Data::Dumper(3) Perl Programmers Reference Guide Data::Dumper(3)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 $Data::Dump::Purity = 1;
eval Data::Dumper->Dump([$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;
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 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.
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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 the section on 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.
$OBJ->Dump or PACKAGE->Dump(ARRAYREF [, ARRAYREF])
Returns the stringified form of the values stored in
the object (preserving the order in which they were
supplied to new), subject to the configuration options
below. In an array context, it returns a list of
strings corresponding to the supplied values.
The second form, for convenience, simply calls the new
method on its arguments before dumping the object
immediately.
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$OBJ->Dumpxs or PACKAGE->Dumpxs(ARRAYREF [, ARRAYREF])
This method is available if you were able to compile
and install the XSUB extension to 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.
$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 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.
$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 an
array context.
DumperX(LIST)
Identical to the 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.
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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 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.
$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.
$Data::Dumper::Purity or $OBJ->Purity([NEWVAL])
Controls the degree to which the output can be evaled
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.
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Since setting this variable imposes a performance
penalty, the default is 0. The Dumpxs() method does
not honor this flag yet.
$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.
$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.
$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])
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.
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Default is bless.
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);
########
# 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';
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}
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;
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,
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,
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instead. See the EXAMPLES manpage above.
The Useqq flag is not honored by Dumpxs() (it always
outputs strings in single quotes).
SCALAR objects have the weirdest looking bless workaround.
AUTHOR
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
Version 2.10 (31 Oct 1998)
SEE ALSOperl(1)16/Sep/1999 perl 5.005, patch 03 9
Data::Dumper(3) Perl Programmers Reference Guide Data::Dumper(3)16/Sep/1999 perl 5.005, patch 03 10
