Logging.jl

@debug message  [key=value | value ...]
@info  message  [key=value | value ...]
@warn  message  [key=value | value ...]
@error message  [key=value | value ...]

@logmsg level message [key=value | value ...]

Create a log record with an informational message. For convenience, four logging macros @debug, @info, @warn and @error are defined which log at the standard severity levels Debug, Info, Warn and Error. @logmsg allows level to be set programmatically to any LogLevel or custom log level types.

message should be an expression which evaluates to a string which is a human readable description of the log event. By convention, this string will be formatted as markdown when presented.

The optional list of key=value pairs supports arbitrary user defined metadata which will be passed through to the logging backend as part of the log record. If only a value expression is supplied, a key representing the expression will be generated using Symbol. For example, x becomes x=x, and foo(10) becomes Symbol("foo(10)")=foo(10). For splatting a list of key value pairs, use the normal splatting syntax, @info "blah" kws....

There are some keys which allow automatically generated log data to be overridden:

• _module=mod can be used to specify a different originating module from the source location of the message.
• _group=symbol can be used to override the message group (this is normally derived from the base name of the source file).
• _id=symbol can be used to override the automatically generated unique message identifier. This is useful if you need to very closely associate messages generated on different source lines.
• _file=string and _line=integer can be used to override the apparent source location of a log message.

There's also some key value pairs which have conventional meaning:

• maxlog=integer should be used as a hint to the backend that the message should be displayed no more than maxlog times.
• exception=ex should be used to transport an exception with a log message, often used with @error. An associated backtrace bt may be attached using the tuple exception=(ex,bt).

Examples

@debug "Verbose debugging information.  Invisible by default"
@info  "An informational message"
@warn  "Something was odd.  You should pay attention"
@error "A non fatal error occurred"

x = 10
@info "Some variables attached to the message" x a=42.0

@debug begin
    sA = sum(A)
    "sum(A) = $sA is an expensive operation, evaluated only when `shouldlog` returns true"
end

for i=1:10000
    @info "With the default backend, you will only see (i = $i) ten times"  maxlog=10
    @debug "Algorithm1" i progress=i/10000
end

SimpleLogger(stream=stderr, min_level=Info)

Simplistic logger for logging all messages with level greater than or equal to min_level to stream.

ConsoleLogger(stream=stderr, min_level=Info; meta_formatter=default_metafmt,
              show_limited=true, right_justify=0)

Logger with formatting optimized for readability in a text console, for example interactive work with the Julia REPL.

Log levels less than min_level are filtered out.

Message formatting can be controlled by setting keyword arguments:

• meta_formatter is a function which takes the log event metadata (level, _module, group, id, file, line) and returns a color (as would be passed to printstyled), prefix and suffix for the log message. The default is to prefix with the log level and a suffix containing the module, file and line location.
• show_limited limits the printing of large data structures to something which can fit on the screen by setting the :limit IOContext key during formatting.
• right_justify is the integer column which log metadata is right justified at. The default is zero (metadata goes on its own line).