log/slog: Logger.With loses handler

[andig]

What version of Go are you using (go version)?

$ go version
go version go1.24.1 darwin/arm64

Does this issue reproduce with the latest release?

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
AR='ar'
CC='cc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='c++'
GCCGO='gccgo'
GO111MODULE=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/andig/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/andig/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/sv/rs_453y57xj86xsbz3kw1mbc0000gn/T/go-build460389641=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/andig/htdocs/slog/go.mod'
GOMODCACHE='/Users/andig/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/andig/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.24.1/libexec'
GOSUMDB='sum.golang.org'
GOTELEMETRY='on'
GOTELEMETRYDIR='/Users/andig/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.24.1/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.24.1'
GOWORK=''
PKG_CONFIG='pkg-config'
GOROOT/bin/go version: go version go1.24.1 darwin/arm64
GOROOT/bin/go tool compile -V: compile version go1.24.1
uname -v: Darwin Kernel Version 24.3.0: Thu Jan  2 20:24:06 PST 2025; root:xnu-11215.81.4~3/RELEASE_ARM64_T8103
ProductName:		macOS
ProductVersion:		15.3.2
BuildVersion:		24D81
lldb --version: lldb-1600.0.39.109
Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)

What did you do?

Consider this play: https://play.golang.com/p/wqZSGx6tIXi

What did you expect to see?

A logger created with custom handler like

l1 := slog.New(handler)

should maintain this handler when creating a derived logger with

l2 := l1.With(attr...)

What did you see instead?

Deriving an slog.Logger using logger.With(...) loses the loggers handler. Instead, a default slog.TextHandler is assigned.

This is surprising and not documented: https://pkg.go.dev/log/slog#Logger.With.

[seankhliao]

With corresponds to WithAttrs in the Handler interface: https://pkg.go.dev/log/slog#Handler

Unlike many projects, the Go project does not use GitHub Issues for general discussion or asking questions. GitHub Issues are used for tracking bugs and proposals only.

For questions please refer to https://github.com/golang/go/wiki/Questions

[gabyhelp]

Related Issues

• log/slog: Custom Handler breaks when using With() and loses parent attributes #71116 (closed)
• slog/log: Concurrent writes to TextHandler and JSONHandler lose log lines. #61321 (closed)
• log/slog: logger discards errors from handler #66579 (closed)
• log/slog: panic: bad kind: 9 in JSONHandler Handler().WithAttrs(LogValuer{}) #59727 (closed)
• log/slog: slog sublog is lost when using a custom handler #65522 (closed)
• log/slog: inconsistent handling of empty groups #61067 (closed)
• testing/slogtest: Parsing example fails when level is Error or Warn #62115 (closed)
• x/exp/slog: TextHandler panics on nil arg #58652 (closed)
• log/slog: LogValue not called pointer receiver vs value #60962 (closed)
• log/slog: log.Fatal() reports wrong source location when both log.Lshortfile or log.Llongfile and slog with source is enabled #67362 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[andig]

With corresponds to WithAttrs in the Handler interface: https://pkg.go.dev/log/slog#Handler

@seankhliao I absolutely appreciate curt answers, but the docs don't convey this. They suggest a logger with additional attributes, not a default logger with attributes. That this may be similar to the handler is absolutely not clear.

Also Handler.WithAttrs is still the same handler, at least that's what I'd expect. For Logger it's not, so not sure what the answer applies here.

Please reopen. Thank you!

[andig]

I understand why this happens but it should be mentioned in the documentation of Logger.With.

[jba]

What should the doc for Logger.With say? Whatever contract it has with its handler it not something that the user of slog cares about.

I think the warning about embedding one handler in another should be in https://github.com/golang/example/blob/master/slog-handler-guide/README.md, and also a reference to that doc should appear in the doc for slog.Handler, not just the package doc.

[jba]

Also, testing/slogtest should catch this problem. Does it?

[gopherbot]

Change https://go.dev/cl/661015 mentions this issue: log/slog: Handler doc points to handler guide

[gopherbot]

Change https://go.dev/cl/660956 mentions this issue: slog-handler-guide: warn against embedding