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Manual Reference Pages  -  TEST::MOCKRANDOM (3)

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Test::MockRandom - Replaces random number generation with non-random number generation



version 1.01


   # intercept rand in another package
   use Test::MockRandom Some::Other::Package;
   use Some::Other::Package; # exports sub foo { return rand }
   foo(); # returns 0.13

   # using a seed list and "oneish"
   srand(0.23, 0.34, oneish() );
   foo(); # returns 0.23
   foo(); # returns 0.34
   foo(); # returns a number just barely less than one
   foo(); # returns 0, as the seed array is empty

   # object-oriented, for use in the current package
   use Test::MockRandom ();
   my $nrng = Test::MockRandom->new(0.42, 0.23);
   $nrng->rand(); # returns 0.42


This perhaps ridiculous-seeming module was created to test routines that manipulate random numbers by providing a known output from rand. Given a list of seeds with srand, it will return each in turn. After seeded random numbers are exhausted, it will always return 0. Seed numbers must be of a form that meets the expected output from rand as called with no arguments — i.e. they must be between 0 (inclusive) and 1 (exclusive). In order to facilitate generating and testing a nearly-one number, this module exports the function oneish, which returns a number just fractionally less than one.

Depending on how this module is called with use, it will export rand to a specified package (e.g. a class being tested) effectively overriding and intercepting calls in that package to the built-in rand. It can also override rand in the current package or even globally. In all of these cases, it also exports srand and oneish to the current package in order to control the output of rand. See USAGE for details.

Alternatively, this module can be used to generate objects, with each object maintaining its own distinct seed array.


By default, Test::MockRandom does not export any functions. This still allows object-oriented use by calling Test::MockRandom->new(@seeds). In order for Test::MockRandom to be more useful, arguments must be provided during the call to use.

    use Test::MockRandom ’Target::Package’

The simplest way to intercept rand in another package is to provide the name(s) of the package(s) for interception as arguments in the use statement. This will export rand to the listed packages and will export srand and oneish to the current package to control the behavior of rand. You <B>mustB> use Test::MockRandom before you use the target package. This is a typical case for testing a module that uses random numbers:

  use Test::More no_plan;
  use Test::MockRandom Some::Package;
  BEGIN { use_ok( Some::Package ) }

  # assume sub foo { return rand } was imported from Some::Package

  is( foo(), 0.5, "is foo() 0.5?") # test gives "ok"

If multiple package names are specified, rand will be exported to all of them.

If you wish to export rand to the current package, simply provide __PACKAGE__ as the parameter for use, or main if importing to a script without a specified package. This can be part of a list provided to use. All of the following idioms work:

  use Test::MockRandom qw( main Some::Package ); # Assumes a script
  use Test::MockRandom __PACKAGE__, Some::Package;

  # The following doesnt interpolate __PACKAGE__ as above, but
  # Test::MockRandom will still DWIM and handle it correctly

  use Test::MockRandom qw( __PACKAGE__ Some::Package );

use Test::MockRandom CW%customized

As an alternative to a package name as an argument to use, Test::MockRandom will also accept a hash reference with a custom set of instructions for how to export functions:

  use Test::MockRandom {
     rand   => [ Some::Package, {Another::Package => random} ],
     srand  => { Another::Package => seed },
     oneish => __PACKAGE__

The keys of the hash may be any of rand, srand, and oneish. The values of the hash give instructions for where to export the symbol corresponding to the key. These are interpreted as follows, depending on their type:
o String: a package to which Test::MockRandom will export the symbol
o Hash Reference: the key is the package to which Test::MockRandom will export the symbol and the value is the name under which it will be exported
o Array Reference: a list of strings or hash references which will be handled as above


In order to intercept the built-in rand in another package, Test::MockRandom must export its own rand function to the target package <B>beforeB> the target package is compiled, thus overriding calls to the built-in. The simple approach (described above) of providing the target package name in the use Test::MockRandom statement accomplishes this because use is equivalent to a require and import within a BEGIN block. To explicitly intercept rand in another package, you can also call export_rand_to, but it must be enclosed in a BEGIN block of its own. The explicit form also support function aliasing just as with the custom approach with use, described above:

  use Test::MockRandom;
  BEGIN {Test::MockRandom->export_rand_to(AnotherPackage=>random)}
  use AnotherPackage;

This BEGIN block must not include a use statement for the package to be intercepted, or perl will compile the package to be intercepted before the export_rand_to function has a chance to execute and intercept calls to the built-in rand. This is very important in testing. The export_rand_to call must be in a separate BEGIN block from a use or use_ok test, which should be enclosed in a BEGIN block of its own:

  use Test::More tests => 1;
  use Test::MockRandom;
  BEGIN { Test::MockRandom->export_rand_to( AnotherPackage ); }
  BEGIN { use_ok( AnotherPackage ); }

Given these cautions, it’s probably best to use either the simple or custom approach with use, which does the right thing in most circumstances. Should additional explicit customization be necessary, Test::MockRandom also provides export_srand_to and export_oneish_to.

Overriding CWrand globally: use Test::MockRandom ’CORE::GLOBAL

This is just like intercepting rand in a package, except that you do it globally by overriding the built-in function in CORE::GLOBAL.

  use Test::MockRandom CORE::GLOBAL;

  # or

  BEGIN { Test::MockRandom->export_rand_to(CORE::GLOBAL) }

You can always access the real, built-in rand by calling it explicitly as CORE::rand.

Intercepting CWrand in a package that also contains a CWrand function

This is tricky as the order in which the symbol table is manipulated will lead to very different results. This can be done safely (maybe) if the module uses the same rand syntax/prototype as the system call but offers them up as method calls which resolve at run-time instead of compile time. In this case, you will need to do an explicit intercept (as above) but do it <B>afterB> importing the package. I.e.:

  use Test::MockRandom SomeRandPackage;
  use SomeRandPackage;
  BEGIN { Test::MockRandom->export_rand_to(SomeRandPackage);

The first line is necessary to get srand and oneish exported to the current package. The second line will define a sub rand in SomeRandPackage, overriding the results of the first line. The third line then re-overrides the rand. You may see warnings about rand being redefined.

Depending on how your rand is written and used, there is a good likelihood that this isn’t going to do what you’re expecting, no matter what. If your package that defines rand relies internally upon the system CORE::GLOBAL::rand function, then you may be best off overriding that instead.



  $obj = new( LIST OF SEEDS );

Returns a new Test::MockRandom object with the specified list of seeds.


  srand( LIST OF SEEDS );
  $obj->srand( LIST OF SEEDS);

If called as a bare function call or package method, sets the seed list for bare/package calls to rand. If called as an object method, sets the seed list for that object only.


  $rv = rand();
  $rv = $obj->rand();
  $rv = rand(3);

If called as a bare or package function, returns the next value from the package seed list. If called as an object method, returns the next value from the object seed list.

If rand is called with a numeric argument, it follows the same behavior as the built-in function — it multiplies the argument with the next value from the seed array (resulting in a random fractional value between 0 and the argument, just like the built-in). If the argument is 0, undef, or non-numeric, it is treated as if the argument is 1.

Using this with an argument in testing may be complicated, as limits in floating point precision mean that direct numeric comparisons are not reliable. E.g.

  rand(3);       # does this return 1.0 or .999999999 etc.


  srand( oneish() );
  if ( rand() == oneish() ) { print "Its almost one." };

A utility function to return a nearly-one value. Equal to ( 2^32 - 1 ) / 2^32. Useful in srand and test functions.


  Test::MockRandom->export_rand_to( Some::Class );
  Test::MockRandom->export_rand_to( Some::Class => random );

This function exports rand into the specified package namespace. It must be called as a class function. If a second argument is provided, it is taken as the symbol name used in the other package as the alias to rand:

  use Test::MockRandom;
  BEGIN { Test::MockRandom->export_rand_to( Some::Class => random ); }
  use Some::Class;
  srand (0.5);
  print Some::Class::random(); # prints 0.5

It can also be used to explicitly intercept rand after Test::MockRandom has been loaded. The effect of this function is highly dependent on when it is called in the compile cycle and should usually called from within a BEGIN block. See USAGE for details.

Most users will not need this function.


  Test::MockRandom->export_srand_to( Some::Class );
  Test::MockRandom->export_srand_to( Some::Class => seed );

This function exports srand into the specified package namespace. It must be called as a class function. If a second argument is provided, it is taken as the symbol name to use in the other package as the alias for srand. This function may be useful if another package wraps srand:

  # In Some/
  package Some::Class;
  sub seed { srand(shift) }
  sub foo  { rand }

  # In a script
  use Test::MockRandom Some::Class;
  BEGIN { Test::MockRandom->export_srand_to( Some::Class ); }
  use Some::Class;
  print foo();   # prints "0.5"

The effect of this function is highly dependent on when it is called in the compile cycle and should usually be called from within a BEGIN block. See USAGE for details.

Most users will not need this function.


  Test::MockRandom->export_oneish_to( Some::Class );
  Test::MockRandom->export_oneish_to( Some::Class => nearly_one );

This function exports oneish into the specified package namespace. It must be called as a class function. If a second argument is provided, it is taken as the symbol name to use in the other package as the alias for oneish. Since oneish is usually only used in a test script, this function is likely only necessary to alias oneish to some other name in the current package:

  use Test::MockRandom Some::Class;
  BEGIN { Test::MockRandom->export_oneish_to( __PACKAGE__, "one" ); }
  use Some::Class;
  seed( one() );
  print foo();   # prints a value very close to one

The effect of this function is highly dependent on when it is called in the compile cycle and should usually be called from within a BEGIN block. See USAGE for details.

Most users will not need this function.


o Test::MockObject
o Test::MockModule


    Bugs / Feature Requests

Please report any bugs or feature requests through the issue tracker at <>. You will be notified automatically of any progress on your issue.

    Source Code

This is open source software. The code repository is available for public review and contribution under the terms of the license.


  git clone


David Golden <>


This software is Copyright (c) 2014 by David Golden.

This is free software, licensed under:

  The Apache License, Version 2.0, January 2004

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perl v5.20.3 TEST::MOCKRANDOM (3) 2014-02-08

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