# Logging

Brian uses a logging system to display warnings and general information messages to the user, as well as writing them to a file with more detailed information, useful for debugging. Each log message has one of the following “log levels”:

`ERROR`

  Only used when an exception is raised, i.e. an error occurs and the current operation is interrupted. Example: You use a variable name in an equation that Brian does not recognize.

`WARNING`

  Brian thinks that something is most likely a bug, but it cannot be sure. Example: You use a `Synapses` object without any synapses in your simulation.

`INFO`

  Brian wants to make the user aware of some automatic choice that it did for the user. Example: You did not specify an integration `method` for a `NeuronGroup` and therefore Brian chose an appropriate method for you.

`DEBUG`

  Additional information that might be useful when a simulation is not working as expected. Example: The integration timestep used during the simulation.

`DIAGNOSTIC`

  Additional information useful when tracking down bugs in Brian itself. Example: The generated code for a `CodeObject`.

By default, all messages with level `DEBUG` or above are written to the log file and all messages of level `INFO` and above are displayed on the console. To change what messages are displayed, see below.

Note

By default, the log file is deleted after a successful simulation run, i.e. when the simulation exited without an error. To keep the log around, set the logging.delete_log_on_exit preference to `False`.

## Logging and multiprocessing

Brian’s logging system is not designed for multiple parallel Brian processes started via Python’s `multiprocessing` module (see the multiprocessing examples). Log messages that get printed from different processes to the console are not printed in a well-defined order and do not contain any indication about which processes they are coming from. You might therefore consider using e.g. `BrianLogger.log_level_error` to only show error messages before starting the processes and avoid cluttering your console with warning and info messages.

To avoid issues when multiple processes try to log to the same log file, file logging is automatically switched off for all processes except for the initial process. If you need a file log for sub-processes, you can call `BrianLogger.initialize` in each sub-process. This way, each process will log to its own file.

## Showing/hiding log messages

If you want to change what messages are displayed on the console, you can call a method of the method of `BrianLogger`:

    BrianLogger.log_level_debug() # now also display debug messages

It is also possible to suppress messages for certain sub-hierarchies by using `BrianLogger.suppress_hierarchy`:

    # Suppress code generation messages on the console
    BrianLogger.suppress_hierarchy('brian2.codegen')
    # Suppress preference messages even in the log file
    BrianLogger.suppress_hierarchy('brian2.core.preferences',
                                   filter_log_file=True)

Similarly, messages ending in a certain name can be suppressed with `BrianLogger.suppress_name`:

    # Suppress resolution conflict warnings
    BrianLogger.suppress_name('resolution_conflict')

These functions should be used with care, as they suppresses messages independent of the level, i.e. even warning and error messages.

## Preferences

You can also change details of the logging system via Brian’s Preferences system. With this mechanism, you can switch the logging to a file off completely (by setting logging.file_log to `False`) or have it log less messages (by setting logging.file_log_level to a level higher than `DEBUG`). To debug details of the code generation system, you can also set logging.file_log_level to `DIAGNOSTIC`. Note that this will make the log file grow quickly in size. To prevent it from filling up the disk, it will only be allowed to grow up to a certain size. You can configure the maximum file size with the logging.file_log_max_size preference.

For a list of all preferences related to logging, see the documentation of the `brian2.utils.logger` module.

Warning

Most of the logging preferences are only taken into account during the initialization of the logging system which takes place as soon as `brian2` is imported. Therefore, if you use e.g. `prefs.logging.file_log = False` in your script, this will not have the intended effect! To make sure these preferences are taken into account, call `BrianLogger.initialize` after setting the preferences. Alternatively, you can set the preferences in a file (see Preferences).