man > Data::Dumper

📖 NAME

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

🚀 Quick Reference

Use Case Command Description
📥 Simple dump print Dumper($foo, $bar); Procedural interface to dump scalars/references
🏷️ Named dump print Data::Dumper->Dump([$foo, $bar], [qw(foo *ary)]); Extended usage with custom variable names
🛠️ OO usage $d = Data::Dumper->new([$foo, $bar], [qw(foo *ary)]);
print $d->Dump;		 Object-oriented interface, allows chaining
🔁 Recursive structures local $Data::Dumper::Purity = 1;
eval Data::Dumper->Dump([$a, $b], [qw(*a b)]);		 Enable extra statements for self-referential data
🔍 Key sorting $Data::Dumper::Sortkeys = 1; Dump hash keys in sorted order
📦 Deep copy $Data::Dumper::Deepcopy = 1; Avoid cross-references (except for cycles)
🔢 Max depth $Data::Dumper::Maxdepth = 3; Limit depth of structure traversal
❄️ Persistence Freezer('Freeze'); Toaster('Thaw'); Serialize/deserialize objects with custom freeze/thaw methods

📋 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 evaled to get back an identical copy of the original reference structure. (Please do consider the security implications of eval'ing code from untrusted sources!)

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 evaled, 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 evaled 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 as a list. When called with a reference to an array of replacement values, returns the object itself. When called with any other type of argument, dies.

$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. When called with an array of replacement names, returns the object itself. If the number of replacement names exceeds the number of values to be named, the excess names will not be used. If the number of replacement names falls short of the number of values to be named, the list of replacement names will be exhausted and remaining values will not be renamed. When called with any other type of argument, dies.

$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 localized 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.

📦 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 main;
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. Even using the Deparse flag will in some cases produce results that behave differently after being passed to eval; see the documentation for B::Deparse.

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) ordering: an even prettier solution might be to use the Sortkeys filter of Data::Dumper.

👤 AUTHOR

Gurusamy Sarathy gsar@activestate.com

Copyright (c) 1996-2019 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 2.179

🔗 SEE ALSO

perl(1)

Data::Dumper □✕
📖 NAME 🚀 Quick Reference 📋 SYNOPSIS 📝 DESCRIPTION
🛠️ Methods 🔧 Functions ⚙️ Configuration Variables or Methods 📦 Exports
💡 EXAMPLES 🐛 BUGS
⚠️ NOTE
👤 AUTHOR 🏷️ VERSION 🔗 SEE ALSO

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