Blog / Standard Library

The Attr type

July 2, 2026

Other than logger methods, the slog.Attr type is probably the thing you’ll use the most in your use of the slog package:

Types

type Attr

type Attr struct {
	Key   string
	Value Value
}

An Attr is a key-value pair.

This type has several type-specific constructors, too, but we’re going to do an abbrevaited look at them, because they’re highly repetitive.

func Any

func Any(key string, value any) Attr

Any returns an Attr for the supplied value. See AnyValue for how values are treated.

Any provides the general-purpose way to create an Attr for any type.

var foo struct  {
  Some   int
  Random string
  Fields float64
}

slog.Any("foo", foo{})
slog.Any("bool", true) // Functionally equivalent to using slog.Bool

Notably, slog.Any can be used for types that also have their own constructors, listed below. The only difference in using the type-specific constructors is that they don’t have to do any runtime type assertion.

func Bool

func Duration

func Float64

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Related Content

Attribute equality

func (Attr) Equal func (a Attr) Equal(b Attr) bool Equal reports whether a and b have equal keys and values. That’s a pretty obvious, and opaque statement. How is equality determined? We have to look at the source to determine: func (a Attr) Equal(b Attr) bool { return a.Key == b.Key && a.Value.Equal(b.Value) } Okay, so it returns true if the keys are equal (that’s simple—they’re just strings) AND if the values are equal.

slog.GroupAttrs

Last time we looked at slog.Group, which accepts the variadic ...any as arguments. But there’s also a version that accepts slog.Attr values: func GroupAttrs func GroupAttrs(key string, attrs ...Attr) Attr GroupAttrs returns an Attr for a Group Value consisting of the given Attrs. GroupAttrs is a more efficient version of Group that accepts only Attr values. But notice there’s no mixed form of this, like there is for the standard logging functions.

slog.Group

The one Attr constructor not yet covered: func Group func Group(key string, args ...any) Attr Group returns an Attr for a Group Value. The first argument is the key; the remaining arguments are converted to Attrs as in Logger.Log. Use Group to collect several key-value pairs under a single key on a log line, or as the result of LogValue in order to log a single value as multiple Attrs.

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