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Data::Dumper(3)
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 $VARn will be undefined, since a recursive structure
cannot be constructed using one Perl statement. You should set the
"Purity" flag to 1 to get additional statements that will correctly fill in
these references.
In the extended usage form, the references to be dumped can be given user-
specified names. If a name begins with a "*", the output will describe the
dereferenced type of the supplied reference for hashes and arrays, and
coderefs. Output of names will be avoided where possible if the "Terse"
flag is set.
In many cases, methods that are used to set the internal state of the
object will return the object itself, so method calls can be conveniently
chained together.
Several styles of output are possible, all controlled by setting the
"Indent" flag. See "Configuration Variables or Methods" below for details.
Methods
PACKAGE->new(ARRAYREF [, ARRAYREF])
Returns a newly created "Data::Dumper" object. The first argument is
an anonymous array of values to be dumped. The optional second
argument is an anonymous array of names for the values. The names need
not have a leading "$" 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.
$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.
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 "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 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.
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 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.
· $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.
· $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.
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
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);
$Data::Dumper::Pair = " : "; # specify hash key/value separator
print Dumper($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)]);
$Data::Dumper::Maxdepth = 3; # no deeper than 3 refs down
print Data::Dumper->Dump([$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;
########
# 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.
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 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 ALSO
perl(1)
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