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Data::Dumper(3pm)                     Perl Programmers Reference Guide                     Data::Dumper(3pm)



NAME
       Data::Dumper - stringified perl data structures, suitable for both printing and "eval"

SYNOPSIS
           use Data::Dumper;

           # simple procedural interface
           print Dumper($foo, $bar);

           # extended usage with names
           print Data::Dumper->Dump([$foo, $bar], [qw(foo *ary)]);

           # configuration variables
           {
             local $Data::Dumper::Purity = 1;
             eval Data::Dumper->Dump([$foo, $bar], [qw(foo *ary)]);
           }

           # OO usage
           $d = Data::Dumper->new([$foo, $bar], [qw(foo *ary)]);
              ...
           print $d->Dump;
              ...
           $d->Purity(1)->Terse(1)->Deepcopy(1);
           eval $d->Dump;

DESCRIPTION
       Given a list of scalars or reference variables, writes out their contents in perl syntax. The
       references can also be objects.  The content of each variable is output in a single Perl statement.
       Handles self-referential structures correctly.

       The return value can be "eval"ed to get back an identical copy of the original reference structure.

       Any references that are the same as one of those passed in will be named $VARn (where n is a numeric
       suffix), and other duplicate references to substructures within $VARn will be appropriately labeled
       using arrow notation.  You can specify names for individual values to be dumped if you use the
       "Dump()" method, or you can change the default $VAR prefix to something else.  See
       $Data::Dumper::Varname and $Data::Dumper::Terse below.

       The default output of self-referential structures can be "eval"ed, but the nested references to $VARn
       will be undefined, since a recursive structure cannot be constructed using one Perl statement.  You
       should set the "Purity" flag to 1 to get additional statements that will correctly fill in these
       references.  Moreover, if "eval"ed when strictures are in effect, you need to ensure that any
       variables it accesses are previously declared.

       In the extended usage form, the references to be dumped can be given user-specified names.  If a name
       begins with a "*", the output will describe the dereferenced type of the supplied reference for
       hashes and arrays, and coderefs.  Output of names will be avoided where possible if the "Terse" flag
       is set.

       In many cases, methods that are used to set the internal state of the object will return the object
       itself, so method calls can be conveniently chained together.

       Several styles of output are possible, all controlled by setting the "Indent" flag.  See
       "Configuration Variables or Methods" below for details.

   Methods
       PACKAGE->new(ARRAYREF [, ARRAYREF])
           Returns a newly created "Data::Dumper" object.  The first argument is an anonymous array of
           values to be dumped.  The optional second argument is an anonymous array of names for the values.
           The names need not have a leading "$" sign, and must be comprised of alphanumeric characters.
           You can begin a name with a "*" to specify that the dereferenced type must be dumped instead of
           the reference itself, for ARRAY and HASH references.

           The prefix specified by $Data::Dumper::Varname will be used with a numeric suffix if the name for
           a value is undefined.

           Data::Dumper will catalog all references encountered while dumping the values. Cross-references
           (in the form of names of substructures in perl syntax) will be inserted at all possible points,
           preserving any structural interdependencies in the original set of values.  Structure traversal
           is depth-first,  and proceeds in order from the first supplied value to the last.

       $OBJ->Dump  or  PACKAGE->Dump(ARRAYREF [, ARRAYREF])
           Returns the stringified form of the values stored in the object (preserving the order in which
           they were supplied to "new"), subject to the configuration options below.  In a list context, it
           returns a list of strings corresponding to the supplied values.

           The second form, for convenience, simply calls the "new" method on its arguments before dumping
           the object immediately.

       $OBJ->Seen([HASHREF])
           Queries or adds to the internal table of already encountered references.  You must use "Reset" to
           explicitly clear the table if needed.  Such references are not dumped; instead, their names are
           inserted wherever they are encountered subsequently.  This is useful especially for properly
           dumping subroutine references.

           Expects an anonymous hash of name => value pairs.  Same rules apply for names as in "new".  If no
           argument is supplied, will return the "seen" list of name => value pairs, in a list context.
           Otherwise, returns the object itself.

       $OBJ->Values([ARRAYREF])
           Queries or replaces the internal array of values that will be dumped.  When called without
           arguments, returns the values.  Otherwise, returns the object itself.

       $OBJ->Names([ARRAYREF])
           Queries or replaces the internal array of user supplied names for the values that will be dumped.
           When called without arguments, returns the names.  Otherwise, returns the object itself.

       $OBJ->Reset
           Clears the internal table of "seen" references and returns the object itself.

   Functions
       Dumper(LIST)
           Returns the stringified form of the values in the list, subject to the configuration options
           below.  The values will be named $VARn in the output, where n is a numeric suffix.  Will return a
           list of strings in a list context.

   Configuration Variables or Methods
       Several configuration variables can be used to control the kind of output generated when using the
       procedural interface.  These variables are usually "local"ized in a block so that other parts of the
       code are not affected by the change.

       These variables determine the default state of the object created by calling the "new" method, but
       cannot be used to alter the state of the object thereafter.  The equivalent method names should be
       used instead to query or set the internal state of the object.

       The method forms return the object itself when called with arguments, so that they can be chained
       together nicely.

       •   $Data::Dumper::Indent  or  $OBJ->Indent([NEWVAL])

           Controls the style of indentation.  It can be set to 0, 1, 2 or 3.  Style 0 spews output without
           any newlines, indentation, or spaces between list items.  It is the most compact format possible
           that can still be called valid perl.  Style 1 outputs a readable form with newlines but no fancy
           indentation (each level in the structure is simply indented by a fixed amount of whitespace).
           Style 2 (the default) outputs a very readable form which takes into account the length of hash
           keys (so the hash value lines up).  Style 3 is like style 2, but also annotates the elements of
           arrays with their index (but the comment is on its own line, so array output consumes twice the
           number of lines).  Style 2 is the default.

       •   $Data::Dumper::Purity  or  $OBJ->Purity([NEWVAL])

           Controls the degree to which the output can be "eval"ed to recreate the supplied reference
           structures.  Setting it to 1 will output additional perl statements that will correctly recreate
           nested references.  The default is 0.

       •   $Data::Dumper::Pad  or  $OBJ->Pad([NEWVAL])

           Specifies the string that will be prefixed to every line of the output.  Empty string by default.

       •   $Data::Dumper::Varname  or  $OBJ->Varname([NEWVAL])

           Contains the prefix to use for tagging variable names in the output. The default is "VAR".

       •   $Data::Dumper::Useqq  or  $OBJ->Useqq([NEWVAL])

           When set, enables the use of double quotes for representing string values.  Whitespace other than
           space will be represented as "[\n\t\r]", "unsafe" characters will be backslashed, and unprintable
           characters will be output as quoted octal integers.  Since setting this variable imposes a
           performance penalty, the default is 0.  "Dump()" will run slower if this flag is set, since the
           fast XSUB implementation doesn't support it yet.

       •   $Data::Dumper::Terse  or  $OBJ->Terse([NEWVAL])

           When set, Data::Dumper will emit single, non-self-referential values as atoms/terms rather than
           statements.  This means that the $VARn names will be avoided where possible, but be advised that
           such output may not always be parseable by "eval".

       •   $Data::Dumper::Freezer  or  $OBJ->Freezer([NEWVAL])

           Can be set to a method name, or to an empty string to disable the feature.  Data::Dumper will
           invoke that method via the object before attempting to stringify it.  This method can alter the
           contents of the object (if, for instance, it contains data allocated from C), and even rebless it
           in a different package.  The client is responsible for making sure the specified method can be
           called via the object, and that the object ends up containing only perl data types after the
           method has been called.  Defaults to an empty string.

           If an object does not support the method specified (determined using UNIVERSAL::can()) then the
           call will be skipped.  If the method dies a warning will be generated.

       •   $Data::Dumper::Toaster  or  $OBJ->Toaster([NEWVAL])

           Can be set to a method name, or to an empty string to disable the feature.  Data::Dumper will
           emit a method call for any objects that are to be dumped using the syntax "bless(DATA,
           CLASS)->METHOD()".  Note that this means that the method specified will have to perform any
           modifications required on the object (like creating new state within it, and/or reblessing it in
           a different package) and then return it.  The client is responsible for making sure the method
           can be called via the object, and that it returns a valid object.  Defaults to an empty string.

       •   $Data::Dumper::Deepcopy  or  $OBJ->Deepcopy([NEWVAL])

           Can be set to a boolean value to enable deep copies of structures.  Cross-referencing will then
           only be done when absolutely essential (i.e., to break reference cycles).  Default is 0.

       •   $Data::Dumper::Quotekeys  or  $OBJ->Quotekeys([NEWVAL])

           Can be set to a boolean value to control whether hash keys are quoted.  A false value will avoid
           quoting hash keys when it looks like a simple string.  Default is 1, which will always enclose
           hash keys in quotes.

       •   $Data::Dumper::Bless  or  $OBJ->Bless([NEWVAL])

           Can be set to a string that specifies an alternative to the "bless" builtin operator used to
           create objects.  A function with the specified name should exist, and should accept the same
           arguments as the builtin.  Default is "bless".

       •   $Data::Dumper::Pair  or  $OBJ->Pair([NEWVAL])

           Can be set to a string that specifies the separator between hash keys and values. To dump nested
           hash, array and scalar values to JavaScript, use: "$Data::Dumper::Pair = ' : ';". Implementing
           "bless" in JavaScript is left as an exercise for the reader.  A function with the specified name
           exists, and accepts the same arguments as the builtin.

           Default is: " => ".

       •   $Data::Dumper::Maxdepth  or  $OBJ->Maxdepth([NEWVAL])

           Can be set to a positive integer that specifies the depth beyond which we don't venture into a
           structure.  Has no effect when "Data::Dumper::Purity" is set.  (Useful in debugger when we often
           don't want to see more than enough).  Default is 0, which means there is no maximum depth.

       •   $Data::Dumper::Useperl  or  $OBJ->Useperl([NEWVAL])

           Can be set to a boolean value which controls whether the pure Perl implementation of
           "Data::Dumper" is used. The "Data::Dumper" module is a dual implementation, with almost all
           functionality written in both pure Perl and also in XS ('C'). Since the XS version is much
           faster, it will always be used if possible. This option lets you override the default behavior,
           usually for testing purposes only. Default is 0, which means the XS implementation will be used
           if possible.

       •   $Data::Dumper::Sortkeys  or  $OBJ->Sortkeys([NEWVAL])

           Can be set to a boolean value to control whether hash keys are dumped in sorted order. A true
           value will cause the keys of all hashes to be dumped in Perl's default sort order. Can also be
           set to a subroutine reference which will be called for each hash that is dumped. In this case
           "Data::Dumper" will call the subroutine once for each hash, passing it the reference of the hash.
           The purpose of the subroutine is to return a reference to an array of the keys that will be
           dumped, in the order that they should be dumped. Using this feature, you can control both the
           order of the keys, and which keys are actually used. In other words, this subroutine acts as a
           filter by which you can exclude certain keys from being dumped. Default is 0, which means that
           hash keys are not sorted.

       •   $Data::Dumper::Deparse  or  $OBJ->Deparse([NEWVAL])

           Can be set to a boolean value to control whether code references are turned into perl source
           code. If set to a true value, "B::Deparse" will be used to get the source of the code reference.
           Using this option will force using the Perl implementation of the dumper, since the fast XSUB
           implementation doesn't support it.

           Caution : use this option only if you know that your coderefs will be properly reconstructed by
           "B::Deparse".

   Exports
       Dumper

EXAMPLES
       Run these code snippets to get a quick feel for the behavior of this module.  When you are through
       with these examples, you may want to add or change the various configuration variables described
       above, to see their behavior.  (See the testsuite in the Data::Dumper distribution for more
       examples.)

           use Data::Dumper;

           package Foo;
           sub new {bless {'a' => 1, 'b' => sub { return "foo" }}, $_[0]};

           package Fuz;                       # a weird REF-REF-SCALAR object
           sub new {bless \($_ = \ 'fu\'z'), $_[0]};

           package main;
           $foo = Foo->new;
           $fuz = Fuz->new;
           $boo = [ 1, [], "abcd", \*foo,
                    {1 => 'a', 023 => 'b', 0x45 => 'c'},
                    \\"p\q\'r", $foo, $fuz];

           ########
           # simple usage
           ########

           $bar = eval(Dumper($boo));
           print($@) if $@;
           print Dumper($boo), Dumper($bar);  # pretty print (no array indices)

           $Data::Dumper::Terse = 1;          # don't output names where feasible
           $Data::Dumper::Indent = 0;         # turn off all pretty print
           print Dumper($boo), "\n";

           $Data::Dumper::Indent = 1;         # mild pretty print
           print Dumper($boo);

           $Data::Dumper::Indent = 3;         # pretty print with array indices
           print Dumper($boo);

           $Data::Dumper::Useqq = 1;          # print strings in double quotes
           print Dumper($boo);

           $Data::Dumper::Pair = " : ";       # specify hash key/value separator
           print Dumper($boo);


           ########
           # recursive structures
           ########

           @c = ('c');
           $c = \@c;
           $b = {};
           $a = [1, $b, $c];
           $b->{a} = $a;
           $b->{b} = $a->[1];
           $b->{c} = $a->[2];
           print Data::Dumper->Dump([$a,$b,$c], [qw(a b c)]);


           $Data::Dumper::Purity = 1;         # fill in the holes for eval
           print Data::Dumper->Dump([$a, $b], [qw(*a b)]); # print as @a
           print Data::Dumper->Dump([$b, $a], [qw(*b a)]); # print as %b


           $Data::Dumper::Deepcopy = 1;       # avoid cross-refs
           print Data::Dumper->Dump([$b, $a], [qw(*b a)]);


           $Data::Dumper::Purity = 0;         # avoid cross-refs
           print Data::Dumper->Dump([$b, $a], [qw(*b a)]);

           ########
           # deep structures
           ########

           $a = "pearl";
           $b = [ $a ];
           $c = { 'b' => $b };
           $d = [ $c ];
           $e = { 'd' => $d };
           $f = { 'e' => $e };
           print Data::Dumper->Dump([$f], [qw(f)]);

           $Data::Dumper::Maxdepth = 3;       # no deeper than 3 refs down
           print Data::Dumper->Dump([$f], [qw(f)]);


           ########
           # object-oriented usage
           ########

           $d = Data::Dumper->new([$a,$b], [qw(a b)]);
           $d->Seen({'*c' => $c});            # stash a ref without printing it
           $d->Indent(3);
           print $d->Dump;
           $d->Reset->Purity(0);              # empty the seen cache
           print join "----\n", $d->Dump;


           ########
           # persistence
           ########

           package Foo;
           sub new { bless { state => 'awake' }, shift }
           sub Freeze {
               my $s = shift;
               print STDERR "preparing to sleep\n";
               $s->{state} = 'asleep';
               return bless $s, 'Foo::ZZZ';
           }

           package Foo::ZZZ;
           sub Thaw {
               my $s = shift;
               print STDERR "waking up\n";
               $s->{state} = 'awake';
               return bless $s, 'Foo';
           }

           package Foo;
           use Data::Dumper;
           $a = Foo->new;
           $b = Data::Dumper->new([$a], ['c']);
           $b->Freezer('Freeze');
           $b->Toaster('Thaw');
           $c = $b->Dump;
           print $c;
           $d = eval $c;
           print Data::Dumper->Dump([$d], ['d']);


           ########
           # symbol substitution (useful for recreating CODE refs)
           ########

           sub foo { print "foo speaking\n" }
           *other = \&foo;
           $bar = [ \&other ];
           $d = Data::Dumper->new([\&other,$bar],['*other','bar']);
           $d->Seen({ '*foo' => \&foo });
           print $d->Dump;


           ########
           # sorting and filtering hash keys
           ########

           $Data::Dumper::Sortkeys = \&my_filter;
           my $foo = { map { (ord, "$_$_$_") } 'I'..'Q' };
           my $bar = { %$foo };
           my $baz = { reverse %$foo };
           print Dumper [ $foo, $bar, $baz ];

           sub my_filter {
               my ($hash) = @_;
               # return an array ref containing the hash keys to dump
               # in the order that you want them to be dumped
               return [
                 # Sort the keys of %$foo in reverse numeric order
                   $hash eq $foo ? (sort {$b <=> $a} keys %$hash) :
                 # Only dump the odd number keys of %$bar
                   $hash eq $bar ? (grep {$_ % 2} keys %$hash) :
                 # Sort keys in default order for all other hashes
                   (sort keys %$hash)
               ];
           }

BUGS
       Due to limitations of Perl subroutine call semantics, you cannot pass an array or hash.  Prepend it
       with a "\" to pass its reference instead.  This will be remedied in time, now that Perl has
       subroutine prototypes.  For now, you need to use the extended usage form, and prepend the name with a
       "*" to output it as a hash or array.

       "Data::Dumper" cheats with CODE references.  If a code reference is encountered in the structure
       being processed (and if you haven't set the "Deparse" flag), an anonymous subroutine that contains
       the string '"DUMMY"' will be inserted in its place, and a warning will be printed if "Purity" is set.
       You can "eval" the result, but bear in mind that the anonymous sub that gets created is just a
       placeholder.  Someday, perl will have a switch to cache-on-demand the string representation of a
       compiled piece of code, I hope.  If you have prior knowledge of all the code refs that your data
       structures are likely to have, you can use the "Seen" method to pre-seed the internal reference table
       and make the dumped output point to them, instead.  See "EXAMPLES" above.

       The "Useqq" and "Deparse" flags makes Dump() run slower, since the XSUB implementation does not
       support them.

       SCALAR objects have the weirdest looking "bless" workaround.

       Pure Perl version of "Data::Dumper" escapes UTF-8 strings correctly only in Perl 5.8.0 and later.

   NOTE
       Starting from Perl 5.8.1 different runs of Perl will have different ordering of hash keys.  The
       change was done for greater security, see "Algorithmic Complexity Attacks" in perlsec.  This means
       that different runs of Perl will have different Data::Dumper outputs if the data contains hashes.  If
       you need to have identical Data::Dumper outputs from different runs of Perl, use the environment
       variable PERL_HASH_SEED, see "PERL_HASH_SEED" in perlrun.  Using this restores the old (platform-specific) (platformspecific)
       specific) ordering: an even prettier solution might be to use the "Sortkeys" filter of Data::Dumper.

AUTHOR
       Gurusamy Sarathy        gsar@activestate.com

       Copyright (c) 1996-98 Gurusamy Sarathy. All rights reserved.  This program is free software; you can
       redistribute it and/or modify it under the same terms as Perl itself.

VERSION
       Version 2.125  (Aug  8 2009)

SEE ALSO
       perl(1)



perl v5.12.5                                     2012-11-03                                Data::Dumper(3pm)