ext::Data::DumperPerlmProgrammers Reext::Data::Dumper::Dumper(3p)NAMEData::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 sin-
gle 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 $VARn will be unde-
fined, since a recursive structure cannot be constructed
using one Perl statement. You should set the "Purity" flag
to 1 to get additional statements that will correctly fill
in these references. Moreover, if "eval"ed when strictures
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are in effect, you need to ensure that any variables it
accesses are previously declared.
In the extended usage form, the references to be dumped can
be given user-specified names. If a name begins with a "*",
the output will describe the dereferenced type of the sup-
plied reference for hashes and arrays, and coderefs. Output
of names will be avoided where possible if the "Terse" flag
is set.
In many cases, methods that are used to set the internal
state of the object will return the object itself, so method
calls can be conveniently chained together.
Several styles of output are possible, all controlled by
setting the "Indent" flag. See "Configuration Variables or
Methods" below for details.
Methods
PACKAGE->new(ARRAYREF [, ARRAYREF])
Returns a newly created "Data::Dumper" object. The
first argument is an anonymous array of values to be
dumped. The optional second argument is an anonymous
array of names for the values. The names need not have
a leading "$" 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 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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immediately.
$OBJ->Seen([HASHREF])
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
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 con-
text.
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
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
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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-
erated.
+ $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
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 exam-
ples.)
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.
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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.
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 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
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 2.121 (Aug 24 2003)
SEE ALSOperl(1)perl v5.8.8 2005-02-05 11
