Follow upon cloudwatch formatters

README.rst

@@ -149,6 +149,8 @@ configuration provided by ``boto3``:
     disable_existing_loggers: False
     formatters:
       json:
+        (): watchtower.CloudWatchJSONFormatter
+        fields: [msg, levelname]
         format: "[%(asctime)s] %(process)d %(levelname)s %(name)s:%(funcName)s:%(lineno)s - %(message)s"
       plaintext:
         format: "[%(asctime)s] %(process)d %(levelname)s %(name)s:%(funcName)s:%(lineno)s - %(message)s"

watchtower/__init__.py

@@ -49,6 +49,59 @@ class WatchtowerWarning(UserWarning):
     pass
 
 
+class CloudWatchFormatter(logging.Formatter):
+    """
+    Log formatter for CloudWatch message. Transforms the logged record message into a compatible message for CloudWatch.
+    This is the default formatter for CloudWatchLogHandler
+
+    :param json_serialize_default:
+        The 'default' function to use when serializing dictionaries as JSON. Refer to the Python standard library
+        documentation on 'json' for more explanation about the 'default' parameter.
+        https://docs.python.org/3/library/json.html#json.dump
+        https://docs.python.org/2/library/json.html#json.dump
+    :type json_serialize_default: Function
+    """
+
+    def __init__(self, *args, json_serialize_default=None, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.json_serialize_default = json_serialize_default or _json_serialize_default
+
+    def format(self, message):
+        if isinstance(message.msg, Mapping):
+            message.msg = json.dumps(message.msg, default=self.json_serialize_default)
+
+        return super().format(message)
+
+
+class CloudWatchJSONFormatter(CloudWatchFormatter):
+    """
+    JSON log formatter for CloudWatch. Transforms the logged record message into a JSON formatted message.
+
+    :param json_serialize_default:
+        The 'default' function to use when serializing dictionaries as JSON. Refer to the Python standard library
+        documentation on 'json' for more explanation about the 'default' parameter.
+        https://docs.python.org/3/library/json.html#json.dump
+        https://docs.python.org/2/library/json.html#json.dump
+    :type json_serialize_default: Function
+    :param fields: A list of fields of the record to include in the CloudWatch Log json object. Defaults to '__all__'.
+    :type fields: list
+    """
+    def __init__(self, *args, fields='__all__', **kwargs):
+        super().__init__(*args, **kwargs)
+        self.fields = fields
+
+    def format(self, message):
+        if self.fields == '__all__':
+            message.msg = dict(message.__dict__)
+        else:
+            message.msg = {k: v for k, v in message.items() if k in self.fields}
+
+        return super().format(message)
+
+
+_default_formatter = CloudWatchFormatter()
+
+
 class CloudWatchLogHandler(logging.Handler):
     """
     Create a new CloudWatch log handler object. This is the main entry point to the functionality of the module. See
@@ -91,6 +144,8 @@ class CloudWatchLogHandler(logging.Handler):
         Create CloudWatch Logs log stream if it does not exist.  **True** by default.
     :type create_log_stream: Boolean
     :param json_serialize_default:
+        **DEPRECATED**: use CloudWatchFormatter for JSON formatting instead.
+
         The 'default' function to use when serializing dictionaries as JSON. Refer to the Python standard library
         documentation on 'json' for more explanation about the 'default' parameter.
         https://docs.python.org/3/library/json.html#json.dump
@@ -131,7 +186,6 @@ def __init__(self, log_group=__name__, stream_name=None, use_queues=True, send_i
         self.stream_name = stream_name
         self.use_queues = use_queues
         self.send_interval = send_interval
-        self.json_serialize_default = json_serialize_default or _json_serialize_default
         self.max_batch_size = max_batch_size
         self.max_batch_count = max_batch_count
         self.max_message_size = max_message_size
@@ -140,6 +194,12 @@ def __init__(self, log_group=__name__, stream_name=None, use_queues=True, send_i
         self.creating_log_stream, self.shutting_down = False, False
         self.create_log_stream = create_log_stream
         self.log_group_retention_days = log_group_retention_days
+        self.json_serialize_default = json_serialize_default
+        if json_serialize_default:
+            warnings.warn(
+                'Specifying json_serialize_default is deprecated, please create a CloudWatchFormatter instance '
+                'which accepts a json_serialize_default and set it as a formatter instead',
+                DeprecationWarning)
 
         # Creating session should be the final call in __init__, after all instance attributes are set.
         # This ensures that failing to create the session will not result in any missing attribtues.
@@ -208,6 +268,19 @@ def _submit_batch(self, batch, stream_name, max_retries=5):
             # from the response
             self.sequence_tokens[stream_name] = response["nextSequenceToken"]
 
+    def format(self, record):
+        """
+        Format the specified record.
+
+        If a formatter is set, use it. Otherwise, use the default formatter for the module. This differs from
+        `logging.Handler.format` as its default is `CloudWatchFormatter`.
+        """
+        if self.formatter:
+            fmt = self.formatter
+        else:
+            fmt = _default_formatter
+        return fmt.format(record)
+
     def emit(self, message):
         if self.creating_log_stream:
             return  # Avoid infinite recursion when asked to log a message as our own side effect
@@ -219,7 +292,7 @@ def emit(self, message):
         if stream_name not in self.sequence_tokens:
             self.sequence_tokens[stream_name] = None
 
-        if isinstance(message.msg, Mapping):
+        if self.json_serialize_default and isinstance(message.msg, Mapping):
             message.msg = json.dumps(message.msg, default=self.json_serialize_default)
 
         cwl_message = dict(timestamp=int(message.created * 1000), message=self.format(message))