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 $d->Dump;
DESCRIPTION
Given a list of scalars or reference variables, writes out
their contents in perl syntax. The references can also be
objects. The contents of each variable is output in a
single Perl statement. Handles self-referential struc
tures correctly.
The return value can be "eval"ed to get back an identical
copy of the original reference structure.
Any references that are the same as one of those passed in
will be named "$VAR"n (where n is a numeric suffix), and
other duplicate references to substructures within "$VAR"n
will be appropriately labeled using arrow notation. You
can specify names for individual values to be dumped if
you use the "Dump()" method, or you can change the default
"$VAR" prefix to something else. See "$Data::Dumper::Var
name" and "$Data::Dumper::Terse" below.
The default output of self-referential structures can be
"eval"ed, but the nested references to "$VAR"n will be
undefined, since a recursive structure cannot be con
structed using one Perl statement. You should set the
"Purity" flag to 1 to get additional statements that will
correctly fill in these references.
In the extended usage form, the references to be dumped
can be given user-specified names. If a name begins with
a "*", the output will describe the dereferenced type of
the supplied reference for hashes and arrays, and
coderefs. Output of names will be avoided where possible
if the "Terse" flag is set.
In many cases, methods that are used to set the internal
state of the object will return the object itself, so
method calls can be conveniently chained together.
Several styles of output are possible, all controlled by
setting the "Indent" flag. See the Configuration Vari
ables or Methods entry elsewhere in this document below
for details.
Methods
PACKAGE->new(ARRAYREF [, ARRAYREF])
Returns a newly created "Data::Dumper" object. The
first argument is an anonymous array of values to be
dumped. The optional second argument is an anonymous
array of names for the values. The names need not
have a leading "$" 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 immediately.
$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 ref
erences.
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 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 sup
plied names for the values that will be dumped. When
called without arguments, returns the names. Other
wise, returns the object itself.
$OBJ->Reset
Clears the internal table of "seen" references and
returns the object itself.
Functions
Dumper(LIST)
Returns the stringified form of the values in the
list, subject to the configuration options below. The
values will be named "$VAR"n 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 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.
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 new
lines, 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
"eval"ed to recreate the supplied reference struc
tures. Setting it to 1 will output additional perl
statements that will correctly recreate nested refer
ences. 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 repre
senting 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 $OBJ->Terse([NEWVAL])
When set, Data::Dumper will emit single, non-self-ref
erential values as atoms/terms rather than statements.
This means that the "$VAR"n 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 creat
ing 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. Default is "bless".
$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.
Exports
Dumper
EXAMPLES
Run these code snippets to get a quick feel for the behav
ior of this module. When you are through with these exam
ples, you may want to add or change the various configura
tion 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)]);
########
# deep structures
########
$a = "pearl";
$b = [ $a ];
$c = { 'b' => $b };
$d = [ $c ];
$e = { 'd' => $d };
$f = { 'e' => $e };
print Data::Dumper->Dump([$f], [qw(f)]);
$Data::Dumper::Maxdepth = 3; # no deeper than 3 refs down
print Data::Dumper->Dump([$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)
########
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 inter
nal reference table and make the dumped output point to
them, instead. See the EXAMPLES manpage above.
The "Useqq" flag makes Dump() run slower, since the XSUB
implementation does not support it.
SCALAR objects have the weirdest looking "bless"
workaround.
AUTHOR
Gurusamy Sarathy gsar@activestate.com
Copyright (c) 1996-98 Gurusamy Sarathy. All rights
reserved. This program is free software; you can redis
tribute it and/or modify it under the same terms as Perl
itself.
VERSION
Version 2.11 (unreleased)
SEE ALSOperl(1)2001-02-22 perl v5.6.1 Data::Dumper(3)
