namespace(3tcl)
NAME
namespace - create and manipulate contexts for commands and variables
SYNOPSIS
namespace ?subcommand? ?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 subcommand are listed below. Note that you can abbreviate the
subcommands.
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 [list 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 does not exist, this
command returns an error. If no namespace names are given, this
command does nothing.
namespace ensemble subcommand ?arg ...?
Creates and manipulates a command that is formed out of an
ensemble of subcommands. See the section ENSEMBLES below for
further details.
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 is not 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, or queries the set of
imported commands in a namespace. When no arguments are
present, namespace import returns the list of commands in the
current namespace that have been imported from other namespaces.
The commands in the returned list are in the format of simple
names, with no namespace qualifiers at all. This format is
suitable for composition with namespace forget (see EXAMPLES
below). When pattern arguments are present, 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 path ?namespaceList?
Returns the command resolution path of the current namespace. If
namespaceList is specified as a list of named namespaces, the
current namespace's command resolution path is set to those
namespaces and returns the empty list. The default command
resolution path is always empty. See the section NAME RESOLUTION
below for an explanation of the rules regarding name resolution.
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 upvar namespace otherVar myVar ?otherVar myVar ...
This command arranges for one or more local variables in the
current procedure to refer to variables in namespace. The
namespace name is resolved as described in section NAME
RESOLUTION. The command namespace upvar $ns a b has the same
behaviour as upvar 0 ${ns}::a b, with the sole exception of the
resolution rules used for qualified namespace or variable names.
namespace upvar returns an empty string.
namespace unknown ?script?
Sets or returns the unknown command handler for the current
namespace. The handler is invoked when a command called from
within the namespace cannot be found (in either the current
namespace or the global namespace). The script argument, if
given, should be a well formed list representing a command name
and optional arguments. When the handler is invoked, the full
invocation line will be appended to the script and the result
evaluated in the context of the namespace. The default handler
for all namespaces is ::unknown. If no argument is given, it
returns the handler for the current namespace.
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 will not 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 basic rules
for looking it up: Variable names are always resolved by looking first
in the current namespace, and then in the global namespace. Command
names are also always resolved by looking in the current namespace
first. If not found there, they are searched for in every namespace on
the current namespace's command path (which is empty by default). If
not found there, command names are looked up in the global namespace
(or, failing that, are processed by the unknown command.) 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 do not 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 add variable b write [namespace code theTraceCallback]
}
set a::b c
When executed, it prints the message:
the value of a::b has changed to c
ENSEMBLES
The namespace ensemble is used to create and manipulate ensemble
commands, which are commands formed by grouping subcommands together.
The commands typically come from the current namespace when the
ensemble was created, though this is configurable. Note that there may
be any number of ensembles associated with any namespace (including
none, which is true of all namespaces by default), though all the
ensembles associated with a namespace are deleted when that namespace
is deleted. The link between an ensemble command and its namespace is
maintained however the ensemble is renamed.
Three subcommands of the namespace ensemble command are defined:
namespace ensemble create ?option value ...?
Creates a new ensemble command linked to the current namespace,
returning the fully qualified name of the command created. The
arguments to namespace ensemble create allow the configuration
of the command as if with the namespace ensemble configure
command. If not overridden with the -command option, this
command creates an ensemble with exactly the same name as the
linked namespace. See the section ENSEMBLE OPTIONS below for a
full list of options supported and their effects.
namespace ensemble configure command ?option? ?value ...?
Retrieves the value of an option associated with the ensemble
command named command, or updates some options associated with
that ensemble command. See the section ENSEMBLE OPTIONS below
for a full list of options supported and their effects.
namespace ensemble exists command
Returns a boolean value that describes whether the command
command exists and is an ensemble command. This command only
ever returns an error if the number of arguments to the command
is wrong.
When called, an ensemble command takes its first argument and looks it
up (according to the rules described below) to discover a list of words
to replace the ensemble command and subcommand with. The resulting
list of words is then evaluated (with no further substitutions) as if
that was what was typed originally (i.e. by passing the list of words
through Tcl_EvalObjv) and returning the result of the command. Note
that it is legal to make the target of an ensemble rewrite be another
(or even the same) ensemble command. The ensemble command will not be
visible through the use of the uplevel or info level commands.
ENSEMBLE OPTIONS
The following options, supported by the namespace ensemble create and
namespace ensemble configure commands, control how an ensemble command
behaves:
-map
When non-empty, this option supplies a dictionary that provides
a mapping from subcommand names to a list of prefix words to
substitute in place of the ensemble command and subcommand words
(in a manner similar to an alias created with interp alias; the
words are not reparsed after substitution); if the first word of
any target is not fully qualified when set, it is assumed to be
relative to the current namespace and changed to be exactly that
(that is, it is always fully qualified when read). When this
option is empty, the mapping will be from the local name of the
subcommand to its fully-qualified name. Note that when this
option is non-empty and the -subcommands option is empty, the
ensemble subcommand names will be exactly those words that have
mappings in the dictionary.
-prefixes
This option (which is enabled by default) controls whether the
ensemble command recognizes unambiguous prefixes of its
subcommands. When turned off, the ensemble command requires
exact matching of subcommand names.
-subcommands
When non-empty, this option lists exactly what subcommands are
in the ensemble. The mapping for each of those commands will be
either whatever is defined in the -map option, or to the command
with the same name in the namespace linked to the ensemble. If
this option is empty, the subcommands of the namespace will
either be the keys of the dictionary listed in the -map option
or the exported commands of the linked namespace at the time of
the invocation of the ensemble command.
-unknown
When non-empty, this option provides a partial command (to which
all the words that are arguments to the ensemble command,
including the fully-qualified name of the ensemble, are
appended) to handle the case where an ensemble subcommand is not
recognized and would otherwise generate an error. When empty
(the default) an error (in the style of Tcl_GetIndexFromObj) is
generated whenever the ensemble is unable to determine how to
implement a particular subcommand. See UNKNOWN HANDLER
BEHAVIOUR for more details.
The following extra option is allowed by namespace ensemble create:
-command
This write-only option allows the name of the ensemble created
by namespace ensemble create to be anything in any existing
namespace. The default value for this option is the fully-
qualified name of the namespace in which the namespace ensemble
create command is invoked.
The following extra option is allowed by namespace ensemble configure:
-namespace
This read-only option allows the retrieval of the fully-
qualified name of the namespace which the ensemble was created
within.
UNKNOWN HANDLER BEHAVIOUR
If an unknown handler is specified for an ensemble, that handler is
called when the ensemble command would otherwise return an error due to
it being unable to decide which subcommand to invoke. The exact
conditions under which that occurs are controlled by the -subcommands,
-map and -prefixes options as described above.
To execute the unknown handler, the ensemble mechanism takes the
specified -unknown option and appends each argument of the attempted
ensemble command invocation (including the ensemble command itself,
expressed as a fully qualified name). It invokes the resulting command
in the scope of the attempted call. If the execution of the unknown
handler terminates normally, the ensemble engine reparses the
subcommand (as described below) and tries to dispatch it again, which
is ideal for when the ensemble's configuration has been updated by the
unknown subcommand handler. Any other kind of termination of the
unknown handler is treated as an error.
The result of the unknown handler is expected to be a list (it is an
error if it is not). If the list is an empty list, the ensemble command
attempts to look up the original subcommand again and, if it is not
found this time, an error will be generated just as if the -unknown
handler was not there (i.e. for any particular invocation of an
ensemble, its unknown handler will be called at most once.) This makes
it easy for the unknown handler to update the ensemble or its backing
namespace so as to provide an implementation of the desired subcommand
and reparse.
When the result is a non-empty list, the words of that list are used to
replace the ensemble command and subcommand, just as if they had been
looked up in the -map. It is up to the unknown handler to supply all
namespace qualifiers if the implementing subcommand is not in the
namespace of the caller of the ensemble command. Also note that when
ensemble commands are chained (e.g. if you make one of the commands
that implement an ensemble subcommand into an ensemble, in a manner
similar to the text widget's tag and mark subcommands) then the rewrite
happens in the context of the caller of the outermost ensemble. That is
to say that ensembles do not in themselves place any namespace contexts
on the Tcl call stack.
Where an empty -unknown handler is given (the default), the ensemble
command will generate an error message based on the list of commands
that the ensemble has defined (formatted similarly to the error message
from Tcl_GetIndexFromObj). This is the error that will be thrown when
the subcommand is still not recognized during reparsing. It is also an
error for an -unknown handler to delete its namespace.
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
# Use the command resolution path to find the name
namespace eval boo {
namespace path ::foo
grill
}
# Import into current namespace, then call local alias
namespace import foo::grill
grill
# Create two ensembles, one with the default name and one with a
# specified name. Then call through the ensembles.
namespace eval foo {
namespace ensemble create
namespace ensemble create -command ::foobar
}
foo grill
foobar grill
Look up where the command imported in the previous example came from:
puts "grill came from [namespace origin grill]"
Remove all imported commands from the current namespace:
namespace forget {*}[namespace import]
SEE ALSO
interp(3tcl), upvar(3tcl), variable(3tcl)
KEYWORDS
command, ensemble, exported, internal, variable
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