# Logging¶

For a description of logging from the users point of view, see Logging.

Logging in Brian is based on the logging module in Python’s standard library.

Every brian module that needs logging should start with the following line, using the get_logger() function to get an instance of BrianLogger:

logger = get_logger(__name__)


In the code, logging can then be done via:

logger.diagnostic('A diagnostic message')
logger.debug('A debug message')
logger.info('An info message')
logger.warn('A warning message')
logger.error('An error message')


If a module logs similar messages in different places or if it might be useful to be able to suppress a subset of messages in a module, add an additional specifier to the logging command, specifying the class or function name, or a method name including the class name (do not include the module name, it will be automatically added as a prefix):

logger.debug('A debug message', 'CodeString')
logger.debug('A debug message', 'NeuronGroup.update')
logger.debug('A debug message', 'reinit')


If you want to log a message only once, e.g. in a function that is called repeatedly, set the optional once keyword to True:

logger.debug('Will only be shown once', once=True)
logger.debug('Will only be shown once', once=True)


The output of debugging looks like this in the log file:

2012-10-02 14:41:41,484 DEBUG    brian2.equations.equations.CodeString: A debug message


and like this on the console (if the log level is set to “debug”):

DEBUG    A debug message [brian2.equations.equations.CodeString]


## Log level recommendations¶

diagnostic
Low-level messages that are not of any interest to the normal user but useful for debugging Brian itself. A typical example is the source code generated by the code generation module.
debug
Messages that are possibly helpful for debugging the user’s code. For example, this shows which objects were included in the network, which clocks the network uses and when simulations start and stop.
info
Messages which are not strictly necessary, but are potentially helpful for the user. In particular, this will show messages about the chosen state updater and other information that might help the user to achieve better performance and/or accuracy in the simulations (e.g. using (event-driven) in synaptic equations, avoiding incompatible dt values between TimedArray and the NeuronGroup using it, …)
warn
Messages that alert the user to a potential mistake in the code, e.g. two possible solutions for an identifier in an equation. It can also be used to make the user aware that he/she is using an experimental feature, an unsupported compiler or similar. In this case, normally the once=True option should be used to raise this warning only once. As a rule of thumb, “common” scripts like the examples provided in the examples folder should normally not lead to any warnings.
error
This log level is not used currently in Brian, an exception should be raised instead. It might be useful in “meta-code”, running scripts and catching any errors that occur.

The default log level shown to the user is info. As a general rule, all messages that the user sees in the default configuration (i.e., info and warn level) should be avoidable by simple changes in the user code, e.g. the renaming of variables, explicitly specifying a state updater instead of relying on the automatic system, adding (clock-driven)/(event-driven) to synaptic equations, etc.

## Testing log messages¶

It is possible to test whether code emits an expected log message using the catch_logs context manager. This is normally not necessary for debug and info messages, but should be part of the unit tests for warning messages (catch_logs by default only catches warning and error messages):

with catch_logs() as logs:
# code that is expected to trigger a warning
# ...
assert len(logs) == 1
# logs contains tuples of (log level, name, message)
assert logs[0][0] == 'WARNING' and logs[0][1].endswith('warning_type')