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NAME
namespace - create and manipulate contexts for commands and variables
SYNOPSIS
namespace ?option? ?arg ...? _________________________________________________________________
DESCRIPTION
The namespace command lets you create, access, and destroy separate contexts for commands and variables.
See the section WHAT IS A NAMESPACE? below for a brief overview of namespaces. The legal values of
option are listed below. Note that you can abbreviate the options.
namespace children ?namespace? ?pattern?
Returns a list of all child namespaces that belong to the namespace namespace. If namespace is
not specified, then the children are returned for the current namespace. This command returns
fully-qualified names, which start with a double colon (::). If the optional pattern is given,
then this command returns only the names that match the glob-style pattern. The actual pattern
used is determined as follows: a pattern that starts with double colon (::) is used directly,
otherwise the namespace namespace (or the fully-qualified name of the current namespace) is
prepended onto the pattern.
namespace code script
Captures the current namespace context for later execution of the script script. It returns a new
script in which script has been wrapped in a namespace inscope command. The new script has two
important properties. First, it can be evaluated in any namespace and will cause script to be
evaluated in the current namespace (the one where the namespace code command was invoked).
Second, additional arguments can be appended to the resulting script and they will be passed to
script as additional arguments. For example, suppose the command set script [namespace code {foo
bar}] is invoked in namespace ::a::b. Then eval "$script x y" can be executed in any namespace
(assuming the value of script has been passed in properly) and will have the same effect as the
command ::namespace eval ::a::b {foo bar x y}. This command is needed because extensions like Tk
normally execute callback scripts in the global namespace. A scoped command captures a command
together with its namespace context in a way that allows it to be executed properly later. See
the section SCOPED SCRIPTS for some examples of how this is used to create callback scripts.
namespace current
Returns the fully-qualified name for the current namespace. The actual name of the global
namespace is ``'' (i.e., an empty string), but this command returns :: for the global namespace as
a convenience to programmers.
namespace delete ?namespace namespace ...?
Each namespace namespace is deleted and all variables, procedures, and child namespaces contained
in the namespace are deleted. If a procedure is currently executing inside the namespace, the
namespace will be kept alive until the procedure returns; however, the namespace is marked to
prevent other code from looking it up by name. If a namespace doesn't exist, this command returns
an error. If no namespace names are given, this command does nothing.
namespace eval namespace arg ?arg ...?
Activates a namespace called namespace and evaluates some code in that context. If the namespace
does not already exist, it is created. If more than one arg argument is specified, the arguments
are concatenated together with a space between each one in the same fashion as the eval command,
and the result is evaluated.
If namespace has leading namespace qualifiers and any leading namespaces do not exist, they are
automatically created.
namespace exists namespace
Returns 1 if namespace is a valid namespace in the current context, returns 0 otherwise.
namespace export ?-clear? ?pattern pattern ...?
Specifies which commands are exported from a namespace. The exported commands are those that can
be later imported into another namespace using a namespace import command. Both commands defined
in a namespace and commands the namespace has previously imported can be exported by a namespace.
The commands do not have to be defined at the time the namespace export command is executed. Each
pattern may contain glob-style special characters, but it may not include any namespace
qualifiers. That is, the pattern can only specify commands in the current (exporting) namespace.
Each pattern is appended onto the namespace's list of export patterns. If the -clear flag is
given, the namespace's export pattern list is reset to empty before any pattern arguments are
appended. If no patterns are given and the -clear flag isn't given, this command returns the
namespace's current export list.
namespace forget ?pattern pattern ...?
Removes previously imported commands from a namespace. Each pattern is a simple or qualified name
such as x, foo::x or a::b::p*. Qualified names contain double colons (::) and qualify a name with
the name of one or more namespaces. Each qualified pattern is qualified with the name of an
exporting namespace and may have glob-style special characters in the command name at the end of
the qualified name. Glob characters may not appear in a namespace name. For each simple pattern
this command deletes the matching commands of the current namespace that were imported from a
different namespace. For qualified patterns, this command first finds the matching exported
commands. It then checks whether any of those commands were previously imported by the current
namespace. If so, this command deletes the corresponding imported commands. In effect, this un-
does the action of a namespace import command.
namespace import ?-force? ?pattern pattern ...?
Imports commands into a namespace. Each pattern is a qualified name like foo::x or a::p*. That
is, it includes the name of an exporting namespace and may have glob-style special characters in
the command name at the end of the qualified name. Glob characters may not appear in a namespace
name. All the commands that match a pattern string and which are currently exported from their
namespace are added to the current namespace. This is done by creating a new command in the
current namespace that points to the exported command in its original namespace; when the new
imported command is called, it invokes the exported command. This command normally returns an
error if an imported command conflicts with an existing command. However, if the -force option is
given, imported commands will silently replace existing commands. The namespace import command
has snapshot semantics: that is, only requested commands that are currently defined in the
exporting namespace are imported. In other words, you can import only the commands that are in a
namespace at the time when the namespace import command is executed. If another command is
defined and exported in this namespace later on, it will not be imported.
namespace inscope namespace script ?arg ...?
Executes a script in the context of the specified namespace. This command is not expected to be
used directly by programmers; calls to it are generated implicitly when applications use namespace
code commands to create callback scripts that the applications then register with, e.g., Tk
widgets. The namespace inscope command is much like the namespace eval command except that the
namespace must already exist, and namespace inscope appends additional args as proper list
elements.
namespace inscope ::foo $script $x $y $z is equivalent to namespace eval ::foo [concat $script
[list $x $y $z]] thus additional arguments will not undergo a second round of substitution, as is
the case with namespace eval.
namespace origin command
Returns the fully-qualified name of the original command to which the imported command command
refers. When a command is imported into a namespace, a new command is created in that namespace
that points to the actual command in the exporting namespace. If a command is imported into a
sequence of namespaces a, b,...,n where each successive namespace just imports the command from
the previous namespace, this command returns the fully-qualified name of the original command in
the first namespace, a. If command does not refer to an imported command, the command's own
fully-qualified name is returned.
namespace parent ?namespace?
Returns the fully-qualified name of the parent namespace for namespace namespace. If namespace is
not specified, the fully-qualified name of the current namespace's parent is returned.
namespace qualifiers string
Returns any leading namespace qualifiers for string. Qualifiers are namespace names separated by
double colons (::). For the string ::foo::bar::x, this command returns ::foo::bar, and for :: it
returns an empty string. This command is the complement of the namespace tail command. Note that
it does not check whether the namespace names are, in fact, the names of currently defined
namespaces.
namespace tail string
Returns the simple name at the end of a qualified string. Qualifiers are namespace names
separated by double colons (::). For the string ::foo::bar::x, this command returns x, and for ::
it returns an empty string. This command is the complement of the namespace qualifiers command.
It does not check whether the namespace names are, in fact, the names of currently defined
namespaces.
namespace which ?-command? ?-variable? name
Looks up name as either a command or variable and returns its fully-qualified name. For example,
if name does not exist in the current namespace but does exist in the global namespace, this
command returns a fully-qualified name in the global namespace. If the command or variable does
not exist, this command returns an empty string. If the variable has been created but not
defined, such as with the variable command or through a trace on the variable, this command will
return the fully-qualified name of the variable. If no flag is given, name is treated as a
command name. See the section NAME RESOLUTION below for an explanation of the rules regarding
name resolution.
WHAT IS A NAMESPACE?
A namespace is a collection of commands and variables. It encapsulates the commands and variables to
ensure that they won't interfere with the commands and variables of other namespaces. Tcl has always had
one such collection, which we refer to as the global namespace. The global namespace holds all global
variables and commands. The namespace eval command lets you create new namespaces. For example,
namespace eval Counter {
namespace export bump
variable num 0
proc bump {} {
variable num
incr num
}
}
creates a new namespace containing the variable num and the procedure bump. The commands and variables
in this namespace are separate from other commands and variables in the same program. If there is a
command named bump in the global namespace, for example, it will be different from the command bump in
the Counter namespace.
Namespace variables resemble global variables in Tcl. They exist outside of the procedures in a
namespace but can be accessed in a procedure via the variable command, as shown in the example above.
Namespaces are dynamic. You can add and delete commands and variables at any time, so you can build up
the contents of a namespace over time using a series of namespace eval commands. For example, the
following series of commands has the same effect as the namespace definition shown above:
namespace eval Counter {
variable num 0
proc bump {} {
variable num
return [incr num]
}
}
namespace eval Counter {
proc test {args} {
return $args
}
}
namespace eval Counter {
rename test ""
}
Note that the test procedure is added to the Counter namespace, and later removed via the rename command.
Namespaces can have other namespaces within them, so they nest hierarchically. A nested namespace is
encapsulated inside its parent namespace and can not interfere with other namespaces.
QUALIFIED NAMES
Each namespace has a textual name such as history or ::safe::interp. Since namespaces may nest,
qualified names are used to refer to commands, variables, and child namespaces contained inside
namespaces. Qualified names are similar to the hierarchical path names for Unix files or Tk widgets,
except that :: is used as the separator instead of / or .. The topmost or global namespace has the name
``'' (i.e., an empty string), although :: is a synonym. As an example, the name ::safe::interp::create
refers to the command create in the namespace interp that is a child of namespace ::safe, which in turn
is a child of the global namespace, ::.
If you want to access commands and variables from another namespace, you must use some extra syntax.
Names must be qualified by the namespace that contains them. From the global namespace, we might access
the Counter procedures like this:
Counter::bump 5
Counter::Reset
We could access the current count like this:
puts "count = $Counter::num"
When one namespace contains another, you may need more than one qualifier to reach its elements. If we
had a namespace Foo that contained the namespace Counter, you could invoke its bump procedure from the
global namespace like this:
Foo::Counter::bump 3
You can also use qualified names when you create and rename commands. For example, you could add a
procedure to the Foo namespace like this:
proc Foo::Test {args} {return $args}
And you could move the same procedure to another namespace like this:
rename Foo::Test Bar::Test
There are a few remaining points about qualified names that we should cover. Namespaces have nonempty
names except for the global namespace. :: is disallowed in simple command, variable, and namespace names
except as a namespace separator. Extra colons in any separator part of a qualified name are ignored;
i.e. two or more colons are treated as a namespace separator. A trailing :: in a qualified variable or
command name refers to the variable or command named {}. However, a trailing :: in a qualified namespace
name is ignored.
NAME RESOLUTION
In general, all Tcl commands that take variable and command names support qualified names. This means
you can give qualified names to such commands as set, proc, rename, and interp alias. If you provide a
fully-qualified name that starts with a ::, there is no question about what command, variable, or
namespace you mean. However, if the name does not start with a :: (i.e., is relative), Tcl follows a
fixed rule for looking it up: Command and variable names are always resolved by looking first in the
current namespace, and then in the global namespace. Namespace names, on the other hand, are always
resolved by looking in only the current namespace.
In the following example,
set traceLevel 0
namespace eval Debug {
printTrace $traceLevel
}
Tcl looks for traceLevel in the namespace Debug and then in the global namespace. It looks up the
command printTrace in the same way. If a variable or command name is not found in either context, the
name is undefined. To make this point absolutely clear, consider the following example:
set traceLevel 0
namespace eval Foo {
variable traceLevel 3
namespace eval Debug {
printTrace $traceLevel
}
}
Here Tcl looks for traceLevel first in the namespace Foo::Debug. Since it is not found there, Tcl then
looks for it in the global namespace. The variable Foo::traceLevel is completely ignored during the name
resolution process.
You can use the namespace which command to clear up any question about name resolution. For example, the
command:
namespace eval Foo::Debug {namespace which -variable traceLevel}
returns ::traceLevel. On the other hand, the command,
namespace eval Foo {namespace which -variable traceLevel}
returns ::Foo::traceLevel.
As mentioned above, namespace names are looked up differently than the names of variables and commands.
Namespace names are always resolved in the current namespace. This means, for example, that a namespace
eval command that creates a new namespace always creates a child of the current namespace unless the new
namespace name begins with ::.
Tcl has no access control to limit what variables, commands, or namespaces you can reference. If you
provide a qualified name that resolves to an element by the name resolution rule above, you can access
the element.
You can access a namespace variable from a procedure in the same namespace by using the variable command.
Much like the global command, this creates a local link to the namespace variable. If necessary, it also
creates the variable in the current namespace and initializes it. Note that the global command only
creates links to variables in the global namespace. It is not necessary to use a variable command if you
always refer to the namespace variable using an appropriate qualified name.
IMPORTING COMMANDS
Namespaces are often used to represent libraries. Some library commands are used so frequently that it
is a nuisance to type their qualified names. For example, suppose that all of the commands in a package
like BLT are contained in a namespace called Blt. Then you might access these commands like this:
Blt::graph .g -background red
Blt::table . .g 0,0
If you use the graph and table commands frequently, you may want to access them without the Blt:: prefix.
You can do this by importing the commands into the current namespace, like this:
namespace import Blt::*
This adds all exported commands from the Blt namespace into the current namespace context, so you can
write code like this:
graph .g -background red
table . .g 0,0
The namespace import command only imports commands from a namespace that that namespace exported with a
namespace export command.
Importing every command from a namespace is generally a bad idea since you don't know what you will get.
It is better to import just the specific commands you need. For example, the command
namespace import Blt::graph Blt::table
imports only the graph and table commands into the current context.
If you try to import a command that already exists, you will get an error. This prevents you from
importing the same command from two different packages. But from time to time (perhaps when debugging),
you may want to get around this restriction. You may want to reissue the namespace import command to
pick up new commands that have appeared in a namespace. In that case, you can use the -force option, and
existing commands will be silently overwritten:
namespace import -force Blt::graph Blt::table
If for some reason, you want to stop using the imported commands, you can remove them with a namespace
forget command, like this:
namespace forget Blt::*
This searches the current namespace for any commands imported from Blt. If it finds any, it removes
them. Otherwise, it does nothing. After this, the Blt commands must be accessed with the Blt:: prefix.
When you delete a command from the exporting namespace like this:
rename Blt::graph ""
the command is automatically removed from all namespaces that import it.
EXPORTING COMMANDS
You can export commands from a namespace like this:
namespace eval Counter {
namespace export bump reset
variable Num 0
variable Max 100
proc bump {{by 1}} {
variable Num
incr Num $by
Check
return $Num
}
proc reset {} {
variable Num
set Num 0
}
proc Check {} {
variable Num
variable Max
if {$Num > $Max} {
error "too high!"
}
}
}
The procedures bump and reset are exported, so they are included when you import from the Counter
namespace, like this:
namespace import Counter::*
However, the Check procedure is not exported, so it is ignored by the import operation.
The namespace import command only imports commands that were declared as exported by their namespace.
The namespace export command specifies what commands may be imported by other namespaces. If a namespace
import command specifies a command that is not exported, the command is not imported.
SCOPED SCRIPTS
The namespace code command is the means by which a script may be packaged for evaluation in a namespace
other than the one in which it was created. It is used most often to create event handlers, Tk bindings,
and traces for evaluation in the global context. For instance, the following code indicates how to
direct a variable trace callback into the current namespace:
namespace eval a {
variable b
proc theTraceCallback { n1 n2 op } {
upvar 1 $n1 var
puts "the value of $n1 has changed to $var"
return
}
trace variable b w [namespace code theTraceCallback]
}
set a::b c
When executed, it prints the message:
the value of a::b has changed to c
EXAMPLES
Create a namespace containing a variable and an exported command:
namespace eval foo {
variable bar 0
proc grill {} {
variable bar
puts "called [incr bar] times"
}
namespace export grill
}
Call the command defined in the previous example in various ways.
# Direct call
foo::grill
# Import into current namespace, then call local alias
namespace import foo::grill
grill
Look up where the command imported in the previous example came from:
puts "grill came from [namespace origin grill]"
SEE ALSO
variable(3tcl)
KEYWORDS
exported, internal, variable