bpo-29863: Add json.COMPACT constant

Doc/library/json.rst

@@ -43,7 +43,7 @@ Encoding basic Python object hierarchies::
 Compact encoding::
 
     >>> import json
-    >>> json.dumps([1,2,3,{'4': 5, '6': 7}], separators=(',', ':'))
+    >>> json.dumps([1,2,3,{'4': 5, '6': 7}], separators=json.COMPACT)
     '[1,2,3,{"4":5,"6":7}]'
 
 Pretty printing::
@@ -170,7 +170,7 @@ Basic Usage
    If specified, *separators* should be an ``(item_separator, key_separator)``
    tuple.  The default is ``(', ', ': ')`` if *indent* is ``None`` and
    ``(',', ': ')`` otherwise.  To get the most compact JSON representation,
-   you should specify ``(',', ':')`` to eliminate whitespace.
+   you should specify :attr:`json.COMPACT` to eliminate whitespace.
 
    .. versionchanged:: 3.4
       Use ``(',', ': ')`` as default if *indent* is not ``None``.
@@ -283,6 +283,17 @@ Basic Usage
       input encoding should be UTF-8, UTF-16 or UTF-32.
 
 
+Constants
+^^^^^^^^^
+
+.. data:: COMPACT
+
+   A constant that can be used as the *separators* argument
+   to emit a compact serialization.
+
+   .. versionadded:: 3.7
+
+
 Encoders and Decoders
 ---------------------
 
@@ -448,7 +459,7 @@ Encoders and Decoders
    If specified, *separators* should be an ``(item_separator, key_separator)``
    tuple.  The default is ``(', ', ': ')`` if *indent* is ``None`` and
    ``(',', ': ')`` otherwise.  To get the most compact JSON representation,
-   you should specify ``(',', ':')`` to eliminate whitespace.
+   you should specify :attr:`json.COMPACT` to eliminate whitespace.
 
    .. versionchanged:: 3.4
       Use ``(',', ': ')`` as default if *indent* is not ``None``.

Lib/json/__init__.py

@@ -30,7 +30,7 @@
     >>> import json
     >>> from collections import OrderedDict
     >>> mydict = OrderedDict([('4', 5), ('6', 7)])
-    >>> json.dumps([1,2,3,mydict], separators=(',', ':'))
+    >>> json.dumps([1,2,3,mydict], separators=json.COMPACT)
     '[1,2,3,{"4":5,"6":7}]'
 
 Pretty printing::
@@ -99,6 +99,7 @@
 __all__ = [
     'dump', 'dumps', 'load', 'loads',
     'JSONDecoder', 'JSONDecodeError', 'JSONEncoder',
+    'COMPACT',
 ]
 
 __author__ = 'Bob Ippolito <bob@redivi.com>'
@@ -107,6 +108,8 @@
 from .encoder import JSONEncoder
 import codecs
 
+COMPACT = (',', ':')
+
 _default_encoder = JSONEncoder(
     skipkeys=False,
     ensure_ascii=True,

Lib/test/test_json/test_dump.py

@@ -47,6 +47,15 @@ def __lt__(self, o):
         d[1337] = "true.dat"
         self.assertEqual(self.dumps(d, sort_keys=True), '{"1337": "true.dat"}')
 
+    def test_compact_dump(self):
+        sio = StringIO()
+        self.json.dump({'name': 'some name', 'value': 'some value'}, sio, separators=self.json.COMPACT)
+        self.assertEqual(sio.getvalue(), '{"name":"some name","value":"some value"}')
+
+    def test_compact_encode(self):
+        encoder = self.json.JSONEncoder(separators=self.json.COMPACT)
+        encoded = encoder.encode({'name': 'some name', 'value': 'some value'})
+        self.assertEqual(encoded, '{"name":"some name","value":"some value"}')
 
 class TestPyDump(TestDump, PyTest): pass