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 $Data::Dump::Purity = 1;
eval Data::Dumper->Dump([$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;
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 evaled 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 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.
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.
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.
new), subject to the configuration options below. In an array 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.
Data::Dumper. It is exactly identical to the Dump method above, only about 4 to 5 times faster, since it is written entirely in
C.
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 a 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 an array context. Otherwise, returns the object itself.
$VARn in the output, where n is a numeric suffix. Will return a list of strings in an array context.
Dumper() function above, but this calls the
XSUB implementation. Only available if you were able to compile and install the
XSUB extensions in
Data::Dumper.
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.
[\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. The Dumpxs() method does not honor this flag yet.
$VARn names will be avoided where possible, but be advised that such output may
not always be parseable by eval.
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.
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);
########
# 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)]);
########
# 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)
########
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;
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, with the
arrival of prototypes in later versions of Perl. 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, 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 flag is not honored by Dumpxs() (it always outputs strings in single quotes).
SCALAR objects have the weirdest looking bless workaround.
Gurusamy Sarathy gsar@umich.edu
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 2.09 (9 July 1998)
perl(1)
If rather than formatting bugs, you encounter substantive content errors in these documents, such as mistakes in the explanations or code, please use the perlbug utility included with the Perl distribution.