Involved Source Filesattr.go Package slog provides structured logging,
in which log records include a message,
a severity level, and various other attributes
expressed as key-value pairs.
It defines a type, [Logger],
which provides several methods (such as [Logger.Info] and [Logger.Error])
for reporting events of interest.
Each Logger is associated with a [Handler].
A Logger output method creates a [Record] from the method arguments
and passes it to the Handler, which decides how to handle it.
There is a default Logger accessible through top-level functions
(such as [Info] and [Error]) that call the corresponding Logger methods.
A log record consists of a time, a level, a message, and a set of key-value
pairs, where the keys are strings and the values may be of any type.
As an example,
slog.Info("hello", "count", 3)
creates a record containing the time of the call,
a level of Info, the message "hello", and a single
pair with key "count" and value 3.
The [Info] top-level function calls the [Logger.Info] method on the default Logger.
In addition to [Logger.Info], there are methods for Debug, Warn and Error levels.
Besides these convenience methods for common levels,
there is also a [Logger.Log] method which takes the level as an argument.
Each of these methods has a corresponding top-level function that uses the
default logger.
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
For more control over the output format, create a logger with a different handler.
This statement uses [New] to create a new logger with a [TextHandler]
that writes structured records in text form to standard error:
logger := slog.New(slog.NewTextHandler(os.Stderr, nil))
[TextHandler] output is a sequence of key=value pairs, easily and unambiguously
parsed by machine. This statement:
logger.Info("hello", "count", 3)
produces this output:
time=2022-11-08T15:28:26.000-05:00 level=INFO msg=hello count=3
The package also provides [JSONHandler], whose output is line-delimited JSON:
logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))
logger.Info("hello", "count", 3)
produces this output:
{"time":"2022-11-08T15:28:26.000000000-05:00","level":"INFO","msg":"hello","count":3}
Both [TextHandler] and [JSONHandler] can be configured with [HandlerOptions].
There are options for setting the minimum level (see Levels, below),
displaying the source file and line of the log call, and
modifying attributes before they are logged.
Setting a logger as the default with
slog.SetDefault(logger)
will cause the top-level functions like [Info] to use it.
[SetDefault] also updates the default logger used by the [log] package,
so that existing applications that use [log.Printf] and related functions
will send log records to the logger's handler without needing to be rewritten.
Some attributes are common to many log calls.
For example, you may wish to include the URL or trace identifier of a server request
with all log events arising from the request.
Rather than repeat the attribute with every log call, you can use [Logger.With]
to construct a new Logger containing the attributes:
logger2 := logger.With("url", r.URL)
The arguments to With are the same key-value pairs used in [Logger.Info].
The result is a new Logger with the same handler as the original, but additional
attributes that will appear in the output of every call.
# Levels
A [Level] is an integer representing the importance or severity of a log event.
The higher the level, the more severe the event.
This package defines constants for the most common levels,
but any int can be used as a level.
In an application, you may wish to log messages only at a certain level or greater.
One common configuration is to log messages at Info or higher levels,
suppressing debug logging until it is needed.
The built-in handlers can be configured with the minimum level to output by
setting [HandlerOptions.Level].
The program's `main` function typically does this.
The default value is LevelInfo.
Setting the [HandlerOptions.Level] field to a [Level] value
fixes the handler's minimum level throughout its lifetime.
Setting it to a [LevelVar] allows the level to be varied dynamically.
A LevelVar holds a Level and is safe to read or write from multiple
goroutines.
To vary the level dynamically for an entire program, first initialize
a global LevelVar:
var programLevel = new(slog.LevelVar) // Info by default
Then use the LevelVar to construct a handler, and make it the default:
h := slog.NewJSONHandler(os.Stderr, &slog.HandlerOptions{Level: programLevel})
slog.SetDefault(slog.New(h))
Now the program can change its logging level with a single statement:
programLevel.Set(slog.LevelDebug)
# Groups
Attributes can be collected into groups.
A group has a name that is used to qualify the names of its attributes.
How this qualification is displayed depends on the handler.
[TextHandler] separates the group and attribute names with a dot.
[JSONHandler] treats each group as a separate JSON object, with the group name as the key.
Use [Group] to create a Group attribute from a name and a list of key-value pairs:
slog.Group("request",
"method", r.Method,
"url", r.URL)
TextHandler would display this group as
request.method=GET request.url=http://example.com
JSONHandler would display it as
"request":{"method":"GET","url":"http://example.com"}
Use [Logger.WithGroup] to qualify all of a Logger's output
with a group name. Calling WithGroup on a Logger results in a
new Logger with the same Handler as the original, but with all
its attributes qualified by the group name.
This can help prevent duplicate attribute keys in large systems,
where subsystems might use the same keys.
Pass each subsystem a different Logger with its own group name so that
potential duplicates are qualified:
logger := slog.Default().With("id", systemID)
parserLogger := logger.WithGroup("parser")
parseInput(input, parserLogger)
When parseInput logs with parserLogger, its keys will be qualified with "parser",
so even if it uses the common key "id", the log line will have distinct keys.
# Contexts
Some handlers may wish to include information from the [context.Context] that is
available at the call site. One example of such information
is the identifier for the current span when tracing is enabled.
The [Logger.Log] and [Logger.LogAttrs] methods take a context as a first
argument, as do their corresponding top-level functions.
Although the convenience methods on Logger (Info and so on) and the
corresponding top-level functions do not take a context, the alternatives ending
in "Context" do. For example,
slog.InfoContext(ctx, "message")
It is recommended to pass a context to an output method if one is available.
# Attrs and Values
An [Attr] is a key-value pair. The Logger output methods accept Attrs as well as
alternating keys and values. The statement
slog.Info("hello", slog.Int("count", 3))
behaves the same as
slog.Info("hello", "count", 3)
There are convenience constructors for [Attr] such as [Int], [String], and [Bool]
for common types, as well as the function [Any] for constructing Attrs of any
type.
The value part of an Attr is a type called [Value].
Like an [any], a Value can hold any Go value,
but it can represent typical values, including all numbers and strings,
without an allocation.
For the most efficient log output, use [Logger.LogAttrs].
It is similar to [Logger.Log] but accepts only Attrs, not alternating
keys and values; this allows it, too, to avoid allocation.
The call
logger.LogAttrs(ctx, slog.LevelInfo, "hello", slog.Int("count", 3))
is the most efficient way to achieve the same output as
slog.InfoContext(ctx, "hello", "count", 3)
# Customizing a type's logging behavior
If a type implements the [LogValuer] interface, the [Value] returned from its LogValue
method is used for logging. You can use this to control how values of the type
appear in logs. For example, you can redact secret information like passwords,
or gather a struct's fields in a Group. See the examples under [LogValuer] for
details.
A LogValue method may return a Value that itself implements [LogValuer]. The [Value.Resolve]
method handles these cases carefully, avoiding infinite loops and unbounded recursion.
Handler authors and others may wish to use [Value.Resolve] instead of calling LogValue directly.
# Wrapping output methods
The logger functions use reflection over the call stack to find the file name
and line number of the logging call within the application. This can produce
incorrect source information for functions that wrap slog. For instance, if you
define this function in file mylog.go:
func Infof(logger *slog.Logger, format string, args ...any) {
logger.Info(fmt.Sprintf(format, args...))
}
and you call it like this in main.go:
Infof(slog.Default(), "hello, %s", "world")
then slog will report the source file as mylog.go, not main.go.
A correct implementation of Infof will obtain the source location
(pc) and pass it to NewRecord.
The Infof function in the package-level example called "wrapping"
demonstrates how to do this.
# Working with Records
Sometimes a Handler will need to modify a Record
before passing it on to another Handler or backend.
A Record contains a mixture of simple public fields (e.g. Time, Level, Message)
and hidden fields that refer to state (such as attributes) indirectly. This
means that modifying a simple copy of a Record (e.g. by calling
[Record.Add] or [Record.AddAttrs] to add attributes)
may have unexpected effects on the original.
Before modifying a Record, use [Record.Clone] to
create a copy that shares no state with the original,
or create a new Record with [NewRecord]
and build up its Attrs by traversing the old ones with [Record.Attrs].
# Performance considerations
If profiling your application demonstrates that logging is taking significant time,
the following suggestions may help.
If many log lines have a common attribute, use [Logger.With] to create a Logger with
that attribute. The built-in handlers will format that attribute only once, at the
call to [Logger.With]. The [Handler] interface is designed to allow that optimization,
and a well-written Handler should take advantage of it.
The arguments to a log call are always evaluated, even if the log event is discarded.
If possible, defer computation so that it happens only if the value is actually logged.
For example, consider the call
slog.Info("starting request", "url", r.URL.String()) // may compute String unnecessarily
The URL.String method will be called even if the logger discards Info-level events.
Instead, pass the URL directly:
slog.Info("starting request", "url", &r.URL) // calls URL.String only if needed
The built-in [TextHandler] will call its String method, but only
if the log event is enabled.
Avoiding the call to String also preserves the structure of the underlying value.
For example [JSONHandler] emits the components of the parsed URL as a JSON object.
If you want to avoid eagerly paying the cost of the String call
without causing the handler to potentially inspect the structure of the value,
wrap the value in a fmt.Stringer implementation that hides its Marshal methods.
You can also use the [LogValuer] interface to avoid unnecessary work in disabled log
calls. Say you need to log some expensive value:
slog.Debug("frobbing", "value", computeExpensiveValue(arg))
Even if this line is disabled, computeExpensiveValue will be called.
To avoid that, define a type implementing LogValuer:
type expensive struct { arg int }
func (e expensive) LogValue() slog.Value {
return slog.AnyValue(computeExpensiveValue(e.arg))
}
Then use a value of that type in log calls:
slog.Debug("frobbing", "value", expensive{arg})
Now computeExpensiveValue will only be called when the line is enabled.
The built-in handlers acquire a lock before calling [io.Writer.Write]
to ensure that each record is written in one piece. User-defined
handlers are responsible for their own locking.
# Writing a handler
For a guide to writing a custom handler, see https://golang.org/s/slog-handler-guide.handler.gojson_handler.golevel.gologger.gorecord.gotext_handler.govalue.go
Code Examples
package main
import (
"log/slog"
"net/http"
"os"
"time"
)
func main() {
r, _ := http.NewRequest("GET", "localhost", nil)
// ...
logger := slog.New(
slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{
ReplaceAttr: func(groups []string, a slog.Attr) slog.Attr {
if a.Key == slog.TimeKey && len(groups) == 0 {
return slog.Attr{}
}
return a
},
}),
)
logger.Info("finished",
slog.Group("req",
slog.String("method", r.Method),
slog.String("url", r.URL.String())),
slog.Int("status", http.StatusOK),
slog.Duration("duration", time.Second))
}
package main
import (
"context"
"log/slog"
"os"
)
func main() {
// Exported constants from a custom logging package.
const (
LevelTrace = slog.Level(-8)
LevelDebug = slog.LevelDebug
LevelInfo = slog.LevelInfo
LevelNotice = slog.Level(2)
LevelWarning = slog.LevelWarn
LevelError = slog.LevelError
LevelEmergency = slog.Level(12)
)
th := slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{
// Set a custom level to show all log output. The default value is
// LevelInfo, which would drop Debug and Trace logs.
Level: LevelTrace,
ReplaceAttr: func(groups []string, a slog.Attr) slog.Attr {
// Remove time from the output for predictable test output.
if a.Key == slog.TimeKey {
return slog.Attr{}
}
// Customize the name of the level key and the output string, including
// custom level values.
if a.Key == slog.LevelKey {
// Rename the level key from "level" to "sev".
a.Key = "sev"
// Handle custom level values.
level := a.Value.Any().(slog.Level)
// This could also look up the name from a map or other structure, but
// this demonstrates using a switch statement to rename levels. For
// maximum performance, the string values should be constants, but this
// example uses the raw strings for readability.
switch {
case level < LevelDebug:
a.Value = slog.StringValue("TRACE")
case level < LevelInfo:
a.Value = slog.StringValue("DEBUG")
case level < LevelNotice:
a.Value = slog.StringValue("INFO")
case level < LevelWarning:
a.Value = slog.StringValue("NOTICE")
case level < LevelError:
a.Value = slog.StringValue("WARNING")
case level < LevelEmergency:
a.Value = slog.StringValue("ERROR")
default:
a.Value = slog.StringValue("EMERGENCY")
}
}
return a
},
})
logger := slog.New(th)
ctx := context.Background()
logger.Log(ctx, LevelEmergency, "missing pilots")
logger.Error("failed to start engines", "err", "missing fuel")
logger.Warn("falling back to default value")
logger.Log(ctx, LevelNotice, "all systems are running")
logger.Info("initiating launch")
logger.Debug("starting background job")
logger.Log(ctx, LevelTrace, "button clicked")
}
package main
import (
"context"
"log/slog"
"log/slog/internal/slogtest"
"os"
)
// A LevelHandler wraps a Handler with an Enabled method
// that returns false for levels below a minimum.
type LevelHandler struct {
level slog.Leveler
handler slog.Handler
}
// NewLevelHandler returns a LevelHandler with the given level.
// All methods except Enabled delegate to h.
func NewLevelHandler(level slog.Leveler, h slog.Handler) *LevelHandler {
// Optimization: avoid chains of LevelHandlers.
if lh, ok := h.(*LevelHandler); ok {
h = lh.Handler()
}
return &LevelHandler{level, h}
}
// Enabled implements Handler.Enabled by reporting whether
// level is at least as large as h's level.
func (h *LevelHandler) Enabled(_ context.Context, level slog.Level) bool {
return level >= h.level.Level()
}
// Handle implements Handler.Handle.
func (h *LevelHandler) Handle(ctx context.Context, r slog.Record) error {
return h.handler.Handle(ctx, r)
}
// WithAttrs implements Handler.WithAttrs.
func (h *LevelHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
return NewLevelHandler(h.level, h.handler.WithAttrs(attrs))
}
// WithGroup implements Handler.WithGroup.
func (h *LevelHandler) WithGroup(name string) slog.Handler {
return NewLevelHandler(h.level, h.handler.WithGroup(name))
}
// Handler returns the Handler wrapped by h.
func (h *LevelHandler) Handler() slog.Handler {
return h.handler
}
// This example shows how to Use a LevelHandler to change the level of an
// existing Handler while preserving its other behavior.
//
// This example demonstrates increasing the log level to reduce a logger's
// output.
//
// Another typical use would be to decrease the log level (to LevelDebug, say)
// during a part of the program that was suspected of containing a bug.
func main() {
th := slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{ReplaceAttr: slogtest.RemoveTime})
logger := slog.New(NewLevelHandler(slog.LevelWarn, th))
logger.Info("not printed")
logger.Warn("printed")
}
package main
import "log/slog"
type Name struct {
First, Last string
}
// LogValue implements slog.LogValuer.
// It returns a group containing the fields of
// the Name, so that they appear together in the log output.
func (n Name) LogValue() slog.Value {
return slog.GroupValue(
slog.String("first", n.First),
slog.String("last", n.Last))
}
func main() {
n := Name{"Perry", "Platypus"}
slog.Info("mission accomplished", "agent", n)
// JSON Output would look in part like:
// {
// ...
// "msg": "mission accomplished",
// "agent": {
// "first": "Perry",
// "last": "Platypus"
// }
// }
}
package main
import (
"log/slog"
"log/slog/internal/slogtest"
"os"
)
// A token is a secret value that grants permissions.
type Token string
// LogValue implements slog.LogValuer.
// It avoids revealing the token.
func (Token) LogValue() slog.Value {
return slog.StringValue("REDACTED_TOKEN")
}
// This example demonstrates a Value that replaces itself
// with an alternative representation to avoid revealing secrets.
func main() {
t := Token("shhhh!")
logger := slog.New(slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{ReplaceAttr: slogtest.RemoveTime}))
logger.Info("permission granted", "user", "Perry", "token", t)
}
package main
import (
"log"
"log/slog"
"os"
)
func main() {
defer log.SetFlags(log.Flags()) // revert changes after the example
log.SetFlags(0)
defer log.SetOutput(log.Writer()) // revert changes after the example
log.SetOutput(os.Stdout)
// Default logging level is slog.LevelInfo.
log.Print("log debug") // log debug
slog.Debug("debug") // no output
slog.Info("info") // INFO info
// Set the default logging level to slog.LevelDebug.
currentLogLevel := slog.SetLogLoggerLevel(slog.LevelDebug)
defer slog.SetLogLoggerLevel(currentLogLevel) // revert changes after the example
log.Print("log debug") // log debug
slog.Debug("debug") // DEBUG debug
slog.Info("info") // INFO info
}
package main
import (
"log"
"log/slog"
"log/slog/internal/slogtest"
"os"
)
func main() {
// Set the default logging level to slog.LevelError.
currentLogLevel := slog.SetLogLoggerLevel(slog.LevelError)
defer slog.SetLogLoggerLevel(currentLogLevel) // revert changes after the example
defer slog.SetDefault(slog.Default()) // revert changes after the example
slog.SetDefault(slog.New(slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{ReplaceAttr: slogtest.RemoveTime})))
log.Print("error") // level=ERROR msg=error
}
package main
import (
"context"
"fmt"
"log/slog"
"os"
"path/filepath"
"runtime"
"time"
)
// Infof is an example of a user-defined logging function that wraps slog.
// The log record contains the source position of the caller of Infof.
func Infof(logger *slog.Logger, format string, args ...any) {
if !logger.Enabled(context.Background(), slog.LevelInfo) {
return
}
var pcs [1]uintptr
runtime.Callers(2, pcs[:]) // skip [Callers, Infof]
r := slog.NewRecord(time.Now(), slog.LevelInfo, fmt.Sprintf(format, args...), pcs[0])
_ = logger.Handler().Handle(context.Background(), r)
}
func main() {
replace := func(groups []string, a slog.Attr) slog.Attr {
// Remove time.
if a.Key == slog.TimeKey && len(groups) == 0 {
return slog.Attr{}
}
// Remove the directory from the source's filename.
if a.Key == slog.SourceKey {
source := a.Value.Any().(*slog.Source)
source.File = filepath.Base(source.File)
}
return a
}
logger := slog.New(slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{AddSource: true, ReplaceAttr: replace}))
Infof(logger, "message, %s", "formatted")
}
A Handler handles log records produced by a Logger.
A typical handler may print log records to standard error,
or write them to a file or database, or perhaps augment them
with additional attributes and pass them on to another handler.
Any of the Handler's methods may be called concurrently with itself
or with other methods. It is the responsibility of the Handler to
manage this concurrency.
Users of the slog package should not invoke Handler methods directly.
They should use the methods of [Logger] instead. Enabled reports whether the handler handles records at the given level.
The handler ignores records whose level is lower.
It is called early, before any arguments are processed,
to save effort if the log event should be discarded.
If called from a Logger method, the first argument is the context
passed to that method, or context.Background() if nil was passed
or the method does not take a context.
The context is passed so Enabled can use its values
to make a decision. Handle handles the Record.
It will only be called when Enabled returns true.
The Context argument is as for Enabled.
It is present solely to provide Handlers access to the context's values.
Canceling the context should not affect record processing.
(Among other things, log messages may be necessary to debug a
cancellation-related problem.)
Handle methods that produce output should observe the following rules:
- If r.Time is the zero time, ignore the time.
- If r.PC is zero, ignore it.
- Attr's values should be resolved.
- If an Attr's key and value are both the zero value, ignore the Attr.
This can be tested with attr.Equal(Attr{}).
- If a group's key is empty, inline the group's Attrs.
- If a group has no Attrs (even if it has a non-empty key),
ignore it. WithAttrs returns a new Handler whose attributes consist of
both the receiver's attributes and the arguments.
The Handler owns the slice: it may retain, modify or discard it. WithGroup returns a new Handler with the given group appended to
the receiver's existing groups.
The keys of all subsequent attributes, whether added by With or in a
Record, should be qualified by the sequence of group names.
How this qualification happens is up to the Handler, so long as
this Handler's attribute keys differ from those of another Handler
with a different sequence of group names.
A Handler should treat WithGroup as starting a Group of Attrs that ends
at the end of the log event. That is,
logger.WithGroup("s").LogAttrs(ctx, level, msg, slog.Int("a", 1), slog.Int("b", 2))
should behave like
logger.LogAttrs(ctx, level, msg, slog.Group("s", slog.Int("a", 1), slog.Int("b", 2)))
If the name is empty, WithGroup returns the receiver.
*JSONHandler
*TextHandler
func Handler.WithAttrs(attrs []Attr) Handler
func Handler.WithGroup(name string) Handler
func (*JSONHandler).WithAttrs(attrs []Attr) Handler
func (*JSONHandler).WithGroup(name string) Handler
func (*Logger).Handler() Handler
func (*TextHandler).WithAttrs(attrs []Attr) Handler
func (*TextHandler).WithGroup(name string) Handler
func New(h Handler) *Logger
func NewLogLogger(h Handler, level Level) *log.Logger
func testing/slogtest.TestHandler(h Handler, results func() []map[string]any) error
HandlerOptions are options for a [TextHandler] or [JSONHandler].
A zero HandlerOptions consists entirely of default values. AddSource causes the handler to compute the source code position
of the log statement and add a SourceKey attribute to the output. 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. ReplaceAttr is called to rewrite each non-group attribute before it is logged.
The attribute's value has been resolved (see [Value.Resolve]).
If ReplaceAttr returns a zero Attr, the attribute is discarded.
The built-in attributes with keys "time", "level", "source", and "msg"
are passed to this function, except that time is omitted
if zero, and source is omitted if AddSource is false.
The first argument is a list of currently open groups that contain the
Attr. It must not be retained or modified. ReplaceAttr is never called
for Group attributes, only their contents. For example, the attribute
list
Int("a", 1), Group("g", Int("b", 2)), Int("c", 3)
results in consecutive calls to ReplaceAttr with the following arguments:
nil, Int("a", 1)
[]string{"g"}, Int("b", 2)
nil, Int("c", 3)
ReplaceAttr can be used to change the default keys of the built-in
attributes, convert types (for example, to replace a `time.Time` with the
integer seconds since the Unix epoch), sanitize personal information, or
remove attributes from the output.
func NewJSONHandler(w io.Writer, opts *HandlerOptions) *JSONHandler
func NewTextHandler(w io.Writer, opts *HandlerOptions) *TextHandler
JSONHandler is a [Handler] that writes Records to an [io.Writer] as
line-delimited JSON objects. Enabled reports whether the handler handles records at the given level.
The handler ignores records whose level is lower. Handle formats its argument [Record] as a JSON object on a single line.
If the Record's time is zero, the time is omitted.
Otherwise, the key is "time"
and the value is output as with json.Marshal.
If the Record's level is zero, the level is omitted.
Otherwise, the key is "level"
and the value of [Level.String] is output.
If the AddSource option is set and source information is available,
the key is "source", and the value is a record of type [Source].
The message's key is "msg".
To modify these or other attributes, or remove them from the output, use
[HandlerOptions.ReplaceAttr].
Values are formatted as with an [encoding/json.Encoder] with SetEscapeHTML(false),
with two exceptions.
First, an Attr whose Value is of type error is formatted as a string, by
calling its Error method. Only errors in Attrs receive this special treatment,
not errors embedded in structs, slices, maps or other data structures that
are processed by the [encoding/json] package.
Second, an encoding failure does not cause Handle to return an error.
Instead, the error message is formatted as a string.
Each call to Handle results in a single serialized call to io.Writer.Write. WithAttrs returns a new [JSONHandler] whose attributes consists
of h's attributes followed by attrs.(*JSONHandler) WithGroup(name string) Handler
*JSONHandler : Handler
func NewJSONHandler(w io.Writer, opts *HandlerOptions) *JSONHandler
A Leveler provides a [Level] value.
As Level itself implements Leveler, clients typically supply
a Level value wherever a Leveler is needed, such as in [HandlerOptions].
Clients who need to vary the level dynamically can provide a more complex
Leveler implementation such as *[LevelVar].( Leveler) Level() LevelLevel
*LevelVar
A Logger records structured information about each call to its
Log, Debug, Info, Warn, and Error methods.
For each call, it creates a [Record] and passes it to a [Handler].
To create a new Logger, call [New] or a Logger method
that begins "With". Debug logs at [LevelDebug]. DebugContext logs at [LevelDebug] with the given context. Enabled reports whether l emits log records at the given context and level. Error logs at [LevelError]. ErrorContext logs at [LevelError] with the given context. Handler returns l's Handler. Info logs at [LevelInfo]. InfoContext logs at [LevelInfo] with the given context. Log emits a log record with the current time and the given level and message.
The Record's Attrs consist of the Logger's attributes followed by
the Attrs specified by args.
The attribute arguments are processed as follows:
- If an argument is an Attr, it is used as is.
- If an argument is a string and this is not the last argument,
the following argument is treated as the value and the two are combined
into an Attr.
- Otherwise, the argument is treated as a value with key "!BADKEY". LogAttrs is a more efficient version of [Logger.Log] that accepts only Attrs. Warn logs at [LevelWarn]. WarnContext logs at [LevelWarn] with the given context. With returns a Logger that includes the given attributes
in each output operation. Arguments are converted to
attributes as if by [Logger.Log]. WithGroup returns a Logger that starts a group, if name is non-empty.
The keys of all attributes added to the Logger will be qualified by the given
name. (How that qualification happens depends on the [Handler.WithGroup]
method of the Logger's Handler.)
If name is empty, WithGroup returns the receiver.
func Default() *Logger
func New(h Handler) *Logger
func With(args ...any) *Logger
func (*Logger).With(args ...any) *Logger
func (*Logger).WithGroup(name string) *Logger
func SetDefault(l *Logger)
A LogValuer is any Go value that can convert itself into a Value for logging.
This mechanism may be used to defer expensive operations until they are
needed, or to expand a single value into a sequence of components.( LogValuer) LogValue() Value
func Value.LogValuer() LogValuer
A Record holds information about a log event.
Copies of a Record share state.
Do not modify a Record after handing out a copy to it.
Call [NewRecord] to create a new Record.
Use [Record.Clone] to create a copy with no shared state. The level of the event. The log message. The program counter at the time the record was constructed, as determined
by runtime.Callers. If zero, no program counter is available.
The only valid use for this value is as an argument to
[runtime.CallersFrames]. In particular, it must not be passed to
[runtime.FuncForPC]. The time at which the output method (Log, Info, etc.) was called. Add converts the args to Attrs as described in [Logger.Log],
then appends the Attrs to the [Record]'s list of Attrs.
It omits empty groups. AddAttrs appends the given Attrs to the [Record]'s list of Attrs.
It omits empty groups. Attrs calls f on each Attr in the [Record].
Iteration stops if f returns false. Clone returns a copy of the record with no shared state.
The original record and the clone can both be modified
without interfering with each other. NumAttrs returns the number of attributes in the [Record].
func NewRecord(t time.Time, level Level, msg string, pc uintptr) Record
func Record.Clone() Record
func Handler.Handle(context.Context, Record) error
func (*JSONHandler).Handle(_ context.Context, r Record) error
func (*TextHandler).Handle(_ context.Context, r Record) error
Source describes the location of a line of source code. File and Line are the file name and line number (1-based) of the source
line. These may be the empty string and zero, respectively, if not known. Function is the package path-qualified function name containing the
source line. If non-empty, this string uniquely identifies a single
function in the program. This may be the empty string if not known.Lineint
TextHandler is a [Handler] that writes Records to an [io.Writer] as a
sequence of key=value pairs separated by spaces and followed by a newline. Enabled reports whether the handler handles records at the given level.
The handler ignores records whose level is lower. Handle formats its argument [Record] as a single line of space-separated
key=value items.
If the Record's time is zero, the time is omitted.
Otherwise, the key is "time"
and the value is output in RFC3339 format with millisecond precision.
If the Record's level is zero, the level is omitted.
Otherwise, the key is "level"
and the value of [Level.String] is output.
If the AddSource option is set and source information is available,
the key is "source" and the value is output as FILE:LINE.
The message's key is "msg".
To modify these or other attributes, or remove them from the output, use
[HandlerOptions.ReplaceAttr].
If a value implements [encoding.TextMarshaler], the result of MarshalText is
written. Otherwise, the result of [fmt.Sprint] is written.
Keys and values are quoted with [strconv.Quote] if they contain Unicode space
characters, non-printing characters, '"' or '='.
Keys inside groups consist of components (keys or group names) separated by
dots. No further escaping is performed.
Thus there is no way to determine from the key "a.b.c" whether there
are two groups "a" and "b" and a key "c", or a single group "a.b" and a key "c",
or single group "a" and a key "b.c".
If it is necessary to reconstruct the group structure of a key
even in the presence of dots inside components, use
[HandlerOptions.ReplaceAttr] to encode that information in the key.
Each call to Handle results in a single serialized call to
io.Writer.Write. WithAttrs returns a new [TextHandler] whose attributes consists
of h's attributes followed by attrs.(*TextHandler) WithGroup(name string) Handler
*TextHandler : Handler
func NewTextHandler(w io.Writer, opts *HandlerOptions) *TextHandler
A Value can represent any Go value, but unlike type any,
it can represent most small values without an allocation.
The zero Value corresponds to nil. Any returns v's value as an any. Bool returns v's value as a bool. It panics
if v is not a bool. Duration returns v's value as a [time.Duration]. It panics
if v is not a time.Duration. Equal reports whether v and w represent the same Go value. Float64 returns v's value as a float64. It panics
if v is not a float64. Group returns v's value as a []Attr.
It panics if v's [Kind] is not [KindGroup]. Int64 returns v's value as an int64. It panics
if v is not a signed integer. Kind returns v's Kind. LogValuer returns v's value as a LogValuer. It panics
if v is not a LogValuer. Resolve repeatedly calls LogValue on v while it implements [LogValuer],
and returns the result.
If v resolves to a group, the group's attributes' values are not recursively
resolved.
If the number of LogValue calls exceeds a threshold, a Value containing an
error is returned.
Resolve's return value is guaranteed not to be of Kind [KindLogValuer]. String returns Value's value as a string, formatted like [fmt.Sprint]. Unlike
the methods Int64, Float64, and so on, which panic if v is of the
wrong kind, String never panics. Time returns v's value as a [time.Time]. It panics
if v is not a time.Time. Uint64 returns v's value as a uint64. It panics
if v is not an unsigned integer.
Value : expvar.Var
Value : fmt.Stringer
Value : math/rand/v2.Source
func AnyValue(v any) Value
func BoolValue(v bool) Value
func DurationValue(v time.Duration) Value
func Float64Value(v float64) Value
func GroupValue(as ...Attr) Value
func Int64Value(v int64) Value
func IntValue(v int) Value
func StringValue(value string) Value
func TimeValue(v time.Time) Value
func Uint64Value(v uint64) Value
func LogValuer.LogValue() Value
func Value.Resolve() (rv Value)
func Value.Equal(w Value) bool
Package-Level Functions (total 39)
Any returns an Attr for the supplied value.
See [AnyValue] for how values are treated.
AnyValue returns a [Value] for the supplied value.
If the supplied value is of type Value, it is returned
unmodified.
Given a value of one of Go's predeclared string, bool, or
(non-complex) numeric types, AnyValue returns a Value of kind
[KindString], [KindBool], [KindUint64], [KindInt64], or [KindFloat64].
The width of the original numeric type is not preserved.
Given a [time.Time] or [time.Duration] value, AnyValue returns a Value of kind
[KindTime] or [KindDuration]. The monotonic time is not preserved.
For nil, or values of all other types, including named types whose
underlying type is numeric, AnyValue returns a value of kind [KindAny].
Float64 returns an Attr for a floating-point number.
Float64Value returns a [Value] for a floating-point number.
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.
GroupValue returns a new [Value] for a list of Attrs.
The caller must not subsequently mutate the argument slice.
New creates a new Logger with the given non-nil Handler.
NewJSONHandler creates a [JSONHandler] that writes to w,
using the given options.
If opts is nil, the default options are used.
NewLogLogger returns a new [log.Logger] such that each call to its Output method
dispatches a Record to the specified handler. The logger acts as a bridge from
the older log API to newer structured logging handlers.
NewRecord creates a [Record] from the given arguments.
Use [Record.AddAttrs] to add attributes to the Record.
NewRecord is intended for logging APIs that want to support a [Handler] as
a backend.
NewTextHandler creates a [TextHandler] that writes to w,
using the given options.
If opts is nil, the default options are used.
SetDefault makes l the default [Logger], which is used by
the top-level functions [Info], [Debug] and so on.
After this call, output from the log package's default Logger
(as with [log.Print], etc.) will be logged using l's Handler,
at a level controlled by [SetLogLoggerLevel].
SetLogLoggerLevel controls the level for the bridge to the [log] package.
Before [SetDefault] is called, slog top-level logging functions call the default [log.Logger].
In that mode, SetLogLoggerLevel sets the minimum level for those calls.
By default, the minimum level is Info, so calls to [Debug]
(as well as top-level logging calls at lower levels)
will not be passed to the log.Logger. After calling
slog.SetLogLoggerLevel(slog.LevelDebug)
calls to [Debug] will be passed to the log.Logger.
After [SetDefault] is called, calls to the default [log.Logger] are passed to the
slog default handler. In that mode,
SetLogLoggerLevel sets the level at which those calls are logged.
That is, after calling
slog.SetLogLoggerLevel(slog.LevelDebug)
A call to [log.Printf] will result in output at level [LevelDebug].
SetLogLoggerLevel returns the previous value.
Names for common levels.
Level numbers are inherently arbitrary,
but we picked them to satisfy three constraints.
Any system can map them to another numbering scheme if it wishes.
First, we wanted the default level to be Info, Since Levels are ints, Info is
the default value for int, zero.
Second, we wanted to make it easy to use levels to specify logger verbosity.
Since a larger level means a more severe event, a logger that accepts events
with smaller (or more negative) level means a more verbose logger. Logger
verbosity is thus the negation of event severity, and the default verbosity
of 0 accepts all events at least as severe as INFO.
Third, we wanted some room between levels to accommodate schemes with named
levels between ours. For example, Google Cloud Logging defines a Notice level
between Info and Warn. Since there are only a few of these intermediate
levels, the gap between the numbers need not be large. Our gap of 4 matches
OpenTelemetry's mapping. Subtracting 9 from an OpenTelemetry level in the
DEBUG, INFO, WARN and ERROR ranges converts it to the corresponding slog
Level range. OpenTelemetry also has the names TRACE and FATAL, which slog
does not. But those OpenTelemetry levels can still be represented as slog
Levels by using the appropriate integers.
Names for common levels.
Level numbers are inherently arbitrary,
but we picked them to satisfy three constraints.
Any system can map them to another numbering scheme if it wishes.
First, we wanted the default level to be Info, Since Levels are ints, Info is
the default value for int, zero.
Second, we wanted to make it easy to use levels to specify logger verbosity.
Since a larger level means a more severe event, a logger that accepts events
with smaller (or more negative) level means a more verbose logger. Logger
verbosity is thus the negation of event severity, and the default verbosity
of 0 accepts all events at least as severe as INFO.
Third, we wanted some room between levels to accommodate schemes with named
levels between ours. For example, Google Cloud Logging defines a Notice level
between Info and Warn. Since there are only a few of these intermediate
levels, the gap between the numbers need not be large. Our gap of 4 matches
OpenTelemetry's mapping. Subtracting 9 from an OpenTelemetry level in the
DEBUG, INFO, WARN and ERROR ranges converts it to the corresponding slog
Level range. OpenTelemetry also has the names TRACE and FATAL, which slog
does not. But those OpenTelemetry levels can still be represented as slog
Levels by using the appropriate integers.
Names for common levels.
Level numbers are inherently arbitrary,
but we picked them to satisfy three constraints.
Any system can map them to another numbering scheme if it wishes.
First, we wanted the default level to be Info, Since Levels are ints, Info is
the default value for int, zero.
Second, we wanted to make it easy to use levels to specify logger verbosity.
Since a larger level means a more severe event, a logger that accepts events
with smaller (or more negative) level means a more verbose logger. Logger
verbosity is thus the negation of event severity, and the default verbosity
of 0 accepts all events at least as severe as INFO.
Third, we wanted some room between levels to accommodate schemes with named
levels between ours. For example, Google Cloud Logging defines a Notice level
between Info and Warn. Since there are only a few of these intermediate
levels, the gap between the numbers need not be large. Our gap of 4 matches
OpenTelemetry's mapping. Subtracting 9 from an OpenTelemetry level in the
DEBUG, INFO, WARN and ERROR ranges converts it to the corresponding slog
Level range. OpenTelemetry also has the names TRACE and FATAL, which slog
does not. But those OpenTelemetry levels can still be represented as slog
Levels by using the appropriate integers.
LevelKey is the key used by the built-in handlers for the level
of the log call. The associated value is a [Level].
Names for common levels.
Level numbers are inherently arbitrary,
but we picked them to satisfy three constraints.
Any system can map them to another numbering scheme if it wishes.
First, we wanted the default level to be Info, Since Levels are ints, Info is
the default value for int, zero.
Second, we wanted to make it easy to use levels to specify logger verbosity.
Since a larger level means a more severe event, a logger that accepts events
with smaller (or more negative) level means a more verbose logger. Logger
verbosity is thus the negation of event severity, and the default verbosity
of 0 accepts all events at least as severe as INFO.
Third, we wanted some room between levels to accommodate schemes with named
levels between ours. For example, Google Cloud Logging defines a Notice level
between Info and Warn. Since there are only a few of these intermediate
levels, the gap between the numbers need not be large. Our gap of 4 matches
OpenTelemetry's mapping. Subtracting 9 from an OpenTelemetry level in the
DEBUG, INFO, WARN and ERROR ranges converts it to the corresponding slog
Level range. OpenTelemetry also has the names TRACE and FATAL, which slog
does not. But those OpenTelemetry levels can still be represented as slog
Levels by using the appropriate integers.
MessageKey is the key used by the built-in handlers for the
message of the log call. The associated value is a string.
SourceKey is the key used by the built-in handlers for the source file
and line of the log call. The associated value is a *[Source].
TimeKey is the key used by the built-in handlers for the time
when the log method is called. The associated Value is a [time.Time].
The pages are generated with Goldsv0.6.9-preview. (GOOS=linux GOARCH=amd64)
Golds is a Go 101 project developed by Tapir Liu.
PR and bug reports are welcome and can be submitted to the issue list.
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