package require

package require evaluates a script when a certain package is required and then ensures that the package is present.


package require ?-exact? package ?version?


package require evaluates a script that must provide the requested package. If a version specification is provided, only a matching version is selected. package require returns the version that was provided, or an error.

If both the -exact and version are provided then only the exact version specified is acceptable. If -exact is omitted but version is specified, any version is acceptable whose major version is the same as version and whose minor version is equal to or greater than version. If both -exact and version are omitted then any version is acceptable. If a matching version has already been provided, its version number is returned and no other action is taken.

To find the required package, package require selects the highest acceptable version in a database that is built from calls to package ifneeded in pkgIndex.tcl files the first time a package routine is called. pkgIndex.tcl files are searched for in directories given in $auto_path, and in their immediate subdirectories.

To provide the required package, package require evaluates the selected script in the global namespace. The script must call package provide with the proper version.

If the package ifneeded database does not contain an acceptable version of the package and a package unknown command has been registered then that command is evaluated in the global namespace, after which package require looks for the package in the database again.

Reloading a package

HD: Here's something I use all the time:

proc reloadPkg pkg {
    eval [package ifneeded $pkg [package require $pkg]]


package require XYZ
# Discover bug in package
# Fix it in editor
reloadPkg XYZ

Origin of a Package

escargo 2003-11-20 PYK 2020-04-28: Is there a way to determine the provenance of a package? I have a script cals package require snit 0.9 and the value returned is 0.82, and it is not at all obvious where that value is coming from.

DGP Use package ifneeded to discover what script was used to load a package.

package ifneeded snit [package require snit]

That will return the script that loaded snit, which likely looks something like:

source /absolute/path/to/snit/snit.tcl

Is that what you were asking?

escargo: Yes, modulo the correction of package provide to package require in the snippet above. I'm running some experimental Snit code which is at version 0.9; the version in tcllib 1.5 is 0.82 which seems to satisfy a package require snit 0.9.

DGP: My original code assumed you already had snit loaded in your interp. In that case, package provide (or even package present) is better than package require.

And yes, 82 > 9, so 0.82 is a later release than 0.9. Any package developer that means 0.8.2 should say so.

But what about the developer who thought that 82 would be less than 90 ?

DGP 82 is less than 90, and package knows it. It also knows that 9 and 90 are different values:

% package vcompare 0.82 0.9
% package vcompare 0.82 0.90
% package vcompare 0.9 0.90
% package vcompare 0.90  0.90

Guidelines for Package Development

package forget removes from an interpreter information that the package is loaded, but does not undo the effects of loading the package. A subsequent package require causes the load script for the package to be evaluated again. This is useful for updating a package in a running interpeter. Due to this feature, a load script should not attmpt to initialize the loaded package, as it may already have been initialized. A package should, however, determine at load time whether a backwards-incompatible version is being load, and make the required changes in-place to any relevant initialized objects. Alternatively, the package could refuse to reload if loading can't safely be performed.


LV: When writing Tcl package A that has dependencies on other packages, where is the best place to put the package require statements? In the pkgIndex.tcl for A, or in the A.tcl, i.e. the file that A's pkgIndex.tcl loads? What are the pros and cons for doing it each way?

Lars H: Code in pkgIndex.tcl is evaluated the first time that Tcl searches for packages. This means that if you put a package require there, and provided the package ifneeded scripts are of the usual kind, then the required packages are loaded as soon as any package (not just yours) is required. That is bad. Thus you should put the [package require] in your A.tcl.

MGS 2004-12-11: I put the pkgIndex.tcl file in the main package directory, then create a Tcl directory to contain all the Tcl files. See TIP 55 .

The pkgIndex.tcl points at tcl/pkginit.tcl which contains a single proc pkginit_ (in the package namespace) which sets package variables, requires dependent packages, then manually adds to the ::auto_index array (from the tcl/tclIndex file) to setup auto-loading for all the package procs, provides the specified package, then deletes its own proc. This way, package require'ing a package does not create any procs (except for initializing auto_loading), and only creates the package namespace with package variables in it. I think the pkgIndex.tcl file should be as simple as possible, and could (if necessary) be auto-created by some package repository. You can see examples of my structure in most of my packages at: [L1 ] and [L2 ]

LV: Here's a followup question. In Img 1.2 , the pkgIndex.tcl says:

 package ifneeded Img 1.2.4 "package require Tk; [list load [file join $dir] Img]"

Based on your comments above, this should have the effect that anyone doing any package require would have to have a $DISPLAY variable set or the statement would fail. But that doesn't seem to have been my experience. Right now I can't duplicate the situation due to having Img 1.3 installed, which does the pkgIndex.tcl differently. Can someone else comment on this situation?

Lars H: OK, perhaps I should have been clearer. In this case, the "package require" is not technically a command in the pkgIndex.tcl file, but merely part of a string. It is tucked away as the package ifneeded script of the Img package and thus does not get evaluated until someone does package require Img, so this is not bad. However had it been

package require Tk
package ifneeded Img 1.2.4 [list load [file join $dir] Img]

then it would have been very bad.

LV: Recently Schelte Bron emailed me after reading a comment I'd posted in comp.lang.tcl, with this little Tcl example:

% package require bwidget
can't find package bwidget
% lsearch -inline -all -regexp [package names] (?iq)bwidget

In other words, if you don't know what sequence of upper and lower case characters a package developer has used, the above lsearch, after a package require has finished, can provide that to you.

uwe posted this proc in response to the discussion about trying to figure out what case a particular extension used:

proc loadwhatimean packname {
    if {[catch {package require $packname} cerr]} {
        set whatwehave [package names]
        set found [lsearch -inline -regexp $whatwehave ***:(?i)$packname]
        puts stderr "required: $packname found : $found"
        if {$found ne {}} {
            return [package require $found]
        } else {
            puts stderr bahh
            # error or return -errorcode ..?
    return $cerr   

LV 2007-10-18:

I'd really like to know what kinds of code you've found helpful in tracking down problems getting packages to load.

Example: I have a package installed in a directory. Other packages in sibling directories are loaded when I run a package require. However, when this one package require is invoked, I get

% package require TclOO
can't find package TclOO

How can I go about determining why this is not loading? There's a pkgIndex.tcl in a sub-directory in the auto_path variable.

LV 2007-10-26

Here's a script that I have started writing. The intent is to do a package require on each package that a particular tclsh knows, reporting the success or failure.

#! /tmp/.lwv/ActiveTcl-8.5/bin/tclsh8.5

catch {package require lwvNotThere}
set plist [package names]

foreach package $plist {
    set rc [catch {set version [package require $package]} result options]
    puts "package require $package load rc: $rc result: $result options: $options"

LV 2008-03-28

Recently on comp.lang.tcl, I read:

> Actually it is a problem in one of my packages. Inside a pkgIndex.tcl, I 
> was doing "catch {package require some-package}". 

Never, never do that. 

No index script should make use of the package require command. 

If you need to test for the presence of a pre-requisite package, use 
package present or package provide to do that. 

See also Tcl Bug 1805928 and  . 

So, I then ran $ find /path/to/ActiveTcl-8.5/lib/. -name pkgIndex.tcl -print | xargs grep 'package.*require' > /tmp/pr.txt

and found a large number of hits for this. I took a look and saw that the predominant use of package require was some variation of

package require Tcl 8.4

However, after that was eliminated, I found a couple of dozen occurrences where Tk was the object of the package require, and then, after that, just over a dozen instances of package requiring img::base, followed by some cases of zlibtcl, or one of the struct packages from tcllib, etc.

I am just wondering whether these cases are unsafe or whether the no index script should make use... statement means something else that I just am missing.

Page Authors

Various editorial changes.