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:
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:
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:
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:
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