MirBSD manpage: Data::Dumper(3p)

ext::Data::DumperPerlmProgrammers Reext::Data::Dumper::Dumper(3p)


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


         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;
         eval $d->Dump;


     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 sin-
     gle Perl statement.  Handles self-referential structures

     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 unde-
     fined, 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

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     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 sup-
     plied 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.


         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

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

         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 struc-
         tural 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

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         Queries or adds to the internal table of already encoun-
         tered 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

         Queries or replaces the internal array of values that
         will be dumped. When called without arguments, returns
         the values.  Otherwise, returns the object itself.

         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.

         Clears the internal table of "seen" references and
         returns the object itself.


         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 con-

     Configuration Variables or Methods

     Several configuration variables can be used to control the
     kind of output generated when using the procedural inter-
     face.  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.

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     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 new-
         lines but no fancy indentation (each level in the struc-
         ture is simply indented by a fixed amount of whi-
         tespace).  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 out-
         put consumes twice the number of lines).  Style 2 is the

     +   $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

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         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 con-
         taining 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 gen-

     +   $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])

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         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 which we don't venture into a struc-
         ture.  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 pur-
         poses only. Default is 0, which means the XS implementa-
         tion 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 con-
         trol both the order of the keys, and which keys are

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         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

     +   $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".




     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 exam-

         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)

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         $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)]);

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         $Data::Dumper::Maxdepth = 3;       # no deeper than 3 refs down
         print Data::Dumper->Dump([$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
         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']);
         $c = $b->Dump;
         print $c;
         $d = eval $c;
         print Data::Dumper->Dump([$d], ['d']);

         # symbol substitution (useful for recreating CODE refs)

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         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)


     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.

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     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.


     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 dif-
     ferent 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 pret-
     tier solution might be to use the "Sortkeys" filter of


     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 2.121  (Aug 24 2003)



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