# Logging

aiohttp uses standard `logging` for tracking the library activity.

We have the following loggers enumerated by names:

- `'aiohttp.access'`

- `'aiohttp.client'`

- `'aiohttp.internal'`

- `'aiohttp.server'`

- `'aiohttp.web'`

- `'aiohttp.websocket'`

You may subscribe to these loggers for getting logging messages. The page does not provide instructions for logging subscribing while the most friendly method is `logging.config.dictConfig()` for configuring whole loggers in your application.

Logging does not work out of the box. It requires at least minimal `'logging'` configuration. Example of minimal working logger setup:

    import logging
    from aiohttp import web

    app = web.Application()
    logging.basicConfig(level=logging.DEBUG)
    web.run_app(app, port=5000)

Added in version 4.0.0.

## Access logs

Access logs are enabled by default. If the debug flag is set, and the default logger `'aiohttp.access'` is used, access logs will be output to `stderr` if no handlers are attached. Furthermore, if the default logger has no log level set, the log level will be set to `logging.DEBUG`.

This logging may be controlled by `aiohttp.web.AppRunner()` and `aiohttp.web.run_app()`.

To override the default logger, pass an instance of `logging.Logger` to override the default logger.

Note

Use `web.run_app(app, access_log=None)` to disable access logs.

In addition, access_log_format may be used to specify the log format.

### Format specification

The library provides custom micro-language to specifying info about request and response:

Option | Meaning
--- | ---
`%%` | The percent sign
`%a` | Remote IP-address (IP-address of proxy if using reverse proxy)
`%t` | Time when the request was started to process
`%P` | The process ID of the child that serviced the request
`%r` | First line of request
`%s` | Response status code
`%b` | Size of response in bytes, including HTTP headers
`%T` | The time taken to serve the request, in seconds
`%Tf` | The time taken to serve the request, in seconds with fraction in %.06f format
`%D` | The time taken to serve the request, in microseconds
`%{FOO}i` | `request.headers['FOO']`
`%{FOO}o` | `response.headers['FOO']`

The default access log format is:

    '%a %t "%r" %s %b "%{Referer}i" "%{User-Agent}i"'

Added in version 2.3.0.

access_log_class introduced.

Example of a drop-in replacement for the default access logger:

    from aiohttp.abc import AbstractAccessLogger

    class AccessLogger(AbstractAccessLogger):

        def log(self, request, response, time):
            self.logger.info(f'{request.remote} '
                             f'"{request.method} {request.path} '
                             f'done in {time}s: {response.status}')

        @property
        def enabled(self):
            """Return True if logger is enabled.

            Override this property if logging is disabled to avoid the
            overhead of calculating details to feed the logger.

            This property may be omitted if logging is always enabled.
            """
            return self.logger.isEnabledFor(logging.INFO)

### Gunicorn access logs

When Gunicorn is used for deployment, its default access log format will be automatically replaced with the default aiohttp’s access log format.

If Gunicorn’s option access_logformat is specified explicitly, it should use aiohttp’s format specification.

Gunicorn’s access log works only if accesslog is specified explicitly in your config or as a command line option. This configuration can be either a path or `'-'`. If the application uses a custom logging setup intercepting the `'gunicorn.access'` logger, accesslog should be set to `'-'` to prevent Gunicorn to create an empty access log file upon every startup.

## Error logs

`aiohttp.web` uses a logger named `'aiohttp.server'` to store errors given on web requests handling.

This log is enabled by default.

To use a different logger name, pass logger (`logging.Logger` instance) to the `aiohttp.web.AppRunner()` constructor.