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# Standard Library Logging

Ideally, structlog should be able to be used as a drop-in replacement for standard library’s `logging` by wrapping it. In other words, you should be able to replace your call to `logging.getLogger()` by a call to `structlog.get_logger()` and things should keep working as before (if structlog is configured right, see Suggested configurations below).

If you run into incompatibilities, it is a bug so please take the time to report it! If you’re a heavy `logging` user, your help to ensure a better compatibility would be highly appreciated!

Important

The quickest way to get started with structlog and `logging` is `structlog.stdlib.recreate_defaults()`. It will recreate the default configuration on top of `logging` and optionally configure `logging` for you.

## Just enough `logging`

If you want to use structlog with `logging`, you should have at least a fleeting understanding on how the standard library operates because structlog will not do any magic things in the background for you. Most importantly you have to configure the `logging` system additionally to configuring structlog.

Usually it is enough to use:

    import logging
    import sys

    logging.basicConfig(
        format="%(message)s",
        stream=sys.stdout,
        level=logging.INFO,
    )

This will send all log messages with the log level `logging.INFO` and above (that means that, for example, `logging.debug` calls are ignored) to standard out without any special formatting by the standard library.

If you require more complex behavior, please refer to the standard library’s `logging` documentation.

## Concrete bound logger

structlog ships a stdlib-specific bound logger that mirrors the log methods of standard library’s `logging.Logger` with correct type hints.

If you want to take advantage of said type hints, you have to either annotate the logger coming from `structlog.get_logger()`, or use `structlog.stdlib.get_logger()` that has the appropriate type hints. Please note though, that it will neither configure nor verify your configuration. It will call `structlog.get_logger()` just like if you would’ve called it – the only difference are the type hints.

See also Type Hints.

### `asyncio`

In `asyncio` applications, you may not want your whole application to block while the processor chain is formatting your log entries.

For that use case structlog comes with a set of non-standard methods that will do all processing in a thread pool executor. They have the same names as the regular methods, except they are prefixed by an `a`. So instead of `logger.info("event!")` you write `await logger.ainfo("event!)`. No extra configuration is necessary and you can mix-and-match both types of methods within the same application.

This means an increased computational cost per log entry, but your application will not block because of logging.

Added in version 23.1.0.

structlog also comes with `structlog.stdlib.AsyncBoundLogger` that blankly makes all logging methods asynchronous (in other words, you have to use `await log.info()` instead of just `log.info()`).

To use it, configure structlog to use `AsyncBoundLogger` as `wrapper_class`.

Added in version 20.2.0.

Deprecated since version 23.1.0.

## Processors

structlog comes with a few standard library-specific processors:

`render_to_log_args_and_kwargs()`:

  Renders the event dictionary into positional and keyword arguments for `logging.Logger` logging methods. This is useful if you want to render your log entries entirely within `logging`.

`render_to_log_kwargs()`:

  Same as above, but does not support passing positional arguments from structlog loggers to `logging.Logger` logging methods as positional arguments. structlog positional arguments are still passed to `logging` under `positional_args` key of `extra` keyword argument.

`filter_by_level()`:

  Checks the log entry’s log level against the configuration of standard library’s logging. Log entries below the threshold get silently dropped. Put it at the beginning of your processing chain to avoid expensive operations from happening in the first place.

`add_logger_name()`:

  Adds the name of the logger to the event dictionary under the key `logger`.

`add_log_level()`:

  Adds the log level to the event dictionary under the key `level`.

`add_log_level_number()`:

  Adds the log level number to the event dictionary under the key `level_number`. Log level numbers map to the log level names. The Python stdlib uses them for filtering logic. This adds the same numbers so users can leverage similar filtering. Compare:

      level in ("warning", "error", "critical")
      level_number >= 30

  The mapping of names to numbers is in `structlog.stdlib._NAME_TO_LEVEL`.

`ExtraAdder`:

  Add extra attributes of `logging.LogRecord` objects to the event dictionary.

This processor can be used for adding data passed in the `extra` parameter of the `logging` module’s log methods to the event dictionary.

`PositionalArgumentsFormatter()`:

  This processes and formats positional arguments (if any) passed to log methods in the same way the `logging` module would do, for example, `logger.info("Hello, %s", name)`.

structlog also comes with `ProcessorFormatter` which is a `logging.Formatter` that enables you to format non-structlog log entries using structlog renderers and multiplex structlog’s output with different renderers (see below for an example).

## Suggested configurations

Note

We do appreciate that fully integrating structlog with standard library’s `logging` is fiddly when done for the first time.

This is the price of flexibility and unfortunately – given the different needs of our users – we can’t make it any simpler without compromising someone’s use-cases. However, once it is set up, you can rely on not having to ever touch it again.

Depending where you’d like to do your formatting, you can take one of four approaches:

### Don’t integrate

The most straight-forward option is to configure standard library `logging` close enough to what structlog is logging and leaving it at that.

Since these are usually log entries from third parties that don’t take advantage of structlog’s features, this is surprisingly often a perfectly adequate approach.

For instance, if you log JSON in production, configure `logging` to use python-json-logger to make it print JSON too, and then tweak the configuration to match their outputs. You can also use `ProcessorFormatter` as a formatter for `logging` to get the same output for both structlog and `logging` log entries – see below for an example.

Important

If you want to use same file (for example, `sys.stdout` or `sys.stderr`) for both structlog and `logging.StreamHandler` output, you must use `WriteLogger` instead of `PrintLogger`.

This is because `PrintLogger` uses `print(log, file=file, flush=True)` to write log, and `print` writes the `log` message and a newline (“\n”) to the stream separately. This can cause interleaving of log entries from structlog and `logging` loggers. `WriteLogger` writes log entries atomically to the file (for example, `file.write(log+"\n")`).

### Rendering within structlog

This is the simplest approach where structlog does all the heavy lifting and passes a fully-formatted string to `logging`. Chances are, this is all you need.

A basic configuration to output structured logs in JSON format looks like this:

    import structlog

    structlog.configure(
        processors=[
            # If log level is too low, abort pipeline and throw away log entry.
            structlog.stdlib.filter_by_level,
            # Add the name of the logger to event dict.
            structlog.stdlib.add_logger_name,
            # Add log level to event dict.
            structlog.stdlib.add_log_level,
            # Perform %-style formatting.
            structlog.stdlib.PositionalArgumentsFormatter(),
            # Add a timestamp in ISO 8601 format.
            structlog.processors.TimeStamper(fmt="iso"),
            # If the "stack_info" key in the event dict is true, remove it and
            # render the current stack trace in the "stack" key.
            structlog.processors.StackInfoRenderer(),
            # If the "exc_info" key in the event dict is either true or a
            # sys.exc_info() tuple, remove "exc_info" and render the exception
            # with traceback into the "exception" key.
            structlog.processors.format_exc_info,
            # If some value is in bytes, decode it to a Unicode str.
            structlog.processors.UnicodeDecoder(),
            # Add callsite parameters.
            structlog.processors.CallsiteParameterAdder(
                {
                    structlog.processors.CallsiteParameter.FILENAME,
                    structlog.processors.CallsiteParameter.FUNC_NAME,
                    structlog.processors.CallsiteParameter.LINENO,
                }
            ),
            # Render the final event dict as JSON.
            structlog.processors.JSONRenderer()
        ],
        # `wrapper_class` is the bound logger that you get back from
        # get_logger(). This one imitates the API of `logging.Logger`.
        wrapper_class=structlog.stdlib.BoundLogger,
        # `logger_factory` is used to create wrapped loggers that are used for
        # OUTPUT. This one returns a `logging.Logger`. The final value (a JSON
        # string) from the final processor (`JSONRenderer`) will be passed to
        # the method of the same name as that you've called on the bound logger.
        logger_factory=structlog.stdlib.LoggerFactory(),
        # Effectively freeze configuration after creating the first bound
        # logger.
        cache_logger_on_first_use=True,
    )

To make your program behave like a proper 12 Factor App that outputs only JSON to `stdout`, configure the `logging` module like this:

    import logging
    import sys

    logging.basicConfig(
        format="%(message)s",
        stream=sys.stdout,
        level=logging.INFO,
    )

In this case only your own logs are formatted as JSON:

    >>> structlog.get_logger("test").warning("hello")
    {"event": "hello", "logger": "test", "level": "warning", "timestamp": "2017-03-06T07:39:09.518720Z"}

    >>> logging.getLogger("test").warning("hello")
    hello

### Rendering using `logging`-based formatters

You can choose to use structlog only for building the event dictionary and leave all formatting – additionally to the output – to the standard library.

    import structlog

    structlog.configure(
        processors=[
            structlog.stdlib.filter_by_level,
            structlog.stdlib.add_logger_name,
            structlog.stdlib.add_log_level,
            structlog.stdlib.PositionalArgumentsFormatter(),
            structlog.processors.StackInfoRenderer(),
            structlog.processors.format_exc_info,
            structlog.processors.UnicodeDecoder(),
            # Transform event dict into `logging.Logger` method arguments.
            # "event" becomes "msg" and the rest is passed as a dict in
            # "extra". IMPORTANT: This means that the standard library MUST
            # render "extra" for the context to appear in log entries! See
            # warning below.
            structlog.stdlib.render_to_log_kwargs,
        ],
        logger_factory=structlog.stdlib.LoggerFactory(),
        wrapper_class=structlog.stdlib.BoundLogger,
        cache_logger_on_first_use=True,
    )

Now you have the event dict available within each log record. If you want all your log entries (meaning: also those not from your application / structlog) to be formatted as JSON, you can use the python-json-logger library:

    import logging
    import sys

    from pythonjsonlogger import jsonlogger

    handler = logging.StreamHandler(sys.stdout)
    handler.setFormatter(jsonlogger.JsonFormatter())
    root_logger = logging.getLogger()
    root_logger.addHandler(handler)

Now both structlog and `logging` will emit JSON logs:

    >>> structlog.get_logger("test").warning("hello")
    {"message": "hello", "logger": "test", "level": "warning"}

    >>> logging.getLogger("test").warning("hello")
    {"message": "hello"}

Warning

With this approach, it’s the standard library `logging` formatter’s duty to do something useful with the event dict. In the above example that’s `jsonlogger.JsonFormatter`.

Keep this in mind if you only get the event name without any context, and exceptions are ostensibly swallowed.

### Rendering using structlog-based formatters within `logging`

Finally, the most ambitious approach. Here, you use structlog’s `ProcessorFormatter` as a `logging.Formatter` for both `logging` as well as structlog log entries.

Consequently, the output is the duty of the standard library too.

`ProcessorFormatter` has two parts to its API:

1. On the structlog side, the processor chain must be configured to end with `structlog.stdlib.ProcessorFormatter.wrap_for_formatter()` as the renderer. It converts the processed event dictionary into something that `ProcessorFormatter` understands.

2. On the `logging` side, you must configure `ProcessorFormatter` as your formatter of choice. `logging` then calls `ProcessorFormatter`’s `format()` method.

   For that, `ProcessorFormatter` wraps a processor chain that is responsible for rendering your log entries to strings.

Warning

When using `ProcessorFormatter`, you must not use `render_to_log_kwargs()` or `render_to_log_args_and_kwargs()` in your processor chain.

You must use `structlog.stdlib.ProcessorFormatter.wrap_for_formatter()` – otherwise you will get puzzling errors from the standard library.

Thus, the simplest possible configuration looks like the following:

    import logging
    import structlog

    structlog.configure(
        processors=[
            # Prepare event dict for `ProcessorFormatter`.
            structlog.stdlib.ProcessorFormatter.wrap_for_formatter,
        ],
        logger_factory=structlog.stdlib.LoggerFactory(),
    )

    formatter = structlog.stdlib.ProcessorFormatter(
        processors=[structlog.dev.ConsoleRenderer()],
    )

    handler = logging.StreamHandler()
    # Use OUR `ProcessorFormatter` to format all `logging` entries.
    handler.setFormatter(formatter)
    root_logger = logging.getLogger()
    root_logger.addHandler(handler)
    root_logger.setLevel(logging.INFO)

which will allow both of these to work in other modules:

    >>> import logging
    >>> import structlog

    >>> logging.getLogger("stdlog").info("woo")
    woo      _from_structlog=False _record=<LogRecord:...>
    >>> structlog.get_logger("structlog").info("amazing", events="oh yes")
    amazing  _from_structlog=True _record=<LogRecord:...> events=oh yes

Of course, you probably want timestamps and log levels in your output. The `ProcessorFormatter` has a `foreign_pre_chain` argument which is responsible for adding properties to events from the standard library – in other words, those that do not originate from a structlog logger – and which should in general match the `processors` argument to `structlog.configure()` so you get a consistent output.

`_from_structlog` and `_record` allow your processors to determine whether the log entry is coming from structlog, and to extract information from `logging.LogRecord`s and add them to the event dictionary. However, you probably don’t want to have them in your log files, thus we’ve added the `ProcessorFormatter.remove_processors_meta` processor to do so conveniently.

For example, to add timestamps, log levels, and traceback handling to your logs without `_from_structlog` and `_record` noise you should do:

    timestamper = structlog.processors.TimeStamper(fmt="%Y-%m-%d %H:%M:%S")
    shared_processors = [
        structlog.stdlib.add_log_level,
        timestamper,
    ]

    structlog.configure(
        processors=shared_processors + [
            structlog.stdlib.ProcessorFormatter.wrap_for_formatter,
        ],
        logger_factory=structlog.stdlib.LoggerFactory(),
        cache_logger_on_first_use=True,
    )

    formatter = structlog.stdlib.ProcessorFormatter(
        # These run ONLY on `logging` entries that do NOT originate within
        # structlog.
        foreign_pre_chain=shared_processors,
        # These run on ALL entries after the pre_chain is done.
        processors=[
            # Remove _record & _from_structlog.
            structlog.stdlib.ProcessorFormatter.remove_processors_meta,
            structlog.dev.ConsoleRenderer(),
        ],
    )

which (given the same `logging.*` calls as in the previous example) will result in:

    >>> logging.getLogger("stdlog").info("woo")
    2021-11-15 11:41:47 [info     ] woo
    >>> structlog.get_logger("structlog").info("amazing", events="oh yes")
    2021-11-15 11:41:47 [info     ] amazing    events=oh yes

This allows you to set up some sophisticated logging configurations. For example, to use the standard library’s `logging.config.dictConfig` to log colored logs to the console and plain logs to a file you could do:

    import logging.config
    import structlog

    timestamper = structlog.processors.TimeStamper(fmt="%Y-%m-%d %H:%M:%S")
    pre_chain = [
        # Add the log level and a timestamp to the event_dict if the log entry
        # is not from structlog.
        structlog.stdlib.add_log_level,
        # Add extra attributes of LogRecord objects to the event dictionary
        # so that values passed in the extra parameter of log methods pass
        # through to log output.
        structlog.stdlib.ExtraAdder(),
        timestamper,
    ]

    def extract_from_record(_, __, event_dict):
        """
        Extract thread and process names and add them to the event dict.
        """
        record = event_dict["_record"]
        event_dict["thread_name"] = record.threadName
        event_dict["process_name"] = record.processName
        return event_dict

    logging.config.dictConfig(
        {
            "version": 1,
            "disable_existing_loggers": False,
            "formatters": {
                "plain": {
                    "()": structlog.stdlib.ProcessorFormatter,
                    "processors": [
                        structlog.stdlib.ProcessorFormatter.remove_processors_meta,
                        structlog.dev.ConsoleRenderer(colors=False),
                    ],
                    "foreign_pre_chain": pre_chain,
                },
                "colored": {
                    "()": structlog.stdlib.ProcessorFormatter,
                    "processors": [
                        extract_from_record,
                        structlog.stdlib.ProcessorFormatter.remove_processors_meta,
                        structlog.dev.ConsoleRenderer(colors=True),
                    ],
                    "foreign_pre_chain": pre_chain,
                },
            },
            "handlers": {
                "default": {
                    "level": "DEBUG",
                    "class": "logging.StreamHandler",
                    "formatter": "colored",
                },
                "file": {
                    "level": "DEBUG",
                    "class": "logging.handlers.WatchedFileHandler",
                    "filename": "test.log",
                    "formatter": "plain",
                },
            },
            "loggers": {
                "": {
                    "handlers": ["default", "file"],
                    "level": "DEBUG",
                    "propagate": True,
                },
            },
        }
    )
    structlog.configure(
        processors=[
            structlog.stdlib.add_log_level,
            structlog.stdlib.PositionalArgumentsFormatter(),
            timestamper,
            structlog.processors.StackInfoRenderer(),
            structlog.processors.format_exc_info,
            structlog.stdlib.ProcessorFormatter.wrap_for_formatter,
        ],
        logger_factory=structlog.stdlib.LoggerFactory(),
        wrapper_class=structlog.stdlib.BoundLogger,
        cache_logger_on_first_use=True,
    )

This defines two formatters: one plain and one colored. Both are run for each log entry. Log entries that do not originate from structlog, are additionally pre-processed using a cached `timestamper` and `add_log_level()`.

Additionally, for both `logging` and structlog – but only for the colorful logger – we also extract some data from `logging.LogRecord`:

    >>> logging.getLogger().warning("bar")
    2021-11-15 13:26:52 [warning  ] bar    process_name=MainProcess thread_name=MainThread

    >>> structlog.get_logger("structlog").warning("foo", x=42)
    2021-11-15 13:26:52 [warning  ] foo    process_name=MainProcess thread_name=MainThread x=42

    >>> pathlib.Path("test.log").read_text()
    2021-11-15 13:26:52 [warning  ] bar
    2021-11-15 13:26:52 [warning  ] foo    x=42

(Sadly, you have to imagine the colors in the first two outputs.)

If you leave `foreign_pre_chain` as `None`, formatting will be left to `logging`. Meaning: you can define a `format` for `ProcessorFormatter` too!