non-hideous dictionary displays

The trailing comma-space that was previously output, while perfectly
legal in every way, is not what most human Python programmers would
write in most situations. We use the same slick idiom as the
`comma_list` method (iterating over the enumeration, but writing the
comma-space before the elements if the index is truthy, which zero is
not) to not put anything extra after the final key-value pair.

astor/codegen.py

@@ -407,8 +407,10 @@ class SourceGenerator(ExplicitNodeVisitor):
 
     @enclose('{}')
     def visit_Dict(self, node):
-        for key, value in zip(node.keys, node.values):
-            self.write(key, ': ', value, ', ')
+        for idx, (key, value) in enumerate(zip(node.keys, node.values)):
+            if idx:
+                self.write(', ')
+            self.write(key, ': ', value)
 
     @enclose('()')
     def visit_BinOp(self, node):

tests/test_codegen.py

@@ -31,6 +31,12 @@ class CodegenTestCase(unittest.TestCase):
         source = "from math import floor"
         self.assertAstSourceEqual(source)
 
+    def test_dictionary_literals(self):
+        source = "{'a': 1, 'b': 2}"
+        self.assertAstSourceEqual(source)
+        another_source = "{'nested': ['structures', {'are': 'important'}]}"
+        self.assertAstSourceEqual(another_source)
+
     def test_try_expect(self):
         source = textwrap.dedent("""\
         try:
@@ -80,10 +86,8 @@ class CodegenTestCase(unittest.TestCase):
                 self.assertRaises(SyntaxError, ast.parse, source)
 
     def test_multiple_unpackings(self):
-        # XXX TODO: no human Pythonista would write the contents of a
-        # dictionary literal with a trailing comma-space like that
         source = textwrap.dedent("""\
-        my_function(*[1], *[2], **{'three': 3, }, **{'four': 'four', })""")
+        my_function(*[1], *[2], **{'three': 3}, **{'four': 'four'})""")
         if sys.version_info >= (3, 5):
             self.assertAstSourceEqual(source)
         else: