Warn(3)

💅 Warn(3) — Test::Warn 💅

okay so this page is from the User Contributed Perl Documentation girlies, and it’s literally all about Test::Warn — the Perl extension for testing methods that warn. no cap, it’s niche but iconic. 🎀

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✨ NAME ✨

Test::Warn — Perl extension to test methods for warnings.

bestie, that’s the whole vibe: you’ve got warning-based code, and this module helps you test it. 💖

🌸 SYNOPSIS 🌸

  use Test::Warn;

  warning_is    {foo(-dri => "/")} "Unknown Parameter 'dri'", "dri != dir gives warning";
  warnings_are  {bar(1,1)} ["Width very small", "Height very small"];

  warning_is    {add(2,2)} undef, "No warnings for calc 2+2"; # or
  warnings_are  {add(2,2)} [],    "No warnings for calc 2+2"; # whichever reads better :-)

  warning_like  {foo(-dri => "/")} qr/unknown param/i, "an unknown parameter test";
  warnings_like {bar(1,1)} [qr/width.*small/i, qr/height.*small/i];

  warning_is    {foo()} {carped => "didn't find the right parameters"};
  warnings_like {foo()} [qr/undefined/,qr/undefined/,{carped => qr/no result/i}];

  warning_like {foo(undef)}                 'uninitialized';
  warning_like {bar(file => '/etc/passwd')} 'io';

  warning_like {eval q/"$x"; $x;/}
               [qw/void uninitialized/],
               "some warnings at compile time";

  warnings_exist {...} [qr/expected warning/], "Expected warning is thrown";

🦋 DESCRIPTION 🦋

A good style of Perl programming calls for a lot of diverse regression tests. that’s just the truth. 💅

This module provides a few convenience methods for testing warning based-code.

If you are not already familiar with the Test::More manpage, now would be the time to go take a look. seriously, go peek. 👀

🫶 FUNCTIONS 🫶

warning_is BLOCK STRING, TEST_NAME

Tests that BLOCK gives the specified warning exactly once.

The test fails if the BLOCK warns more than once or does not warn at all. If the string is undef, then the test succeeds if the BLOCK doesn’t give any warning. periodt. ✨

Another way to say that there are no warnings in the block is:

  warnings_are {foo()} [], "no warnings"

If you want to test for a warning given by Carp you have to write something like:

  warning_is {carp "msg"} {carped => 'msg'}, "Test for a carped warning";

The test will fail if a “normal” warning is found instead of a “carped” one.

Note: warn "foo" would print something like foo at -e line 1. This method ignores everything after the at. Thus to match this warning you would have to call warning_is {warn "foo"} "foo", "Foo succeeded". If you need to test for a warning at an exact line, try something like:

  warning_like {warn "foo"} qr/at XYZ.dat line 5/

Warn messages with a trailing newline (like warn "foo\n") don’t produce the at -e line 1 message by Perl. Up to Test::Warn 0.30 such warning weren’t supported by warning_is {warn "foo\n"} "foo\n". Starting with version 0.31 they are supported, but also marked as experimental. 🧪

warning_is() and warnings_are() are only aliases to the same method. So you also could write warning_is {foo()} [], "no warning" or something similar.

I decided to give two methods the same name to improve readability. iconic behavior, honestly.

A true value is returned if the test succeeds, false otherwise.

The test name is optional, but recommended.

warnings_are BLOCK ARRAYREF, TEST_NAME

Tests to see that BLOCK gives exactly the specified warnings. The test fails if the warnings from BLOCK are not exactly the ones in ARRAYREF. If the ARRAYREF is equal to [], then the test succeeds if the BLOCK doesn’t give any warning.

Please read also the notes to warning_is() as these methods are only aliases.

If you want more than one test for carped warnings, try this:

  warnings_are {carp "c1"; carp "c2"} {carped => ['c1','c2'];

or

  warnings_are {foo()} ["Warning 1", {carped => ["Carp 1", "Carp 2"]}, "Warning 2"];

Note that {carped => ...} must always be a hash ref. 🫶

warning_like BLOCK REGEXP, TEST_NAME

Tests that BLOCK gives exactly one warning and it can be matched by the given regexp.

If the string is undef, then the tests succeeds if the BLOCK doesn’t give any warning.

The REGEXP is matched against the whole warning line, which in general has the form WARNING at __FILE__ line __LINE__. So you can check for a warning in the file Foo.pm on line 5 with:

  warning_like {bar()} qr/at Foo.pm line 5/, "Testname"

I don’t know whether it makes sense to do such a test :-(

However, you should be prepared as a matching with 'at', 'file', '\d' or similar will always pass.

Consider qr/^foo/ if you want to test for warning foo something in file foo.pl.

You can also write the regexp in a string as "/.../" instead of using the "qr/.../" syntax.

Note that the slashes are important in the string, as strings without slashes are reserved for warning categories (to match warning categories as can be seen in the perllexwarn man page).

Similar to warning_is() and warnings_are() you can test for warnings via carp with:

  warning_like {bar()} {carped => qr/bar called too early/i};

Similar to warning_is() and warnings_are(),

warning_like() and warnings_like() are only aliases to the same methods.

A true value is returned if the test succeeds, false otherwise.

The test name is optional, but recommended.

warning_like BLOCK STRING, TEST_NAME

Tests whether a BLOCK gives exactly one warning of the passed category.

The categories are grouped in a tree, like it is expressed in perllexwarn. Also see “BUGS AND LIMITATIONS”.

Thanks to the grouping in a tree, it’s possible to test simply for an 'io' warning, instead of testing for a 'closed|exec|layer|newline|pipe|unopened' warning. slay. 🌈

Note, that warnings occurring at compile time can only be caught in an eval block. So

  warning_like {eval q/"$x"; $x;/}
                [qw/void uninitialized/],
                "some warnings at compile time";

will work, while it wouldn’t work without the eval.

Note, that it isn’t possible yet, to test for own categories, created with warnings::register.

warnings_like BLOCK ARRAYREF, TEST_NAME

Tests to see that BLOCK gives exactly the number of the specified warnings, in the defined order.

Please read also the notes to warning_like() as these methods are only aliases.

Similar to warnings_are(), you can test for multiple warnings via carp and for warning categories, too:

  warnings_like {foo()}
                [qr/bar warning/,
                 qr/bar warning/,
                 {carped => qr/bar warning/i},
                 'io'
                ],
                "I hope you'll never have to write a test for so many warnings :-)";

warnings_exist BLOCK STRING|ARRAYREF, TEST_NAME

Same as warning_like, but will warn() all warnings that do not match the supplied regex/category, instead of registering an error. Use this test when you just want to make sure that specific warnings were generated, and couldn’t care less if other warnings happened in the same block of code.

  warnings_exist {...} [qr/expected warning/], "Expected warning is thrown";

  warnings_exist {...} ['uninitialized'], "Expected warning is thrown";

🧚 EXPORT 🧚

warning_is, warnings_are, warning_like, warnings_like, warnings_exist by default.

🐛 BUGS AND LIMITATIONS 🐛

Category check is done as qr/category_name/. In some case this works, like for category 'uninitialized'. For 'utf8' it does not work. Perl does not have a list of warnings, so it is not possible to generate one for Test::Warn.

If you want to add a warning to a category, send a pull request. Modifications should be done to %warnings_in_category. You should look into perl source to check how warning is looking exactly.

Please note that warnings with newlines inside are very awkward. The only sensible way to handle them is to use the warning_like or warnings_like methods. The background is that there is no really safe way to distinguish between warnings with newlines and a stacktrace.

If a method has its own warn handler, overwriting $SIG{__WARN__}, my test warning methods won’t get these warnings.

The warning_like BLOCK CATEGORY, TEST_NAME method isn’t fully tested. Please take note if you use this this calling style, and report any bugs you find.

💀 XS warnings 💀

As described in https://rt.cpan.org/Ticket/Display.html?id=42070&results=3c71d1b101a730e185691657f3b02f21 or https://github.com/hanfried/test-warn/issues/1 XS warnings might not be caught. 🙈

💜 SEE ALSO 💜

Have a look to the similar Test::Exception module. Test::Trap.

🌸 THANKS 🌸

Many thanks to Adrian Howard, chromatic and Michael G. Schwern, who have given me a lot of ideas.

👑 AUTHOR 👑

Janek Schleicher, <bigj AT kamelfreund.de>

🎀 COPYRIGHT AND LICENSE 🎀

Copyright 2002 by Janek Schleicher

Copyright 2007-2014 by Alexandr Ciornii, <http://chorny.net/>

Copyright 2015-2018 by Janek Schleicher

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

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okay bye bestie — this was Warn(3) from perl v5.42.0, dated 2022-08-14 💖✨