Disallow []= on ImmutableDict

Fixes #88.

edn_format/immutable_dict.py

@@ -15,11 +15,6 @@ def __init__(self, somedict):
     def __getitem__(self, key):
         return self.dict[key]
 
-    def __setitem__(self, key, value):
-        modifiable = dict(self.dict)
-        modifiable[key] = value
-        return ImmutableDict(modifiable)
-
     def __repr__(self):
         return self.dict.__repr__()
 

tests.py

@@ -2,20 +2,19 @@
 # TODO: Tests pass on Python 3.6, Disabled to not break tests on 2.7 :-(
 # from __future__ import absolute_import, division, print_function, unicode_literals
 
-from collections import OrderedDict
-from uuid import uuid4, UUID
-import random
 import datetime
 import fractions
+import random
 import unittest
+from collections import OrderedDict
+from uuid import uuid4, UUID
 
 import pytz
 
 from edn_format import edn_lex, edn_parse, \
     loads, dumps, Keyword, Symbol, ImmutableDict, ImmutableList, Char, \
     TaggedElement, add_tag, remove_tag, tag, \
     EDNDecodeError
-
 from edn_format.compat import _PY3, unicode
 
 
@@ -632,6 +631,18 @@ def test_equality(self):
         self.assertTrue(Symbol("db/id") == Symbol("db/id"))
 
 
+class ImmutableDictTest(unittest.TestCase):
+    def test_mutation(self):
+        x = ImmutableDict({})
+
+        def mutate():
+            nonlocal x
+            # noinspection PyUnresolvedReferences
+            x["foo"] = "bar"
+
+        self.assertRaises(TypeError, mutate)
+
+
 class ImmutableListTest(unittest.TestCase):
     def test_list(self):
         x = ImmutableList([1, 2, 3])