


ext::Data-Dumper:PerlpProgrammers Refext::Data-Dumper::Dumper(3p)


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 contents 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.



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       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



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           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.

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




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           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.

       o   $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.

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

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

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

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

       o   $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.

       o   $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".

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



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           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.

       o   $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.

       o   $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.

       o   $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.

       o   $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".




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       o   $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: " => ".

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

           Can be set to a positive integer that specifies the
           depth beyond which 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.

       o   $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.

       o   $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.

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



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           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);



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




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           $d = Data::Dumper->new([$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;


           ########



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           # 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.



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   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) 1996-98 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 2.124  (Jun 13 2009)

SEE ALSO
       perl(1)































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