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 content 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. (Please do consider the security implications of
eval'ing code from untrusted sources!)
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::Varname and $Data::Dumper::Terse
below.
The default output of self-referential structures can be "eval"ed, but
the nested references to $VAR
n will be undefined, since a recursive
structure cannot be constructed using one Perl statement. You should set the
"Purity" flag to 1 to get additional statements that will correctly
fill in these references. Moreover, if "eval"ed when strictures are
in effect, you need to ensure that any variables it accesses are previously
declared.
In the extended usage form, the references to be dumped can be given
user-specified names. If a name begins with a "*", the output will
describe the dereferenced type of the supplied reference for hashes and
arrays, and coderefs. Output of names will be avoided where possible if the
"Terse" flag is set.
In many cases, methods that are used to set the internal state of the object
will return the object itself, so method calls can be conveniently chained
together.
Several styles of output are possible, all controlled by setting the
"Indent" flag. See "Configuration Variables or Methods"
below for details.
Methods¶
- PACKAGE->new(ARRAYREF [, ARRAYREF])
- Returns a newly created "Data::Dumper" object. The first
argument is an anonymous array of values to be dumped. The optional second
argument is an anonymous array of names for the values. The names need not
have a leading "$" 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 as a list. When called with a
reference to an array of replacement values, returns the object itself.
When called with any other type of argument, dies.
- $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. When called with an array of replacement names, returns the object
itself. If the number of replacement names exceeds the number of values to
be named, the excess names will not be used. If the number of replacement
names falls short of the number of values to be named, the list of
replacement names will be exhausted and remaining values will not be
renamed. When called with any other type of argument, dies.
- $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 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 $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.
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 generated.
- •
- $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
defined 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 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::Maxrecurse or
$OBJ->Maxrecurse([NEWVAL])
Can be set to a positive integer that specifies the depth beyond which
recursion into a structure will throw an exception. This is intended as a
security measure to prevent perl running out of stack space when dumping
an excessively deep structure. Can be set to 0 to remove the limit.
Default is 1000.
- •
- $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".
- •
- $Data::Dumper::Sparseseen or
$OBJ->Sparseseen([NEWVAL])
By default, Data::Dumper builds up the "seen" hash of scalars that
it has encountered during serialization. This is very expensive. This seen
hash is necessary to support and even just detect circular references. It
is exposed to the user via the "Seen()" call both for writing
and reading.
If you, as a user, do not need explicit access to the "seen" hash,
then you can set the "Sparseseen" option to allow Data::Dumper
to eschew building the "seen" hash for scalars that are known
not to possess more than one reference. This speeds up serialization
considerably if you use the XS implementation.
Note: If you turn on "Sparseseen", then you must not rely on the
content of the seen hash since its contents will be an implementation
detail!
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 main;
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) 1996-2014 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 2.151_01 (January 8 2015)
SEE ALSO¶
perl(1)