Add JSON decoder example (see erikrose#18)

example_json.py

@@ -0,0 +1,112 @@
+r"""# JSON grammar
+
+value = space (string / number / object / array / true_false_null) space
+
+object = "{" members "}"
+members = (pair ("," pair)*)?
+pair = string ":" value
+array = "[" elements "]"
+elements = (value ("," value)*)?
+true_false_null = "true" / "false" / "null"
+
+string = space "\"" char* "\"" space
+char = ~"[^\"]"  # TODO implement the real thing
+number = (int frac exp) / (int exp) / (int frac) / int
+int = "-"? ((digit1to9 digits) / digit)
+frac = "." digits
+exp = e digits
+digits = digit+
+e = "e+" / "e-" / "e" / "E+" / "E-" / "E"
+
+digit1to9 = ~"[1-9]"
+digit = ~"[0-9]"
+space = ~"\s*"
+
+"""
+from parsimonious.grammar import Grammar
+
+
+class JSONDecoder(object):
+
+    def loads(self, node):
+        if isinstance(node, str):
+            node = Grammar(__doc__).parse(node)
+        method = getattr(self, node.expr_name, self.default)
+        return method(node, [self.loads(n) for n in node])
+
+    def default(self, node, children):
+        return children
+
+    def value(self, node, children):
+        return children[1][0]
+
+    def object(self, node, children):
+        return children[1]
+
+    def members(self, node, children):
+        if not children:
+            return {}
+        head, tail = children[0][:1], children[0][1:][0]
+        return dict(head + [el[1] for el in tail])
+
+    def pair(self, node, children):
+        return children[0], children[2]
+
+    def array(self, node, children):
+        return children[1]
+
+    def elements(self, node, children):
+        if not children:
+            return []
+        head, tail = children[0][:1], children[0][1:][0]
+        return head + [el[1] for el in tail]
+
+    def true_false_null(self, node, children):
+        return {'true': True, 'false': False, 'null': None}[node.text.strip()]
+
+    def number(self, node, children):
+        return float(node.text.strip())
+
+    def string(self, node, children):
+        return node.text.strip().strip('"')  # TODO implement the real thing
+
+
+json = JSONDecoder()
+
+
+def test_numbers():
+    assert json.loads('5') == 5
+    assert json.loads('55') == 55
+    assert json.loads('-5') == -5
+    assert json.loads('5.5') == 5.5
+    assert json.loads('5e-2') == 5e-2
+    assert json.loads('5.3e-2') == 5.3e-2
+
+
+def test_strings():
+    assert json.loads('""') == ''
+    assert json.loads('"hai"') == 'hai'
+
+
+def test_true_false_null():
+    assert json.loads('true') == True
+    assert json.loads('false') == False
+    assert json.loads('null') == None
+
+
+def test_arrays():
+    assert json.loads('[]') == []
+    assert json.loads('[1]') == [1]
+    assert json.loads('["a",1,2,3]') == ['a', 1, 2, 3]
+    assert json.loads('["a",[1,2],3]') == ['a', [1, 2], 3]
+
+
+def test_objects():
+    assert json.loads('{}') == {}
+    assert json.loads('{"a":1}') == {'a': 1}
+    assert json.loads('{"a":1,"b":2}') == {'a': 1, 'b': 2}
+
+
+def test_whitespace():
+    source = ' { "a" : 1 , "b" : [ 2 , 3 ] } '
+    assert json.loads(source) == {'a': 1, 'b': [2, 3]}