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Michael G Schwern > Test-Sims > Test::Sims



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Test::Sims - Helps build semi-random data for testing


    package My::Sims;

    use Test::Sims;

    # Creates rand_name() and exported on demand.
    make_rand name => [
        qw(Mal Zoe Jayne Kaylee Inara River Simon Wash Zoe Book)

    # Automatically exported
    sub sim_character {
        my %defaults = (
            name   => rand_name(),
            series => "Firefly",

        require Character;
        return Character->new(
            %defaults, @_;


THIS IS AN EARLY RELEASE! While very well tested behaviors may change. The interface is not stable.

This is a module to help building semi-random data for testing and to create large, nested, interesting data structures.

This module contains no new assertions, but it does tie in with Test::Builder.

It does two things. It contains functions which make generating random data easier and it allows you to write repeatable, yet random, test data.


    my $code = make_rand $name => \@list;
    my $code = make_rand $name => sub { ... };

Creates a subroutine called <rand_$name> and exports it on request.

If a @list is given it will generate a subroutine which returns elements out of @list at random. It takes min and max arguments to control how many.

    my @items = rand_$name(
        min => $min_random_items,
        max => $max_random_items

min and max both default to 1. So by default you get 1 item.

If a subroutine is given it will simply give that routine a name. This is just to get the convenience of adding it to the exports.

Also adds it to a "rand" export tag.

        package Sim::Firefly;

        make_rand crew => [
            qw(Zoe Wash Mal River Simon Book Jayne Kaylee Inara)


        use Sim::Firefly ":rand";

        my $crew = rand_crew;             # 1 name
        my @crew = rand_crew( max => 3 ); # 1, 2 or 3 names



A utility function which causes your module to export all the functions called <sims_*>. It also creates an export tag called "sims".

You should call this at the end of your Sim package.

Controlling randomness

You can control the random seed used by Test::Sims by setting the TEST_SIMS_SEED environment variable. This is handy to make test runs repeatable.

    TEST_SIMS_SEED=12345 perl -Ilib t/some_test.t

Test::Sims will output the seed used at the end of each test run. If the test failed it will be visible to the user, otherwise it will be a TAP comment and only visible if the test is run verbosely.

If having new data every run is too chaotic for you, you can set TEST_SIMS_SEED to something which will remain fixed during a development session. Perhaps the PID of your shell or your uid or the date (20090704, for example).

sim functions

Test::Sims doesn't do anything with functions named sim_* but export them. Generally we recommend they're written like so:

    sub sim_thing {
        my %defaults = (
            name        => rand_name(),
            age         => rand_age(),
            motto       => rand_text(),
            picture     => rand_image(),

        return Thing->new( %defaults, @_ );

This way you can get a completely random Thing.

    my $thing = sim_thing();

Or you can lock down the bits you need leaving the rest to float free.

    # Joe's motto and picture remain random
    my $joe = sim_thing(
        name => "Joe",
        age  => 64


Here's an example of making a simple package to generate random dates.

    package Sim::Date;

    use strict;
    use warnings;

    require DateTime;
    use Test::Sims;

    make_rand year  => [1800..2100];

    sub sim_datetime {
        my %args = @_;

        my $year = $args{year} || rand_year();
        my $date = DateTime->new( year => $year );

        my $days_in_year = $date->is_leap_year ? 366 : 365;
        my $secs = rand( $days_in_year * 24 * 60 * 60 );
        $date->add( seconds => $secs );

        $date->set( %args );

        return $date;


And then using it.

    use Sim::Date;

    # Random date.
    my $date = sim_datetime;

    # Random date in July 2009
    my $date = sim_datetime(
        year  => 2009,
        month => 7,



If defined its value will be used to make tests repeatable. See "Controlling randomness".


"Generating Test Data with The Sims" is a set of slides outlining the Sims testing technique which this module is supporting.

Data::Random for common rand_* routines.

Data::Generate to generate random data from a set of rules.


The source code repository can be found at

The latest release can be found at


Please report bugs, problems, rough corners, feedback and suggestions to

Report early, report often.


Thanks go to the folks at Blackstar and Grant Street Group for helping to develop this technique.


Copyright 2009 Michael G Schwern ><

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


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