Configuration

Global Defaults

To make logging as unintrusive and straight-forward to use as possible, structlog comes with a plethora of configuration options and convenience functions. Let’s start at the end and introduce the ultimate convenience function that relies purely on configuration: structlog.get_logger().

The goal is to reduce your per-file logging boilerplate to:

from structlog import get_logger
logger = get_logger()

while still giving you the full power via configuration.

To that end you’ll have to call structlog.configure() on app initialization. Thus the example from the previous chapter could have been written as following:

>>> configure(processors=[proc], context_class=dict)
>>> log = get_logger()
>>> log.msg("hello world")
I got called with {'event': 'hello world'}
{'event': 'hello world'}

because PrintLogger is the default LoggerFactory (see Logger Factories).

Configuration also applies to wrap_logger() because that’s what’s used internally:

>>> configure(processors=[proc], context_class=dict)
>>> log = wrap_logger(PrintLogger())
>>> log.msg("hello world")
I got called with {'event': 'hello world'}
{'event': 'hello world'}

You can call structlog.configure() repeatedly and only set one or more settings – the rest will not be affected.

structlog tries to behave in the least surprising way when it comes to handling defaults and configuration:

  1. Arguments passed to structlog.wrap_logger() always take the highest precedence over configuration. That means that you can overwrite whatever you’ve configured for each logger respectively.

  2. If you leave them on None, structlog will check whether you’ve configured default values using structlog.configure() and uses them if so.

  3. If you haven’t configured or passed anything at all, the default fallback values try to be convenient and development-friendly.

If necessary, you can always reset your global configuration back to default values using structlog.reset_defaults(). That can be handy in tests.

At any time, you can check whether and how structlog is configured:

>>> structlog.is_configured()
False
>>> class MyDict(dict): pass
>>> structlog.configure(context_class=MyDict)
>>> structlog.is_configured()
True
>>> cfg = structlog.get_config()
>>> cfg["context_class"]
<class 'MyDict'>

Note

Since you will call structlog.get_logger most likely in module scope, they run at import time before you had a chance to configure structlog. Therefore they return a lazy proxy that returns a correct wrapped logger on first bind()/new().

Thus, you must never call new() or bind() in module or class scope because otherwise you will receive a logger configured with structlog’s default values. Use get_logger()‘s initial_values to achieve pre-populated contexts.

To enable you to log with the module-global logger, it will create a temporary BoundLogger and relay the log calls to it on each call. Therefore if you have nothing to bind but intend to do lots of log calls in a function, it makes sense performance-wise to create a local logger by calling bind() or new() without any parameters. See also Performance.

Logger Factories

To make structlog.get_logger work, one needs one more option that hasn’t been discussed yet: logger_factory.

It is a callable that returns the logger that gets wrapped and returned. In the simplest case, it’s a function that returns a logger – or just a class. But you can also pass in an instance of a class with a __call__ method for more complicated setups.

New in version 0.4.0: structlog.get_logger can optionally take positional parameters.

These will be passed to the logger factories. For example, if you use run structlog.get_logger("a name") and configure structlog to use the standard library LoggerFactory which has support for positional parameters, the returned logger will have the name "a name".

When writing custom logger factories, they should always accept positional parameters even if they don’t use them. That makes sure that loggers are interchangeable.

For the common cases of standard library logging and Twisted logging, structlog comes with two factories built right in:

So all it takes to use structlog with standard library logging is this:

>>> from structlog import get_logger, configure
>>> from structlog.stdlib import LoggerFactory
>>> configure(logger_factory=LoggerFactory())
>>> log = get_logger()
>>> log.critical("this is too easy!")
event='this is too easy!'

By using structlog’s structlog.stdlib.LoggerFactory, it is also ensured that variables like function names and line numbers are expanded correctly in your log format.

The Twisted example shows how easy it is for Twisted.

Note

LoggerFactory-style factories always need to get passed as instances like in the examples above. While neither allows for customization using parameters yet, they may do so in the future.

Calling structlog.get_logger without configuration gives you a perfectly useful structlog.PrintLogger. We don’t believe silent loggers are a sensible default.

Where to Configure

The best place to perform your configuration varies with applications and frameworks. Ideally as late as possible but before non-framework (i.e. your) code is executed. If you use standard library’s logging, it makes sense to configure them next to each other.

Django

See Third-Party Extensions in the wiki.

Flask

See Logging.

Pyramid

Application constructor.

Twisted

The plugin definition is the best place. If your app is not a plugin, put it into your tac file (and then learn about plugins).

If you have no choice but have to configure on import time in module-global scope, or can’t rule out for other reasons that that your structlog.configure gets called more than once, structlog offers structlog.configure_once that raises a warning if structlog has been configured before (no matter whether using structlog.configure or configure_once()) but doesn’t change anything.