Fix uiri#217: dumps unable to detect circular refs

If an object is passed to dumps, and that object has a circular
reference, then dumps will raise a ValueError because an object with
circular references cannot be encoded as valid TOML.

tests/test_api.py

@@ -51,6 +51,19 @@ def test_valid_tests():
         toml.dumps(toml.load(open(os.path.join(valid_dir, f))))
 
 
+def test_circular_ref():
+    a = {}
+    b = {}
+    b['c'] = 4
+    b['self'] = b
+    a['b'] = b
+    with pytest.raises(ValueError):
+        toml.dumps(a)
+
+    with pytest.raises(ValueError):
+        toml.dumps(b)
+
+
 def test__dict():
     class TestDict(dict):
         pass

toml/encoder.py

@@ -46,7 +46,13 @@ def dumps(o, encoder=None):
         encoder = TomlEncoder(o.__class__)
     addtoretval, sections = encoder.dump_sections(o, "")
     retval += addtoretval
+    outer_objs = [id(o)]
     while sections:
+        section_ids = [id(section) for section in sections]
+        for outer_obj in outer_objs:
+            if outer_obj in section_ids:
+                raise ValueError("Circular reference detected")
+        outer_objs += section_ids
         newsections = encoder.get_empty_table()
         for section in sections:
             addtoretval, addtosections = encoder.dump_sections(