The true power of structlog lies in its combinable log processors. A log processor is a regular callable or in other words: A function or an instance of a class with a __call__() method.


The processor chain is a list of processors. Each processors receives three positional arguments:


Your wrapped logger object. For example logging.Logger or structlog.typing.FilteringBoundLogger (default).


The name of the wrapped method. If you called log.warning("foo"), it will be "warning".


Current context together with the current event. If the context was {"a": 42} and the event is "foo", the initial event_dict will be {"a":42, "event": "foo"}.

The return value of each processor is passed on to the next one as event_dict until finally the return value of the last processor gets passed into the wrapped logging method.


structlog only looks at the return value of the last processor. That means that as long as you control the next processor in the chain (the processor that will get your return value passed as an argument), you can return whatever you want.

Returning a modified event dictionary from your processors is just a convention to make processors composable.


If you set up your logger like:

structlog.configure(processors=[f1, f2, f3])
log = structlog.get_logger().bind(x=42)

and call"some_event", y=23), it results in the following call chain:
   f3(wrapped_logger, "info",
      f2(wrapped_logger, "info",
         f1(wrapped_logger, "info", {"event": "some_event", "x": 42, "y": 23})

In this case, f3 has to make sure it returns something can handle (see Adapting and Rendering). For the example with PrintLogger above, this means f3 must return a string.

The simplest modification a processor can make is adding new values to the event_dict. Parsing human-readable timestamps is tedious, not so UNIX timestamps – let’s add one to each log entry:

import calendar
import time

def timestamper(logger, log_method, event_dict):
    event_dict["timestamp"] = calendar.timegm(time.gmtime())
    return event_dict


You’re explicitly allowed to modify the event_dict parameter, because a copy has been created before calling the first processor.

Please note that structlog comes with such a processor built in: TimeStamper.


If a processor raises structlog.DropEvent, the event is silently dropped.

Therefore, the following processor drops every entry:

from structlog import DropEvent

def dropper(logger, method_name, event_dict):
    raise DropEvent

But we can do better than that!

How about dropping only log entries that are marked as coming from a certain peer (for example, monitoring)?

class ConditionalDropper:
    def __init__(self, peer_to_ignore):
        self._peer_to_ignore = peer_to_ignore

    def __call__(self, logger, method_name, event_dict):
        >>> cd = ConditionalDropper("")
        >>> cd(None, None, {"event": "foo", "peer": ""})
        {'peer': '', 'event': 'foo'}
        >>> cd(None, None, {"event": "foo", "peer": ""})
        Traceback (most recent call last):
        if event_dict.get("peer") == self._peer_to_ignore:
            raise DropEvent

        return event_dict

Since it’s so common to filter by the log level, structlog comes with structlog.make_filtering_bound_logger() that filters log entries before they even enter the processor chain. It does not use the standard library, but it does use its names and order of log levels.

Adapting and Rendering#

An important role is played by the last processor because its duty is to adapt the event_dict into something the logging methods of the wrapped logger understand. With that, it’s also the only processor that needs to know anything about the underlying system.

It can return one of three types:

  • An Unicode string (str), a bytes string (bytes), or a bytearray that is passed as the first (and only) positional argument to the underlying logger.

  • A tuple of (args, kwargs) that are passed as log_method(*args, **kwargs).

  • A dictionary which is passed as log_method(**kwargs).

Therefore return "hello world" is a shortcut for return (("hello world",), {}) (the example in Chains assumes this shortcut has been taken).

This should give you enough power to use structlog with any logging system while writing agnostic processors that operate on dictionaries.

Changed in version 14.0.0: Allow final processor to return a dict.

Changed in version 20.2.0: Allow final processor to return a bytes.

Changed in version 21.2.0: Allow final processor to return a bytearray.


The probably most useful formatter for string based loggers is structlog.processors.JSONRenderer. Advanced log aggregation and analysis tools like Logstash offer features like telling them “this is JSON, deal with it” instead of fiddling with regular expressions.

For a list of shipped processors, check out the API documentation.

Third-Party Packages#

structlog was specifically designed to be as composable and reusable as possible, so whatever you’re missing: chances are, you can solve it with a processor! Since processors are self-contained callables, it’s easy to write your own and to share them in separate packages.

We collect those packages in our GitHub Wiki and encourage you to add your package too!