Configuring Logging Output

You can register “destinations” to handle logging output; a destination is a callable that takes a message dictionary. For example, if we want to just print each new message:

import json, sys
from eliot import add_destinations

def stdout(message):

Before destinations are added

Up to a 1000 messages will be buffered in memory until the first set of destinations are added, at which point those messages will be delivered to newly added set of destinations. This ensures that no messages will be lost if logging happens during configuration but before a destination is added.

Outputting JSON to a file

Since JSON is a common output format, Eliot provides a utility class that logs to a file, eliot.FileDestination(file=yourfile). Each Eliot message will be encoded in JSON and written on a new line. As a short hand you can call eliot.to_file, which will create the destination and then add it automatically. For example:

from eliot import to_file
to_file(open("eliot.log", "ab"))


This destination is blocking: if writing to a file takes a long time your code will not be able to proceed until writing is done. If you’re using Twisted you can wrap a eliot.FileDestination with a non-blocking eliot.logwriter.ThreadedWriter. This allows you to log to a file without blocking the Twisted reactor.

Customizing JSON Encoding

If you’re using Eliot’s JSON output you may wish to customize encoding. By default Eliot uses eliot.json.EliotJSONEncoder (a subclass of json.JSONEncoder) to encode objects. You can customize encoding by passing a custom subclass to either eliot.FileDestination or eliot.to_file:

from eliot.json import EliotJSONEncoder
from eliot import to_file

class MyClass:
    def __init__(self, x):
        self.x = x

class MyEncoder(EliotJSONEncoder):
    def default(self, obj):
        if isinstance(obj, MyClass):
            return {"x": obj.x}
        return EliotJSONEncoder.default(self, obj)

to_file(open("eliot.log", "ab"), encoder=MyEncoder)

For more details on JSON encoding see the Python JSON documentation.

Adding Fields to All Messages

Sometimes you want to add a field to all messages output by your process, regardless of destination. For example if you’re aggregating logs from multiple processes into a central location you might want to include a field process_id that records the name and process id of your process in every log message. Use the eliot.add_global_fields API to do so, e.g.:

import os, sys
from eliot import add_global_fields

add_global_fields(process_id="%s:%d" % (sys.argv[0], os.getpid()))

You should call add_global_fields before add_destinations to ensure all messages get the global fields.