String stringNode(StringNode node) {
         @Override
         public String arrayNode(ArrayNode node) {
             if (node.getElements().isEmpty()) {
-                return "[]";
+                return "()";
             }
 
-            StringBuilder builder = new StringBuilder("[\n");
+            StringBuilder builder = new StringBuilder("(\n");
             var oldIndent = indent;
             indent += getIndentText();
             for (Node element : node.getElements()) {
@@ -392,17 +400,18 @@ public String arrayNode(ArrayNode node) {
             }
             indent = oldIndent;
             builder.append(indent);
-            builder.append(']');
+            builder.append(')');
             return builder.toString();
         }
 
         @Override
         public String objectNode(ObjectNode node) {
+            writer.addStdlibImport("types", "MappingProxyType");
             if (node.getMembers().isEmpty()) {
-                return "{}";
+                return "MappingProxyType({})";
             }
 
-            StringBuilder builder = new StringBuilder("{\n");
+            StringBuilder builder = new StringBuilder("MappingProxyType({\n");
             var oldIndent = indent;
             indent += getIndentText();
             for (Map.Entry<StringNode, Node> member : node.getMembers().entrySet()) {
@@ -414,7 +423,7 @@ public String objectNode(ObjectNode node) {
             }
             indent = oldIndent;
             builder.append(indent);
-            builder.append('}');
+            builder.append("})");
             return builder.toString();
         }
     }