namespace
namespace(n) Tcl Built-In Commands namespace(n)
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NAME
namespace - create and manipulate contexts for commands and variables
SYNOPSIS
namespace ?option? ?arg ...?
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DESCRIPTION
The namespace command lets you create, access, and destroy separate
contexts for commands and variables. See the section WHAT IS A NAMES-
PACE? below for a brief overview of namespaces. The legal option's are
listed below. Note that you can abbreviate the option's.
namespace children ?namespace? ?pattern?
Returns a list of all child namespaces that belong to the names-
pace namespace. If namespace is not specified, then the chil-
dren are returned for the current namespace. This command
returns fully-qualified names, which start with ::. 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 :: is
used directly, otherwise the namespace namespace (or the fully-
qualified name of the current namespace) is prepended onto the
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 code 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 [names-
pace 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 exe-
cute 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 VALUES 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, proce-
dures, 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 argu-
ments are concatenated together with a space between each one in
the same fashion as the eval command, and the result is evalu-
ated.
If namespace has leading namespace qualifiers and any leading
namespaces do not exist, they are automatically created.
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 com-
mands defined in a namespace and commands the namespace has pre-
viously imported can be exported by a namespace. The commands
do not have to be defined at the time the namespace export com-
mand 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 names-
pace'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 names-
pace's current export list.
namespace forget ?pattern pattern ...?
Removes previously imported commands from a namespace. Each
pattern is a qualified name such as foo::x or a::b::p*. Quali-
fied names contain ::s and qualify a name with the name of one
or more namespaces. Each pattern is qualified with the name of
an exporting namespace and may have glob-style special charac-
ters in the command name at the end of the qualified name. Glob
characters may not appear in a namespace name. This command
first finds the matching exported commands. It then checks
whether any of those those commands were previously imported by
the current namespace. If so, this command deletes the corre-
sponding 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 com-
mand. 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 arg ?arg ...?
Executes a script in the context of a particular 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 names-
pace inscope command is much like the namespace eval command
except that it has lappend semantics and the namespace must
already exist. It treats the first argument as a list, and
appends any arguments after the first onto the end as proper
list elements. namespace inscope ::foo a x y z is equivalent to
namespace eval ::foo [concat a [list x y z]] This lappend seman-
tics is important because many callback scripts are actually
prefixes.
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 names-
paces 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 ::s. 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. Qual-
ifiers are namespace names separated by ::s. 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 names-
pace. 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-quali-
fied 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 com-
mand named bump in the global namespace, for example, it will be dif-
ferent from the command bump in the Counter namespace.
Namespace variables resemble global variables in Tcl. They exist out-
side of the procedures in a namespace but can be accessed in a proce-
dure 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 hierar-
chically. A nested namespace is encapsulated inside its parent names-
pace 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 com-
mands, 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 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 names-
pace 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 quali-
fier 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 :s in a qualified name are
ignored; that is, two or more :s 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 sup-
port 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 names-
pace. Namespace names, on the other hand, are always resolved by look-
ing 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 res-
olution 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 a ::.
Tcl has no access control to limit what variables, commands, or names-
paces 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 names-
pace 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 com-
mands are used so frequently that it is a nuisance to type their quali-
fied 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 spe-
cific 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 dif-
ferent 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 an 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.
SEE ALSO
variable(n)
KEYWORDS
exported, internal, variable
Tcl 8.0 namespace(n)
