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Author image Curtis "Ovid" Poe


Test::Differences - Test strings and data structures and show differences if not ok




   use Test;    ## Or use Test::More
   use Test::Differences;

   eq_or_diff $got,  "a\nb\nc\n",   "testing strings";
   eq_or_diff \@got, [qw( a b c )], "testing arrays";

   ## Passing options:
   eq_or_diff $got, $expected, $name, { context => 300 };  ## options

   ## Using with DBI-like data structures

   use DBI;

   ... open connection & prepare statement and @expected_... here...

   eq_or_diff $sth->fetchall_arrayref, \@expected_arrays  "testing DBI arrays";
   eq_or_diff $sth->fetchall_hashref,  \@expected_hashes, "testing DBI hashes";

   ## To force textual or data line numbering (text lines are numbered 1..):
   eq_or_diff_text ...;
   eq_or_diff_data ...;


This module exports three test functions and four diff-style functions:

  • Test functions

    • eq_or_diff

    • eq_or_diff_data

    • eq_or_diff_text

  • Diff style functions

    • table_diff (the default)

    • unified_diff

    • oldstyle_diff

    • context_diff


When the code you're testing returns multiple lines, records or data structures and they're just plain wrong, an equivalent to the Unix diff utility may be just what's needed. Here's output from an example test script that checks two text documents and then two (trivial) data structures:

 not ok 1 - differences in text
 #     Failed test ((eval 2) at line 14)
 #     +---+----------------+----------------+
 #     | Ln|Got             |Expected        |
 #     +---+----------------+----------------+
 #     |  1|this is line 1  |this is line 1  |
 #     *  2|this is line 2  |this is line b  *
 #     |  3|this is line 3  |this is line 3  |
 #     +---+----------------+----------------+
 not ok 2 - differences in whitespace
 #     Failed test ((eval 2) at line 20)
 #     +---+------------------+------------------+
 #     | Ln|Got               |Expected          |
 #     +---+------------------+------------------+
 #     |  1|        indented  |        indented  |
 #     *  2|        indented  |\tindented        *
 #     |  3|        indented  |        indented  |
 #     +---+------------------+------------------+
 not ok 3
 #     Failed test ((eval 2) at line 22)
 #     +----+-------------------------------------+----------------------------+
 #     | Elt|Got                                  |Expected                    |
 #     +----+-------------------------------------+----------------------------+
 #     *   0|bless( [                             |[                           *
 #     *   1|  'Move along, nothing to see here'  |  'Dry, humorless message'  *
 #     *   2|], 'Test::Builder' )                 |]                           *
 #     +----+-------------------------------------+----------------------------+
 # Looks like you failed 3 tests of 3.

eq_or_diff_...() compares two strings or (limited) data structures and either emits an ok indication or a side-by-side diff. Test::Differences is designed to be used with Test.pm and with Test::Simple, Test::More, and other Test::Builder based testing modules. As the SYNOPSIS shows, another testing module must be used as the basis for your test suite.

These functions assume that you are presenting it with "flat" records, looking like:

   - scalars composed of record-per-line
   - arrays of scalars,
   - arrays of arrays of scalars,
   - arrays of hashes containing only scalars

All of these are flattened in to single strings which are then compared for differences. Differently data structures can be compared, as long as they flatten identically.

All other data structures are run through Data::Dumper first. This is a bit dangerous, as some versions of perl shipped with Data::Dumpers that could do the oddest things with unexpected, like core dump. Only as of 5.8.0 does Data::Dumper sort hash keys, which is necessary for HASH dumps to be fully predictable. This will be changed when this bites somebody or I get some free time.

eq_or_diff() starts counting records at 0 unless you pass it two text strings:

   eq_or_diff $a, $b;   ## First line is line number 1
   eq_or_diff @a, @b;   ## First element is element 0
   eq_or_diff $a, @b;   ## First line/element is element 0

If you want to force a first record number of 0, use eq_or_diff_data. If you want to force a first record number of 1, use eq_or_diff_text. I chose this over passing in an options hash because it's clearer and simpler this way. YMMV.


The options to eq_or_diff give some fine-grained control over the output.

  • context

    This allows you to control the amount of context shown:

       eq_or_diff $got, $expected, $name, { context => 50000 };

    will show you lots and lots of context. Normally, eq_or_diff() uses some heuristics to determine whether to show 3 lines of context (like a normal unified diff) or 25 lines.

  • data_type

    text or data. See eq_or_diff_text and eq_or_diff_data to understand this. You can usually ignore this.


For extremely long strings, a table diff can wrap on your screen and be hard to read. If you are comfortable with different diff formats, you can switch to a format more suitable for your data. These are the four formats supported by the Text::Diff module and are set with the following functions:

  • table_diff (the default)

  • unified_diff

  • oldstyle_diff

  • context_diff

You can run the following to understand the different diff output styles:

 use Test::More 'no_plan';
 use Test::Differences;

 my $long_string = join '' => 1..40;

 TODO: {
     local $TODO = 'Testing diff styles';

     # this is the default and does not need to explicitly set unless you need
     # to reset it back from another diff type
     eq_or_diff $long_string, "-$long_string", 'table diff';

     eq_or_diff $long_string, "-$long_string", 'unified diff';

     eq_or_diff $long_string, "-$long_string", 'context diff';

     eq_or_diff $long_string, "-$long_string", 'oldstyle diff';


There are several basic ways of deploying Test::Differences requiring more or less labor by you or your users.

  • Fallback to is_deeply.

    This is your best option if you want this module to be optional.

     use Test::More;
     BEGIN {
         if (!eval q{ use Test::Differences; 1 }) {
             *eq_or_diff = \&is_deeply;
  •  eval "use Test::Differences";

    If you want to detect the presence of Test::Differences on the fly, something like the following code might do the trick for you:

        use Test qw( !ok );   ## get all syms *except* ok
        eval "use Test::Differences";
        use Data::Dumper;
        sub ok {
            goto &eq_or_diff if defined &eq_or_diff && @_ > 1;
            @_ = map ref $_ ? Dumper( @_ ) : $_, @_;
            goto Test::&ok;
        plan tests => 1;
        ok "a", "b";
  • PREREQ_PM => { .... "Test::Differences" => 0, ... }

    This method will let CPAN and CPANPLUS users download it automatically. It will discomfit those users who choose/have to download all packages manually.

  • t/lib/Test/Differences.pm, t/lib/Text/Diff.pm, ...

    By placing Test::Differences and its prerequisites in the t/lib directory, you avoid forcing your users to download the Test::Differences manually if they aren't using CPAN or CPANPLUS.

    If you put a use lib "t/lib"; in the top of each test suite before the use Test::Differences;, make test should work well.

    You might want to check once in a while for new Test::Differences releases if you do this.


Test or Test::More

This module "mixes in" with Test.pm or any of the test libraries based on Test::Builder (Test::Simple, Test::More, etc). It does this by peeking to see whether Test.pm or Test/Builder.pm is in %INC, so if you are not using one of those, it will print a warning and play dumb by not emitting test numbers (or incrementing them). If you are using one of these, it should interoperate nicely.


Exports all 3 functions by default (and by design). Use

    use Test::Differences ();

to suppress this behavior if you don't like the namespace pollution.

This module will not override functions like ok(), is(), is_deeply(), etc. If it did, then you could eval "use Test::Differences qw( is_deeply );" to get automatic upgrading to diffing behaviors without the sub my_ok shown above. Test::Differences intentionally does not provide this behavior because this would mean that Test::Differences would need to emulate every popular test module out there, which would require far more coding and maintenance that I'm willing to do. Use the eval and my_ok deployment shown above if you want some level of automation.


Perls before 5.6.0 don't support characters > 255 at all, and 5.6.0 seems broken. This means that you might get odd results using perl5.6.0 with unicode strings.

Data::Dumper and older Perls.

Relies on Data::Dumper (for now), which, prior to perl5.8, will not always report hashes in the same order. $Data::Dumper::SortKeys is set to 1, so on more recent versions of Data::Dumper, this should not occur. Check CPAN to see if it's been peeled out of the main perl distribution and backported. Reported by Ilya Martynov <ilya@martynov.org>, although the SortKeys "future perfect" workaround has been set in anticipation of a new Data::Dumper for a while. Note that the two hashes should report the same here:

    not ok 5
    #     Failed test (t/ctrl/05-home.t at line 51)
    # +----+------------------------+----+------------------------+
    # | Elt|Got                     | Elt|Expected                |
    # +----+------------------------+----+------------------------+
    # |   0|{                       |   0|{                       |
    # |   1|  'password' => '',     |   1|  'password' => '',     |
    # *   2|  'method' => 'login',  *    |                        |
    # |   3|  'ctrl' => 'home',     |   2|  'ctrl' => 'home',     |
    # |    |                        *   3|  'method' => 'login',  *
    # |   4|  'email' => 'test'     |   4|  'email' => 'test'     |
    # |   5|}                       |   5|}                       |
    # +----+------------------------+----+------------------------+

Data::Dumper also overlooks the difference between

    $a[0] = \$a[1];
    $a[1] = \$a[0];   # $a[0] = \$a[1]


    $x = \$y;
    $y = \$x;
    @a = ( $x, $y );  # $a[0] = \$y, not \$a[1]

The former involves two scalars, the latter 4: $x, $y, and @a[0,1]. This was carefully explained to me in words of two syllables or less by Yves Orton <demerphq@hotmail.com>. The plan to address this is to allow you to select Data::Denter or some other module of your choice as an option.


    Barrie Slaymaker <barries@slaysys.com>


    Curtis "Ovid" Poe <ovid@cpan.org>


Copyright 2001-2008 Barrie Slaymaker, All Rights Reserved.

You may use this software under the terms of the GNU public license, any version, or the Artistic license.