man Namespace (3): 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]"

KEYWORDS

exported, internal, variable