Handle empty patterns specially

implementations/python/preserves/schema.py

@@ -136,7 +136,10 @@ class SchemaEntity:
     def __repr__(self):
         n = self._constructor_name()
         if self.SIMPLE:
-            return n + '(' + repr(self.value) + ')'
+            if self.EMPTY:
+                return n + '()'
+            else:
+                return n + '(' + repr(self.value) + ')'
         else:
             return n + ' ' + repr(self._as_dict())
 
@@ -180,6 +183,7 @@ def safesetattr(o, k, v):
         setattr(o, k, v)
 
 class Definition(SchemaEntity):
+    EMPTY = False
     SIMPLE = False
     FIELD_NAMES = []
     ENUMERATION = None
@@ -193,9 +197,13 @@ class Definition(SchemaEntity):
     def __init__(self, *args):
         self._fields = args
         if self.SIMPLE:
-            if len(args) != 1:
-                raise Exception('%s needs exactly one argument' % (self._constructor_name(),))
-            self.value = args[0]
+            if self.EMPTY:
+                if len(args) != 0:
+                    raise Exception('%s takes no arguments' % (self._constructor_name(),))
+            else:
+                if len(args) != 1:
+                    raise Exception('%s needs exactly one argument' % (self._constructor_name(),))
+                self.value = args[0]
         else:
             if len(args) != len(self.FIELD_NAMES):
                 raise Exception('%s needs argument(s) %r' % (self._constructor_name(), self.FIELD_NAMES))
@@ -225,6 +233,7 @@ class Definition(SchemaEntity):
         cls.SCHEMA = schema
         cls.MODULE_PATH = module_path
         cls.NAME = name
+        cls.EMPTY = is_empty_pattern(schema)
         cls.SIMPLE = is_simple_pattern(schema)
         cls.FIELD_NAMES = []
         cls.VARIANT = variant
@@ -235,7 +244,11 @@ class Definition(SchemaEntity):
     def try_decode(cls, v):
         if cls.SIMPLE:
             i = cls.parse(cls.SCHEMA, v, [])
-            if i is not None: return cls(i)
+            if i is not None:
+                if cls.EMPTY:
+                    return cls()
+                else:
+                    return cls(i)
         else:
             args = []
             if cls.parse(cls.SCHEMA, v, args) is not None: return cls(*args)
@@ -260,6 +273,9 @@ SIMPLE_PATTERN_KEYS = [ATOM, EMBEDDED, LIT, SEQOF, SETOF, DICTOF, REF]
 def is_simple_pattern(p):
     return p == ANY or (isinstance(p, Record) and p.key in SIMPLE_PATTERN_KEYS)
 
+def is_empty_pattern(p):
+    return isinstance(p, Record) and p.key == LIT
+
 def gather_defined_field_names(s, acc):
     if is_simple_pattern(s):
         pass