Adds a way to register custom formatters

Author: sdispater

pendulum/formatting/__init__.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 
+from .formatter import Formatter
 from .classic_formatter import ClassicFormatter
 from .alternative_formatter import AlternativeFormatter
 
@@ -8,3 +9,27 @@
     'classic': ClassicFormatter(),
     'alternative': AlternativeFormatter(),
 }
+
+
+def register_formatter(name, formatter):
+    """
+    Register a new formatter.
+
+    :param name: The name of the formatter.
+    :type name: str
+
+    :param formatter: The formatter instance
+    :type formatter: Formatter
+
+    :rtype: None
+    """
+    if name in FORMATTERS:
+        raise ValueError('Formatter [{}] already exists'.format(name))
+
+    if not isinstance(formatter, Formatter):
+        raise ValueError(
+            'The formatter instance '
+            'must be an instance of Formatter'
+        )
+
+    FORMATTERS[name] = formatter

tests/formatting_tests/test_module.py

@@ -0,0 +1,34 @@
+# -*- coding: utf-8 -*-
+
+import pendulum
+from .. import AbstractTestCase
+
+from pendulum.formatting import register_formatter, Formatter
+
+
+class ModuleTestCase(AbstractTestCase):
+
+    def test_register_formatter(self):
+        register_formatter('test', TestFormatter())
+
+        self.assertEqual('foo', pendulum.now().format('', formatter='test'))
+
+    def test_register_formatter_existing_name(self):
+        self.assertRaises(ValueError, register_formatter, 'classic', TestFormatter())
+
+    def test_register_formatter_not_formatter_instance(self):
+        self.assertRaises(ValueError, register_formatter, 'test', FooFormatter())
+
+
+class TestFormatter(Formatter):
+
+    def format(self, dt, fmt, locale=None):
+        return 'foo'
+
+
+class FooFormatter(object):
+
+    def format(self, dt, fmt, locale=None):
+        return 'foo'
+
+