Better logging for go
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About this package

This package is designed to support better logging for Go. Specifically, this project aims to support different levels of logging and the ability to customize log output via custom implementations of the interfaces provided in the package. In addition, all logged messages are wrapped in closures and are only evaluated and rendered if they will be outputed.

You can install this package via:


Using this package

Introductory usage

The easiest way to start using this package is to use the Global PackageLogger and the exported global namespace wrapper functions. For example:

package mypackage

import ""

func Foo() {
    golog.Info("Hello, world")
    golog.Warningf("Error %d", 4)
    golog.Errorc(func() { return verySlowStringFunction() })
    golog.Fatal("Error opening file:", err)

The Global PackageLogger outputs to default files set by flags. For example, to log to stderr and to temp.log at log level WARNING, invoke the binary using this package as follows: ./mybinary --golog.logfile=/dev/stderr --golog.logfile=temp.log --golog.minloglevel=1

This package also makes it easy to log to a testing harness in addition to files. To do this, invoke StartTestLogging(t) at the start of every test and StopTestLogging() at the end. For example:

package mypackage

import (

func TestFoo(t *testing.T) {
    golog.StartTestLogging(t); defer golog.StopTestLogging()

    // Test the Foo() function.

While in test logging mode, calls to Fatal() (and DefaultLogger.FailNow()) will call testing.(*T).FailNow() rather than exiting the program abruptly.

Another common way to use this pacakge is to create a local PackageLogger. This can either be declared on the package level or passed in by value.

Advanced usage

This package is highly modular and configurable; different components can be plugged in to modify the behavior. For example, to speed up logging, an advanced user could try creating a LocationLogger using the NoLocation function, or even create a custom location function.

Advanced users can further take advantage of the modularity of the package to implement and control individual parts. For example, logging in XML format should be done by writing a proper LogOuter.

Understanding this package

This package was designed to be highly modular, with different interfaces for each logical component. The important types are:

  • A LogMessage is a logged message with associated metadata.

  • A LogOuter controls the formatted output of a LogMessage.

  • A MultiLogOuter multiplexes an outputted message to a set of keyed LogOuters. The associated MultiLogOuterFlag automatically add logfiles to the associated set of LogOuters.

  • A Logger decides whether or not to log a message, and if so renders the message and outputs it.

  • A LocationLogger is a wrapper for a Logger that generates a closure to return a LogMessage with the associate metadata and is the first easily usable entrypoint into this package.

  • A StringLogger is a wrapper for a LocationLogger that exports methods for logging at semantic levels.

  • A PackageLogger has a set of functions designed be quickly useful and is the expected entry point into this package.

For additional documenation, see the godoc output for this package.