Blog / Standard Library

The default handler

April 7, 2026

Much like the older log package, log/slog ships with a “default logger”. You can use this logger without doing any configuration, just by calling any of the package-level logging functions:

slog.Error("oh noes!")

While you’d probably never want to use the default logger in a serious server application, it can be a convenience for small or throw-away utilities. But how does it work? That’s today’s topic!

Overview

…

The default handler formats the log record’s message, time, level, and attributes as a string and passes it to the log package.

2022/11/08 15:28:26 INFO hello count=3

So that’s doubly convenient—not only does it log things in a reasonable format, but it also passes them to the log package for backward compatibility. This means that if your application already takes advantage of the default log package’s logger, you don’t need to do any additional configuration to start using log/slog. Sweet!

But wait… how does the default log package’s logger work? Well, it’s pretty limited (which is, of course, largely why log/slog was added to replace it!) By default, it simply writes to STDERR. Though you can configure the default log.Logger… a little bit—mainly by redirecting where those logs go (e.g. to a file). But that’s a good segue into the default logger configuration we do care about—you can also configure the log/slog default logger, which we’ll talk about tomorrow!

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Test logging with ReplaceAttr

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Handler level

Each handler has an optional Level value, which controls which logs to drop, and which to actually log: type HandlerOptions type HandlerOptions struct { … // Level reports the minimum record level that will be logged. // The handler discards records with lower levels. // If Level is nil, the handler assumes LevelInfo. // The handler calls Level.Level for each record processed; // to adjust the minimum level dynamically, use a LevelVar.

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