Run Format

Package template

import "text/template"
Overview
Index
Examples
Subdirectories

Overview ▾

Package template implements data-driven templates for generating textual output.

To generate HTML output, see package html/template, which has the same interface as this package but automatically secures HTML output against certain attacks.

Templates are executed by applying them to a data structure. Annotations in the template refer to elements of the data structure (typically a field of a struct or a key in a map) to control execution and derive values to be displayed. Execution of the template walks the structure and sets the cursor, represented by a period '.' and called "dot", to the value at the current location in the structure as execution proceeds.

The input text for a template is UTF-8-encoded text in any format. "Actions"--data evaluations or control structures--are delimited by "{{" and "}}"; all text outside actions is copied to the output unchanged. Actions may not span newlines, although comments can.

Once constructed, a template may be executed safely in parallel.

Here is a trivial example that prints "17 items are made of wool".

type Inventory struct {
	Material string
	Count    uint
}
sweaters := Inventory{"wool", 17}
tmpl, err := template.New("test").Parse("{{.Count}} items are made of {{.Material}}")
if err != nil { panic(err) }
err = tmpl.Execute(os.Stdout, sweaters)
if err != nil { panic(err) }

More intricate examples appear below.

Actions

Here is the list of actions. "Arguments" and "pipelines" are evaluations of data, defined in detail below.

{{/* a comment */}}
	A comment; discarded. May contain newlines.
	Comments do not nest and must start and end at the
	delimiters, as shown here.

{{pipeline}}
	The default textual representation of the value of the pipeline
	is copied to the output.

{{if pipeline}} T1 {{end}}
	If the value of the pipeline is empty, no output is generated;
	otherwise, T1 is executed.  The empty values are false, 0, any
	nil pointer or interface value, and any array, slice, map, or
	string of length zero.
	Dot is unaffected.

{{if pipeline}} T1 {{else}} T0 {{end}}
	If the value of the pipeline is empty, T0 is executed;
	otherwise, T1 is executed.  Dot is unaffected.

{{if pipeline}} T1 {{else if pipeline}} T0 {{end}}
	To simplify the appearance of if-else chains, the else action
	of an if may include another if directly; the effect is exactly
	the same as writing
		{{if pipeline}} T1 {{else}}{{if pipeline}} T0 {{end}}{{end}}

{{range pipeline}} T1 {{end}}
	The value of the pipeline must be an array, slice, map, or channel.
	If the value of the pipeline has length zero, nothing is output;
	otherwise, dot is set to the successive elements of the array,
	slice, or map and T1 is executed. If the value is a map and the
	keys are of basic type with a defined order ("comparable"), the
	elements will be visited in sorted key order.

{{range pipeline}} T1 {{else}} T0 {{end}}
	The value of the pipeline must be an array, slice, map, or channel.
	If the value of the pipeline has length zero, dot is unaffected and
	T0 is executed; otherwise, dot is set to the successive elements
	of the array, slice, or map and T1 is executed.

{{template "name"}}
	The template with the specified name is executed with nil data.

{{template "name" pipeline}}
	The template with the specified name is executed with dot set
	to the value of the pipeline.

{{with pipeline}} T1 {{end}}
	If the value of the pipeline is empty, no output is generated;
	otherwise, dot is set to the value of the pipeline and T1 is
	executed.

{{with pipeline}} T1 {{else}} T0 {{end}}
	If the value of the pipeline is empty, dot is unaffected and T0
	is executed; otherwise, dot is set to the value of the pipeline
	and T1 is executed.

Arguments

An argument is a simple value, denoted by one of the following.

- A boolean, string, character, integer, floating-point, imaginary
  or complex constant in Go syntax. These behave like Go's untyped
  constants, although raw strings may not span newlines.
- The keyword nil, representing an untyped Go nil.
- The character '.' (period):
	.
  The result is the value of dot.
- A variable name, which is a (possibly empty) alphanumeric string
  preceded by a dollar sign, such as
	$piOver2
  or
	$
  The result is the value of the variable.
  Variables are described below.
- The name of a field of the data, which must be a struct, preceded
  by a period, such as
	.Field
  The result is the value of the field. Field invocations may be
  chained:
    .Field1.Field2
  Fields can also be evaluated on variables, including chaining:
    $x.Field1.Field2
- The name of a key of the data, which must be a map, preceded
  by a period, such as
	.Key
  The result is the map element value indexed by the key.
  Key invocations may be chained and combined with fields to any
  depth:
    .Field1.Key1.Field2.Key2
  Although the key must be an alphanumeric identifier, unlike with
  field names they do not need to start with an upper case letter.
  Keys can also be evaluated on variables, including chaining:
    $x.key1.key2
- The name of a niladic method of the data, preceded by a period,
  such as
	.Method
  The result is the value of invoking the method with dot as the
  receiver, dot.Method(). Such a method must have one return value (of
  any type) or two return values, the second of which is an error.
  If it has two and the returned error is non-nil, execution terminates
  and an error is returned to the caller as the value of Execute.
  Method invocations may be chained and combined with fields and keys
  to any depth:
    .Field1.Key1.Method1.Field2.Key2.Method2
  Methods can also be evaluated on variables, including chaining:
    $x.Method1.Field
- The name of a niladic function, such as
	fun
  The result is the value of invoking the function, fun(). The return
  types and values behave as in methods. Functions and function
  names are described below.
- A parenthesized instance of one the above, for grouping. The result
  may be accessed by a field or map key invocation.
	print (.F1 arg1) (.F2 arg2)
	(.StructValuedMethod "arg").Field

Arguments may evaluate to any type; if they are pointers the implementation automatically indirects to the base type when required. If an evaluation yields a function value, such as a function-valued field of a struct, the function is not invoked automatically, but it can be used as a truth value for an if action and the like. To invoke it, use the call function, defined below.

A pipeline is a possibly chained sequence of "commands". A command is a simple value (argument) or a function or method call, possibly with multiple arguments:

Argument
	The result is the value of evaluating the argument.
.Method [Argument...]
	The method can be alone or the last element of a chain but,
	unlike methods in the middle of a chain, it can take arguments.
	The result is the value of calling the method with the
	arguments:
		dot.Method(Argument1, etc.)
functionName [Argument...]
	The result is the value of calling the function associated
	with the name:
		function(Argument1, etc.)
	Functions and function names are described below.

Pipelines

A pipeline may be "chained" by separating a sequence of commands with pipeline characters '|'. In a chained pipeline, the result of the each command is passed as the last argument of the following command. The output of the final command in the pipeline is the value of the pipeline.

The output of a command will be either one value or two values, the second of which has type error. If that second value is present and evaluates to non-nil, execution terminates and the error is returned to the caller of Execute.

Variables

A pipeline inside an action may initialize a variable to capture the result. The initialization has syntax

$variable := pipeline

where $variable is the name of the variable. An action that declares a variable produces no output.

If a "range" action initializes a variable, the variable is set to the successive elements of the iteration. Also, a "range" may declare two variables, separated by a comma:

range $index, $element := pipeline

in which case $index and $element are set to the successive values of the array/slice index or map key and element, respectively. Note that if there is only one variable, it is assigned the element; this is opposite to the convention in Go range clauses.

A variable's scope extends to the "end" action of the control structure ("if", "with", or "range") in which it is declared, or to the end of the template if there is no such control structure. A template invocation does not inherit variables from the point of its invocation.

When execution begins, $ is set to the data argument passed to Execute, that is, to the starting value of dot.

Examples

Here are some example one-line templates demonstrating pipelines and variables. All produce the quoted word "output":

{{"\"output\""}}
	A string constant.
{{`"output"`}}
	A raw string constant.
{{printf "%q" "output"}}
	A function call.
{{"output" | printf "%q"}}
	A function call whose final argument comes from the previous
	command.
{{printf "%q" (print "out" "put")}}
	A parenthesized argument.
{{"put" | printf "%s%s" "out" | printf "%q"}}
	A more elaborate call.
{{"output" | printf "%s" | printf "%q"}}
	A longer chain.
{{with "output"}}{{printf "%q" .}}{{end}}
	A with action using dot.
{{with $x := "output" | printf "%q"}}{{$x}}{{end}}
	A with action that creates and uses a variable.
{{with $x := "output"}}{{printf "%q" $x}}{{end}}
	A with action that uses the variable in another action.
{{with $x := "output"}}{{$x | printf "%q"}}{{end}}
	The same, but pipelined.

Functions

During execution functions are found in two function maps: first in the template, then in the global function map. By default, no functions are defined in the template but the Funcs method can be used to add them.

Predefined global functions are named as follows.

and
	Returns the boolean AND of its arguments by returning the
	first empty argument or the last argument, that is,
	"and x y" behaves as "if x then y else x". All the
	arguments are evaluated.
call
	Returns the result of calling the first argument, which
	must be a function, with the remaining arguments as parameters.
	Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where
	Y is a func-valued field, map entry, or the like.
	The first argument must be the result of an evaluation
	that yields a value of function type (as distinct from
	a predefined function such as print). The function must
	return either one or two result values, the second of which
	is of type error. If the arguments don't match the function
	or the returned error value is non-nil, execution stops.
html
	Returns the escaped HTML equivalent of the textual
	representation of its arguments.
index
	Returns the result of indexing its first argument by the
	following arguments. Thus "index x 1 2 3" is, in Go syntax,
	x[1][2][3]. Each indexed item must be a map, slice, or array.
js
	Returns the escaped JavaScript equivalent of the textual
	representation of its arguments.
len
	Returns the integer length of its argument.
not
	Returns the boolean negation of its single argument.
or
	Returns the boolean OR of its arguments by returning the
	first non-empty argument or the last argument, that is,
	"or x y" behaves as "if x then x else y". All the
	arguments are evaluated.
print
	An alias for fmt.Sprint
printf
	An alias for fmt.Sprintf
println
	An alias for fmt.Sprintln
urlquery
	Returns the escaped value of the textual representation of
	its arguments in a form suitable for embedding in a URL query.

The boolean functions take any zero value to be false and a non-zero value to be true.

There is also a set of binary comparison operators defined as functions:

eq
	Returns the boolean truth of arg1 == arg2
ne
	Returns the boolean truth of arg1 != arg2
lt
	Returns the boolean truth of arg1 < arg2
le
	Returns the boolean truth of arg1 <= arg2
gt
	Returns the boolean truth of arg1 > arg2
ge
	Returns the boolean truth of arg1 >= arg2

For simpler multi-way equality tests, eq (only) accepts two or more arguments and compares the second and subsequent to the first, returning in effect

arg1==arg2 || arg1==arg3 || arg1==arg4 ...

(Unlike with || in Go, however, eq is a function call and all the arguments will be evaluated.)

The comparison functions work on basic types only (or named basic types, such as "type Celsius float32"). They implement the Go rules for comparison of values, except that size and exact type are ignored, so any integer value may be compared with any other integer value, any unsigned integer value may be compared with any other unsigned integer value, and so on. However, as usual, one may not compare an int with a float32 and so on.

Associated templates

Each template is named by a string specified when it is created. Also, each template is associated with zero or more other templates that it may invoke by name; such associations are transitive and form a name space of templates.

A template may use a template invocation to instantiate another associated template; see the explanation of the "template" action above. The name must be that of a template associated with the template that contains the invocation.

Nested template definitions

When parsing a template, another template may be defined and associated with the template being parsed. Template definitions must appear at the top level of the template, much like global variables in a Go program.

The syntax of such definitions is to surround each template declaration with a "define" and "end" action.

The define action names the template being created by providing a string constant. Here is a simple example:

`{{define "T1"}}ONE{{end}}
{{define "T2"}}TWO{{end}}
{{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}}
{{template "T3"}}`

This defines two templates, T1 and T2, and a third T3 that invokes the other two when it is executed. Finally it invokes T3. If executed this template will produce the text

ONE TWO

By construction, a template may reside in only one association. If it's necessary to have a template addressable from multiple associations, the template definition must be parsed multiple times to create distinct *Template values, or must be copied with the Clone or AddParseTree method.

Parse may be called multiple times to assemble the various associated templates; see the ParseFiles and ParseGlob functions and methods for simple ways to parse related templates stored in files.

A template may be executed directly or through ExecuteTemplate, which executes an associated template identified by name. To invoke our example above, we might write,

err := tmpl.Execute(os.Stdout, "no data needed")
if err != nil {
	log.Fatalf("execution failed: %s", err)
}

or to invoke a particular template explicitly by name,

err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed")
if err != nil {
	log.Fatalf("execution failed: %s", err)
}

Index ▾

Package files

doc.go exec.go funcs.go helper.go template.go

func HTMLEscape

func HTMLEscape(w io.Writer, b []byte)

HTMLEscape writes to w the escaped HTML equivalent of the plain text data b.

func HTMLEscapeString

func HTMLEscapeString(s string) string

HTMLEscapeString returns the escaped HTML equivalent of the plain text data s.

func HTMLEscaper

func HTMLEscaper(args ...interface{}) string

HTMLEscaper returns the escaped HTML equivalent of the textual representation of its arguments.

func JSEscape

func JSEscape(w io.Writer, b []byte)

JSEscape writes to w the escaped JavaScript equivalent of the plain text data b.

func JSEscapeString

func JSEscapeString(s string) string

JSEscapeString returns the escaped JavaScript equivalent of the plain text data s.

func JSEscaper

func JSEscaper(args ...interface{}) string

JSEscaper returns the escaped JavaScript equivalent of the textual representation of its arguments.

func URLQueryEscaper

func URLQueryEscaper(args ...interface{}) string

URLQueryEscaper returns the escaped value of the textual representation of its arguments in a form suitable for embedding in a URL query.

type FuncMap

type FuncMap map[string]interface{}

FuncMap is the type of the map defining the mapping from names to functions. Each function must have either a single return value, or two return values of which the second has type error. In that case, if the second (error) return value evaluates to non-nil during execution, execution terminates and Execute returns that error.

type Template

type Template struct {
        *parse.Tree
        // contains filtered or unexported fields
}

Template is the representation of a parsed template. The *parse.Tree field is exported only for use by html/template and should be treated as unexported by all other clients.

Example

Dear Aunt Mildred,

It was a pleasure to see you at the wedding.
Thank you for the lovely bone china tea set.

Best wishes,
Josie

Dear Uncle John,

It is a shame you couldn't make it to the wedding.
Thank you for the lovely moleskin pants.

Best wishes,
Josie

Dear Cousin Rodney,

It is a shame you couldn't make it to the wedding.

Best wishes,
Josie

Example (Func)

This example demonstrates a custom function to process template text. It installs the strings.Title function and uses it to Make Title Text Look Good In Our Template's Output.

Input: "the go programming language"
Output 0: The Go Programming Language
Output 1: "The Go Programming Language"
Output 2: "The Go Programming Language"

Example (Glob)

Here we demonstrate loading a set of templates from a directory.

Code:

// Here we create a temporary directory and populate it with our sample
    // template definition files; usually the template files would already
    // exist in some location known to the program.
    dir := createTestDir([]templateFile{
            // T0.tmpl is a plain template file that just invokes T1.
            {"T0.tmpl", `T0 invokes T1: ({{template "T1"}})`},
            // T1.tmpl defines a template, T1 that invokes T2.
            {"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
            // T2.tmpl defines a template T2.
            {"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
    })
    // Clean up after the test; another quirk of running as an example.
    defer os.RemoveAll(dir)

    // pattern is the glob pattern used to find all the template files.
    pattern := filepath.Join(dir, "*.tmpl")

    // Here starts the example proper.
    // T0.tmpl is the first name matched, so it becomes the starting template,
    // the value returned by ParseGlob.
    tmpl := template.Must(template.ParseGlob(pattern))

    err := tmpl.Execute(os.Stdout, nil)
    if err != nil {
            log.Fatalf("template execution: %s", err)
    }
    

Output:

T0 invokes T1: (T1 invokes T2: (This is T2))

Example (Helpers)

This example demonstrates one way to share some templates and use them in different contexts. In this variant we add multiple driver templates by hand to an existing bundle of templates.

Code:

// Here we create a temporary directory and populate it with our sample
    // template definition files; usually the template files would already
    // exist in some location known to the program.
    dir := createTestDir([]templateFile{
            // T1.tmpl defines a template, T1 that invokes T2.
            {"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
            // T2.tmpl defines a template T2.
            {"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
    })
    // Clean up after the test; another quirk of running as an example.
    defer os.RemoveAll(dir)

    // pattern is the glob pattern used to find all the template files.
    pattern := filepath.Join(dir, "*.tmpl")

    // Here starts the example proper.
    // Load the helpers.
    templates := template.Must(template.ParseGlob(pattern))
    // Add one driver template to the bunch; we do this with an explicit template definition.
    _, err := templates.Parse("{{define `driver1`}}Driver 1 calls T1: ({{template `T1`}})\n{{end}}")
    if err != nil {
            log.Fatal("parsing driver1: ", err)
    }
    // Add another driver template.
    _, err = templates.Parse("{{define `driver2`}}Driver 2 calls T2: ({{template `T2`}})\n{{end}}")
    if err != nil {
            log.Fatal("parsing driver2: ", err)
    }
    // We load all the templates before execution. This package does not require
    // that behavior but html/template's escaping does, so it's a good habit.
    err = templates.ExecuteTemplate(os.Stdout, "driver1", nil)
    if err != nil {
            log.Fatalf("driver1 execution: %s", err)
    }
    err = templates.ExecuteTemplate(os.Stdout, "driver2", nil)
    if err != nil {
            log.Fatalf("driver2 execution: %s", err)
    }
    

Output:

Driver 1 calls T1: (T1 invokes T2: (This is T2))
Driver 2 calls T2: (This is T2)

Example (Share)

This example demonstrates how to use one group of driver templates with distinct sets of helper templates.

Code:

// Here we create a temporary directory and populate it with our sample
    // template definition files; usually the template files would already
    // exist in some location known to the program.
    dir := createTestDir([]templateFile{
            // T0.tmpl is a plain template file that just invokes T1.
            {"T0.tmpl", "T0 ({{.}} version) invokes T1: ({{template `T1`}})\n"},
            // T1.tmpl defines a template, T1 that invokes T2. Note T2 is not defined
            {"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
    })
    // Clean up after the test; another quirk of running as an example.
    defer os.RemoveAll(dir)

    // pattern is the glob pattern used to find all the template files.
    pattern := filepath.Join(dir, "*.tmpl")

    // Here starts the example proper.
    // Load the drivers.
    drivers := template.Must(template.ParseGlob(pattern))

    // We must define an implementation of the T2 template. First we clone
    // the drivers, then add a definition of T2 to the template name space.

    // 1. Clone the helper set to create a new name space from which to run them.
    first, err := drivers.Clone()
    if err != nil {
            log.Fatal("cloning helpers: ", err)
    }
    // 2. Define T2, version A, and parse it.
    _, err = first.Parse("{{define `T2`}}T2, version A{{end}}")
    if err != nil {
            log.Fatal("parsing T2: ", err)
    }

    // Now repeat the whole thing, using a different version of T2.
    // 1. Clone the drivers.
    second, err := drivers.Clone()
    if err != nil {
            log.Fatal("cloning drivers: ", err)
    }
    // 2. Define T2, version B, and parse it.
    _, err = second.Parse("{{define `T2`}}T2, version B{{end}}")
    if err != nil {
            log.Fatal("parsing T2: ", err)
    }

    // Execute the templates in the reverse order to verify the
    // first is unaffected by the second.
    err = second.ExecuteTemplate(os.Stdout, "T0.tmpl", "second")
    if err != nil {
            log.Fatalf("second execution: %s", err)
    }
    err = first.ExecuteTemplate(os.Stdout, "T0.tmpl", "first")
    if err != nil {
            log.Fatalf("first: execution: %s", err)
    }

    

Output:

T0 (second version) invokes T1: (T1 invokes T2: (T2, version B))
T0 (first version) invokes T1: (T1 invokes T2: (T2, version A))

func Must

func Must(t *Template, err error) *Template

Must is a helper that wraps a call to a function returning (*Template, error) and panics if the error is non-nil. It is intended for use in variable initializations such as

var t = template.Must(template.New("name").Parse("text"))

func New

func New(name string) *Template

New allocates a new template with the given name.

func ParseFiles

func ParseFiles(filenames ...string) (*Template, error)

ParseFiles creates a new Template and parses the template definitions from the named files. The returned template's name will have the (base) name and (parsed) contents of the first file. There must be at least one file. If an error occurs, parsing stops and the returned *Template is nil.

func ParseGlob

func ParseGlob(pattern string) (*Template, error)

ParseGlob creates a new Template and parses the template definitions from the files identified by the pattern, which must match at least one file. The returned template will have the (base) name and (parsed) contents of the first file matched by the pattern. ParseGlob is equivalent to calling ParseFiles with the list of files matched by the pattern.

func (*Template) AddParseTree

func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error)

AddParseTree creates a new template with the name and parse tree and associates it with t.

func (*Template) Clone

func (t *Template) Clone() (*Template, error)

Clone returns a duplicate of the template, including all associated templates. The actual representation is not copied, but the name space of associated templates is, so further calls to Parse in the copy will add templates to the copy but not to the original. Clone can be used to prepare common templates and use them with variant definitions for other templates by adding the variants after the clone is made.

func (*Template) Delims

func (t *Template) Delims(left, right string) *Template

Delims sets the action delimiters to the specified strings, to be used in subsequent calls to Parse, ParseFiles, or ParseGlob. Nested template definitions will inherit the settings. An empty delimiter stands for the corresponding default: {{ or }}. The return value is the template, so calls can be chained.

func (*Template) Execute

func (t *Template) Execute(wr io.Writer, data interface{}) (err error)

Execute applies a parsed template to the specified data object, and writes the output to wr.

func (*Template) ExecuteTemplate

func (t *Template) ExecuteTemplate(wr io.Writer, name string, data interface{}) error

ExecuteTemplate applies the template associated with t that has the given name to the specified data object and writes the output to wr.

func (*Template) Funcs

func (t *Template) Funcs(funcMap FuncMap) *Template

Funcs adds the elements of the argument map to the template's function map. It panics if a value in the map is not a function with appropriate return type. However, it is legal to overwrite elements of the map. The return value is the template, so calls can be chained.

func (*Template) Lookup

func (t *Template) Lookup(name string) *Template

Lookup returns the template with the given name that is associated with t, or nil if there is no such template.

func (*Template) Name

func (t *Template) Name() string

Name returns the name of the template.

func (*Template) New

func (t *Template) New(name string) *Template

New allocates a new template associated with the given one and with the same delimiters. The association, which is transitive, allows one template to invoke another with a {{template}} action.

func (*Template) Parse

func (t *Template) Parse(text string) (*Template, error)

Parse parses a string into a template. Nested template definitions will be associated with the top-level template t. Parse may be called multiple times to parse definitions of templates to associate with t. It is an error if a resulting template is non-empty (contains content other than template definitions) and would replace a non-empty template with the same name. (In multiple calls to Parse with the same receiver template, only one call can contain text other than space, comments, and template definitions.)

func (*Template) ParseFiles

func (t *Template) ParseFiles(filenames ...string) (*Template, error)

ParseFiles parses the named files and associates the resulting templates with t. If an error occurs, parsing stops and the returned template is nil; otherwise it is t. There must be at least one file.

func (*Template) ParseGlob

func (t *Template) ParseGlob(pattern string) (*Template, error)

ParseGlob parses the template definitions in the files identified by the pattern and associates the resulting templates with t. The pattern is processed by filepath.Glob and must match at least one file. ParseGlob is equivalent to calling t.ParseFiles with the list of files matched by the pattern.

func (*Template) Templates

func (t *Template) Templates() []*Template

Templates returns a slice of the templates associated with t, including t itself.

Subdirectories

Name      Synopsis
..
parse      Package parse builds parse trees for templates as defined by text/template and html/template.