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Test::MockModule(3pm) User Contributed Perl Documentation Test::MockModule(3pm)


Test::MockModule - Override subroutines in a module for unit testing


        use Module::Name;
        use Test::MockModule;
                my $module = Test::MockModule->new('Module::Name');
                $module->mock('subroutine', sub { ... });
                Module::Name::subroutine(@args); # mocked
                # Same effect, but this will die() if other_subroutine()
                # doesn't already exist, which is often desirable.
                $module->redefine('other_subroutine', sub { ... });
                # This will die() if another_subroutine() is defined.
                $module->define('another_subroutine', sub { ... });
                # you can also chain new/mock/redefine/define
                ->mock( one_subroutine => sub { ... })
                ->redefine( other_subroutine => sub { ... } )
                ->define( a_new_sub => 1234 );
        Module::Name::subroutine(@args); # original subroutine
        # Working with objects
        use Foo;
        use Test::MockModule;
                my $mock = Test::MockModule->new('Foo');
                $mock->mock(foo => sub { print "Foo!\n"; });
                my $foo = Foo->new();
                $foo->foo(); # prints "Foo!\n"
    # If you want to prevent noop and mock from working, you can
    # load Test::MockModule in strict mode.
    use Test::MockModule qw/strict/;
    my $module = Test::MockModule->new('Module::Name');
    # Redefined the other_subroutine or dies if it's not there.
    $module->redefine('other_subroutine', sub { ... });
    # Dies since you specified you wanted strict mode.
    $module->mock('subroutine', sub { ... });
    # Turn strictness off in this lexical scope
        use Test::MockModule 'nostrict';
        # ->mock() works now
        $module->mock('subroutine', sub { ... });
    # Back in the strict scope, so mock() dies here
    $module->mock('subroutine', sub { ... });


"Test::MockModule" lets you temporarily redefine subroutines in other packages for the purposes of unit testing.

A "Test::MockModule" object is set up to mock subroutines for a given module. The object remembers the original subroutine so it can be easily restored. This happens automatically when all MockModule objects for the given module go out of scope, or when you unmock() the subroutine.


One of the weaknesses of testing using mocks is that the implementation of the interface that you are mocking might change, while your mocks get left alone. You are not now mocking what you thought you were, and your mocks might now be hiding bugs that will only be spotted in production. To help prevent this you can load Test::MockModule in 'strict' mode:

    use Test::MockModule qw(strict);

This will disable use of the mock() method, making it a fatal runtime error. You should instead define mocks using redefine(), which will only mock things that already exist and die if you try to redefine something that doesn't exist.

Strictness is lexically scoped, so you can do this in one file:

    use Test::MockModule qw(strict);

and this in another:

    use Test::MockModule; # the default is nostrict

You can even mix n match at different places in a single file thus:

    use Test::MockModule qw(strict);
    # here mock() dies
        use Test::MockModule qw(nostrict);
        # here mock() works
    # here mock() goes back to dieing
    use Test::MockModule qw(nostrict);
    # and from here on mock() works again

NB that strictness must be defined at compile-time, and set using "use". If you think you're going to try and be clever by calling Test::MockModule's import() method at runtime then what happens in undefined, with results differing from one version of perl to another. What larks!


Returns an object that will mock subroutines in the specified $package.

If there is no $VERSION defined in $package, the module will be automatically loaded. You can override this behaviour by setting the "no_auto" option:

        my $mock = Test::MockModule->new('Module::Name', no_auto => 1);
Returns the target package name for the mocked subroutines
Returns a boolean value indicating whether or not the subroutine is currently mocked
Temporarily replaces one or more subroutines in the mocked module. A subroutine can be mocked with a code reference or a scalar. A scalar will be recast as a subroutine that returns the scalar.

Returns the current "Test::MockModule" object, so you can chain new with mock.

        my $mock = Test::MockModule->new->(...)->mock(...);

The following statements are equivalent:

        $module->mock(purge => 'purged');
        $module->mock(purge => sub { return 'purged'});

When dealing with references, things behave slightly differently. The following statements are NOT equivalent:

        # Returns the same arrayref each time, with the localtime() at time of mocking
        $module->mock(updated => [localtime()]);
        # Returns a new arrayref each time, with up-to-date localtime() value
        $module->mock(updated => sub { return [localtime()]});

The following statements are in fact equivalent:

        my $array_ref = [localtime()]
        $module->mock(updated => $array_ref)
        $module->mock(updated => sub { return $array_ref });

However, "undef" is a special case. If you mock a subroutine with "undef" it will install an empty subroutine

        $module->mock(purge => undef);
        $module->mock(purge => sub { });

rather than a subroutine that returns "undef":

        $module->mock(purge => sub { undef });

You can call mock() for the same subroutine many times, but when you call unmock(), the original subroutine is restored (not the last mocked instance).


If you are trying to mock a subroutine exported from another module, this may not behave as you initially would expect, since Test::MockModule is only mocking at the target module, not anything importing that module. If you mock the local package, or use a fully qualified function name, you will get the behavior you desire:

        use Test::MockModule;
        use Test::More;
        use POSIX qw/strftime/;
        my $posix = Test::MockModule->new("POSIX");
        $posix->mock("strftime", "Yesterday");
        is strftime("%D", localtime(time)), "Yesterday", "`strftime` was mocked successfully"; # Fails
        is POSIX::strftime("%D", localtime(time)), "Yesterday", "`strftime` was mocked successfully"; # Succeeds
        my $main = Test::MockModule->new("main", no_auto => 1);
        $main->mock("strftime", "today");
        is strftime("%D", localtime(time)), "today", "`strftime` was mocked successfully"; # Succeeds

If you are trying to mock a subroutine that was exported into a module that you're trying to test, rather than mocking the subroutine in its originating module, you can instead mock it in the module you are testing:

        package MyModule;
        use POSIX qw/strftime/;
        sub minus_twentyfour
                return strftime("%a, %b %d, %Y", localtime(time - 86400));
        package main;
        use Test::More;
        use Test::MockModule;
        my $posix = Test::MockModule->new("POSIX");
        $posix->mock("strftime", "Yesterday");
        is MyModule::minus_twentyfour(), "Yesterday", "`minus-twentyfour` got mocked"; # fails
        my $mymodule = Test::MockModule->new("MyModule", no_auto => 1);
        $mymodule->mock("strftime", "Yesterday");
        is MyModule::minus_twentyfour(), "Yesterday", "`minus-twentyfour` got mocked"; # succeeds
The same behavior as mock(), but this will preemptively check to be sure that all passed subroutines actually exist. This is useful to ensure that if a mocked module's interface changes the test doesn't just keep on testing a code path that no longer behaves consistently with the mocked behavior.

Note that redefine is also now checking if one of the parent provides the sub and will not die if it's available in the chain.

Returns the current "Test::MockModule" object, so you can chain new with redefine.

        my $mock = Test::MockModule->new->(...)->redefine(...);
The reverse of redefine, this will fail if the passed subroutine exists. While this use case is rare, there are times where the perl code you are testing is inspecting a package and adding a missing subroutine is actually what you want to do.

By using define, you're asserting that the subroutine you want to be mocked should not exist in advance.

Note: define does not check for inheritance like redefine.

Returns the current "Test::MockModule" object, so you can chain new with define.

        my $mock = Test::MockModule->new->(...)->define(...);
Returns the original (unmocked) subroutine

Here is a sample how to wrap a function with custom arguments using the original subroutine. This is useful when you cannot (do not) want to alter the original code to abstract one hardcoded argument pass to a function.

        package MyModule;
        sub sample {
                return get_path_for("/a/b/c/d");
        sub get_path_for {
                ... # anything goes there...
        package main;
        use Test::MockModule;
        my $mock = Test::MockModule->new("MyModule");
        # replace all calls to get_path_for using a different argument
        $mock->redefine("get_path_for", sub {
                return $mock->original("get_path_for")->("/my/custom/path");
        # or
        $mock->redefine("get_path_for", sub {
                my $path = shift;
                if ( $path && $path eq "/a/b/c/d" ) {
                        # only alter calls with path set to "/a/b/c/d"
                        return $mock->original("get_path_for")->("/my/custom/path");
                } else { # preserve the original arguments
                        return $mock->original("get_path_for")->($path, @_);
Restores the original $subroutine. You can specify a list of subroutines to unmock() in one go.
Restores all the subroutines in the package that were mocked. This is automatically called when all "Test::MockObject" objects for the given package go out of scope.
Given a list of subroutine names, mocks each of them with a no-op subroutine. Handy for mocking methods you want to ignore!

    # Neuter a list of methods in one go
    $module->noop('purge', 'updated');
A stub for Log::Trace
A stub for Log::Trace





Current Maintainer: Geoff Franks <>

Original Author: Simon Flack <simonflk _AT_>

Lexical scoping of strictness: David Cantrell <>


Copyright 2004 Simon Flack <simonflk _AT_>. All rights reserved

You may distribute under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl README file.

2021-09-08 perl v5.38.2