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man : Data::Dumper(3p)
ext::Data::DumperPerlmProgrammers Reext::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.
$OBJ->Dump or PACKAGE->Dump(ARRAYREF [, ARRAYREF])
Returns the stringified form of the values stored in
the object (preserving the order in which they were
supplied to "new"), subject to the configuration
options below. In a list context, it returns a list
of strings corresponding to the supplied values.
The second form, for convenience, simply calls the
"new" method on its arguments before dumping the
object immediately.
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$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.
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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])
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
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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".
o $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.
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])
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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".
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.
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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])
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($bar); # pretty print (no array indices)
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$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
$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)
########
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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)
];
}
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.
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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) ordering:
an even prettier solution might be to use the "Sortkeys"
filter of Data::Dumper.
AUTHOR
Gurusamy Sarathy gsarATactivestate.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 2.121 (Aug 24 2003)
SEE ALSO
perl(1)
perl v5.10.0 2008-09-30 11
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