1Data::Dumper(3pm) Perl Programmers Reference Guide Data::Dumper(3pm)
2
6 Data::Dumper - stringified perl data structures, suitable for both
7 printing and "eval"
8
10 use Data::Dumper;
11 # simple procedural interface
13 print Dumper($foo, $bar);
14 # extended usage with names
16 print Data::Dumper->Dump([$foo, $bar], [qw(foo *ary)]);
17 # configuration variables
19 {
20 local $Data::Dumper::Purity = 1;
21 eval Data::Dumper->Dump([$foo, $bar], [qw(foo *ary)]);
22 }
23 # OO usage
25 $d = Data::Dumper->new([$foo, $bar], [qw(foo *ary)]);
26 ...
27 print $d->Dump;
28 ...
29 $d->Purity(1)->Terse(1)->Deepcopy(1);
30 eval $d->Dump;
31
33 Given a list of scalars or reference variables, writes out their con‐
34 tents in perl syntax. The references can also be objects. The contents
35 of each variable is output in a single Perl statement. Handles self-
36 referential structures correctly.
37 The return value can be "eval"ed to get back an identical copy of the
39 original reference structure.
40 Any references that are the same as one of those passed in will be
42 named $VARn (where n is a numeric suffix), and other duplicate refer‐
43 ences to substructures within $VARn will be appropriately labeled using
44 arrow notation. You can specify names for individual values to be
45 dumped if you use the "Dump()" method, or you can change the default
46 $VAR prefix to something else. See $Data::Dumper::Varname and
47 $Data::Dumper::Terse below.
48 The default output of self-referential structures can be "eval"ed, but
50 the nested references to $VARn will be undefined, since a recursive
51 structure cannot be constructed using one Perl statement. You should
52 set the "Purity" flag to 1 to get additional statements that will cor‐
53 rectly fill in these references. Moreover, if "eval"ed when strictures
54 are in effect, you need to ensure that any variables it accesses are
55 previously declared.
56 In the extended usage form, the references to be dumped can be given
58 user-specified names. If a name begins with a "*", the output will
59 describe the dereferenced type of the supplied reference for hashes and
60 arrays, and coderefs. Output of names will be avoided where possible
61 if the "Terse" flag is set.
62 In many cases, methods that are used to set the internal state of the
64 object will return the object itself, so method calls can be conve‐
65 niently chained together.
66 Several styles of output are possible, all controlled by setting the
68 "Indent" flag. See "Configuration Variables or Methods" below for
69 details.
70 Methods
72 PACKAGE->new(ARRAYREF [, ARRAYREF])
74 Returns a newly created "Data::Dumper" object. The first argument
75 is an anonymous array of values to be dumped. The optional second
76 argument is an anonymous array of names for the values. The names
77 need not have a leading "$" sign, and must be comprised of alphanu‐
78 meric characters. You can begin a name with a "*" to specify that
79 the dereferenced type must be dumped instead of the reference
80 itself, for ARRAY and HASH references.
81 The prefix specified by $Data::Dumper::Varname will be used with a
83 numeric suffix if the name for a value is undefined.
84 Data::Dumper will catalog all references encountered while dumping
86 the values. Cross-references (in the form of names of substructures
87 in perl syntax) will be inserted at all possible points, preserving
88 any structural interdependencies in the original set of values.
89 Structure traversal is depth-first, and proceeds in order from the
90 first supplied value to the last.
91 $OBJ->Dump or PACKAGE->Dump(ARRAYREF [, ARRAYREF])
93 Returns the stringified form of the values stored in the object
94 (preserving the order in which they were supplied to "new"), sub‐
95 ject to the configuration options below. In a list context, it
96 returns a list of strings corresponding to the supplied values.
97 The second form, for convenience, simply calls the "new" method on
99 its arguments before dumping the object immediately.
100 $OBJ->Seen([HASHREF])
102 Queries or adds to the internal table of already encountered refer‐
103 ences. You must use "Reset" to explicitly clear the table if
104 needed. Such references are not dumped; instead, their names are
105 inserted wherever they are encountered subsequently. This is use‐
106 ful especially for properly dumping subroutine references.
107 Expects an anonymous hash of name => value pairs. Same rules apply
109 for names as in "new". If no argument is supplied, will return the
110 "seen" list of name => value pairs, in a list context. Otherwise,
111 returns the object itself.
112 $OBJ->Values([ARRAYREF])
114 Queries or replaces the internal array of values that will be
115 dumped. When called without arguments, returns the values. Other‐
116 wise, returns the object itself.
117 $OBJ->Names([ARRAYREF])
119 Queries or replaces the internal array of user supplied names for
120 the values that will be dumped. When called without arguments,
121 returns the names. Otherwise, returns the object itself.
122 $OBJ->Reset
124 Clears the internal table of "seen" references and returns the
125 object itself.
126 Functions
128 Dumper(LIST)
130 Returns the stringified form of the values in the list, subject to
131 the configuration options below. The values will be named $VARn in
132 the output, where n is a numeric suffix. Will return a list of
133 strings in a list context.
134 Configuration Variables or Methods
136 Several configuration variables can be used to control the kind of out‐
138 put generated when using the procedural interface. These variables are
139 usually "local"ized in a block so that other parts of the code are not
140 affected by the change.
141 These variables determine the default state of the object created by
143 calling the "new" method, but cannot be used to alter the state of the
144 object thereafter. The equivalent method names should be used instead
145 to query or set the internal state of the object.
146 The method forms return the object itself when called with arguments,
148 so that they can be chained together nicely.
149 · $Data::Dumper::Indent or $OBJ->Indent([NEWVAL])
151 Controls the style of indentation. It can be set to 0, 1, 2 or 3.
153 Style 0 spews output without any newlines, indentation, or spaces
154 between list items. It is the most compact format possible that
155 can still be called valid perl. Style 1 outputs a readable form
156 with newlines but no fancy indentation (each level in the structure
157 is simply indented by a fixed amount of whitespace). Style 2 (the
158 default) outputs a very readable form which takes into account the
159 length of hash keys (so the hash value lines up). Style 3 is like
160 style 2, but also annotates the elements of arrays with their index
161 (but the comment is on its own line, so array output consumes twice
162 the number of lines). Style 2 is the default.
163 · $Data::Dumper::Purity or $OBJ->Purity([NEWVAL])
165 Controls the degree to which the output can be "eval"ed to recreate
167 the supplied reference structures. Setting it to 1 will output
168 additional perl statements that will correctly recreate nested ref‐
169 erences. The default is 0.
170 · $Data::Dumper::Pad or $OBJ->Pad([NEWVAL])
172 Specifies the string that will be prefixed to every line of the
174 output. Empty string by default.
175 · $Data::Dumper::Varname or $OBJ->Varname([NEWVAL])
177 Contains the prefix to use for tagging variable names in the out‐
179 put. The default is "VAR".
180 · $Data::Dumper::Useqq or $OBJ->Useqq([NEWVAL])
182 When set, enables the use of double quotes for representing string
184 values. Whitespace other than space will be represented as
185 "[\n\t\r]", "unsafe" characters will be backslashed, and unprint‐
186 able characters will be output as quoted octal integers. Since
187 setting this variable imposes a performance penalty, the default is
188 0. "Dump()" will run slower if this flag is set, since the fast
189 XSUB implementation doesn't support it yet.
190 · $Data::Dumper::Terse or $OBJ->Terse([NEWVAL])
192 When set, Data::Dumper will emit single, non-self-referential val‐
194 ues as atoms/terms rather than statements. This means that the
195 $VARn names will be avoided where possible, but be advised that
196 such output may not always be parseable by "eval".
197 · $Data::Dumper::Freezer or $OBJ->Freezer([NEWVAL])
199 Can be set to a method name, or to an empty string to disable the
201 feature. Data::Dumper will invoke that method via the object
202 before attempting to stringify it. This method can alter the con‐
203 tents of the object (if, for instance, it contains data allocated
204 from C), and even rebless it in a different package. The client is
205 responsible for making sure the specified method can be called via
206 the object, and that the object ends up containing only perl data
207 types after the method has been called. Defaults to an empty
208 string.
209 If an object does not support the method specified (determined
211 using UNIVERSAL::can()) then the call will be skipped. If the
212 method dies a warning will be generated.
213 · $Data::Dumper::Toaster or $OBJ->Toaster([NEWVAL])
215 Can be set to a method name, or to an empty string to disable the
217 feature. Data::Dumper will emit a method call for any objects that
218 are to be dumped using the syntax "bless(DATA, CLASS)->METHOD()".
219 Note that this means that the method specified will have to perform
220 any modifications required on the object (like creating new state
221 within it, and/or reblessing it in a different package) and then
222 return it. The client is responsible for making sure the method
223 can be called via the object, and that it returns a valid object.
224 Defaults to an empty string.
225 · $Data::Dumper::Deepcopy or $OBJ->Deepcopy([NEWVAL])
227 Can be set to a boolean value to enable deep copies of structures.
229 Cross-referencing will then only be done when absolutely essential
230 (i.e., to break reference cycles). Default is 0.
231 · $Data::Dumper::Quotekeys or $OBJ->Quotekeys([NEWVAL])
233 Can be set to a boolean value to control whether hash keys are
235 quoted. A false value will avoid quoting hash keys when it looks
236 like a simple string. Default is 1, which will always enclose hash
237 keys in quotes.
238 · $Data::Dumper::Bless or $OBJ->Bless([NEWVAL])
240 Can be set to a string that specifies an alternative to the "bless"
242 builtin operator used to create objects. A function with the spec‐
243 ified name should exist, and should accept the same arguments as
244 the builtin. Default is "bless".
245 · $Data::Dumper::Pair or $OBJ->Pair([NEWVAL])
247 Can be set to a string that specifies the separator between hash
249 keys and values. To dump nested hash, array and scalar values to
250 JavaScript, use: "$Data::Dumper::Pair = ' : ';". Implementing
251 "bless" in JavaScript is left as an exercise for the reader. A
252 function with the specified name exists, and accepts the same argu‐
253 ments as the builtin.
254 Default is: " => ".
256 · $Data::Dumper::Maxdepth or $OBJ->Maxdepth([NEWVAL])
258 Can be set to a positive integer that specifies the depth beyond
260 which which we don't venture into a structure. Has no effect when
261 "Data::Dumper::Purity" is set. (Useful in debugger when we often
262 don't want to see more than enough). Default is 0, which means
263 there is no maximum depth.
264 · $Data::Dumper::Useperl or $OBJ->Useperl([NEWVAL])
266 Can be set to a boolean value which controls whether the pure Perl
268 implementation of "Data::Dumper" is used. The "Data::Dumper" module
269 is a dual implementation, with almost all functionality written in
270 both pure Perl and also in XS ('C'). Since the XS version is much
271 faster, it will always be used if possible. This option lets you
272 override the default behavior, usually for testing purposes only.
273 Default is 0, which means the XS implementation will be used if
274 possible.
275 · $Data::Dumper::Sortkeys or $OBJ->Sortkeys([NEWVAL])
277 Can be set to a boolean value to control whether hash keys are
279 dumped in sorted order. A true value will cause the keys of all
280 hashes to be dumped in Perl's default sort order. Can also be set
281 to a subroutine reference which will be called for each hash that
282 is dumped. In this case "Data::Dumper" will call the subroutine
283 once for each hash, passing it the reference of the hash. The pur‐
284 pose of the subroutine is to return a reference to an array of the
285 keys that will be dumped, in the order that they should be dumped.
286 Using this feature, you can control both the order of the keys, and
287 which keys are actually used. In other words, this subroutine acts
288 as a filter by which you can exclude certain keys from being
289 dumped. Default is 0, which means that hash keys are not sorted.
290 · $Data::Dumper::Deparse or $OBJ->Deparse([NEWVAL])
292 Can be set to a boolean value to control whether code references
294 are turned into perl source code. If set to a true value,
295 "B::Deparse" will be used to get the source of the code reference.
296 Using this option will force using the Perl implementation of the
297 dumper, since the fast XSUB implementation doesn't support it.
298 Caution : use this option only if you know that your coderefs will
300 be properly reconstructed by "B::Deparse".
301 Exports
303 Dumper
305
307 Run these code snippets to get a quick feel for the behavior of this
308 module. When you are through with these examples, you may want to add
309 or change the various configuration variables described above, to see
310 their behavior. (See the testsuite in the Data::Dumper distribution
311 for more examples.)
312 use Data::Dumper;
314 package Foo;
316 sub new {bless {'a' => 1, 'b' => sub { return "foo" }}, $_[0]};
317 package Fuz; # a weird REF-REF-SCALAR object
319 sub new {bless \($_ = \ 'fu\'z'), $_[0]};
320 package main;
322 $foo = Foo->new;
323 $fuz = Fuz->new;
324 $boo = [ 1, [], "abcd", \*foo,
325 {1 => 'a', 023 => 'b', 0x45 => 'c'},
326 \\"p\q\'r", $foo, $fuz];
327 ########
329 # simple usage
330 ########
331 $bar = eval(Dumper($boo));
333 print($@) if $@;
334 print Dumper($boo), Dumper($bar); # pretty print (no array indices)
335 $Data::Dumper::Terse = 1; # don't output names where feasible
337 $Data::Dumper::Indent = 0; # turn off all pretty print
338 print Dumper($boo), "\n";
339 $Data::Dumper::Indent = 1; # mild pretty print
341 print Dumper($boo);
342 $Data::Dumper::Indent = 3; # pretty print with array indices
344 print Dumper($boo);
345 $Data::Dumper::Useqq = 1; # print strings in double quotes
347 print Dumper($boo);
348 $Data::Dumper::Pair = " : "; # specify hash key/value separator
350 print Dumper($boo);
351 ########
353 # recursive structures
354 ########
355 @c = ('c');
357 $c = \@c;
358 $b = {};
359 $a = [1, $b, $c];
360 $b->{a} = $a;
361 $b->{b} = $a->[1];
362 $b->{c} = $a->[2];
363 print Data::Dumper->Dump([$a,$b,$c], [qw(a b c)]);
364 $Data::Dumper::Purity = 1; # fill in the holes for eval
366 print Data::Dumper->Dump([$a, $b], [qw(*a b)]); # print as @a
367 print Data::Dumper->Dump([$b, $a], [qw(*b a)]); # print as %b
368 $Data::Dumper::Deepcopy = 1; # avoid cross-refs
370 print Data::Dumper->Dump([$b, $a], [qw(*b a)]);
371 $Data::Dumper::Purity = 0; # avoid cross-refs
373 print Data::Dumper->Dump([$b, $a], [qw(*b a)]);
374 ########
376 # deep structures
377 ########
378 $a = "pearl";
380 $b = [ $a ];
381 $c = { 'b' => $b };
382 $d = [ $c ];
383 $e = { 'd' => $d };
384 $f = { 'e' => $e };
385 print Data::Dumper->Dump([$f], [qw(f)]);
386 $Data::Dumper::Maxdepth = 3; # no deeper than 3 refs down
388 print Data::Dumper->Dump([$f], [qw(f)]);
389 ########
391 # object-oriented usage
392 ########
393 $d = Data::Dumper->new([$a,$b], [qw(a b)]);
395 $d->Seen({'*c' => $c}); # stash a ref without printing it
396 $d->Indent(3);
397 print $d->Dump;
398 $d->Reset->Purity(0); # empty the seen cache
399 print join "----\n", $d->Dump;
400 ########
402 # persistence
403 ########
404 package Foo;
406 sub new { bless { state => 'awake' }, shift }
407 sub Freeze {
408 my $s = shift;
409 print STDERR "preparing to sleep\n";
410 $s->{state} = 'asleep';
411 return bless $s, 'Foo::ZZZ';
412 }
413 package Foo::ZZZ;
415 sub Thaw {
416 my $s = shift;
417 print STDERR "waking up\n";
418 $s->{state} = 'awake';
419 return bless $s, 'Foo';
420 }
421 package Foo;
423 use Data::Dumper;
424 $a = Foo->new;
425 $b = Data::Dumper->new([$a], ['c']);
426 $b->Freezer('Freeze');
427 $b->Toaster('Thaw');
428 $c = $b->Dump;
429 print $c;
430 $d = eval $c;
431 print Data::Dumper->Dump([$d], ['d']);
432 ########
434 # symbol substitution (useful for recreating CODE refs)
435 ########
436 sub foo { print "foo speaking\n" }
438 *other = \&foo;
439 $bar = [ \&other ];
440 $d = Data::Dumper->new([\&other,$bar],['*other','bar']);
441 $d->Seen({ '*foo' => \&foo });
442 print $d->Dump;
443 ########
445 # sorting and filtering hash keys
446 ########
447 $Data::Dumper::Sortkeys = \&my_filter;
449 my $foo = { map { (ord, "$_$_$_") } 'I'..'Q' };
450 my $bar = { %$foo };
451 my $baz = { reverse %$foo };
452 print Dumper [ $foo, $bar, $baz ];
453 sub my_filter {
455 my ($hash) = @_;
456 # return an array ref containing the hash keys to dump
457 # in the order that you want them to be dumped
458 return [
459 # Sort the keys of %$foo in reverse numeric order
460 $hash eq $foo ? (sort {$b <=> $a} keys %$hash) :
461 # Only dump the odd number keys of %$bar
462 $hash eq $bar ? (grep {$_ % 2} keys %$hash) :
463 # Sort keys in default order for all other hashes
464 (sort keys %$hash)
465 ];
466 }
467
469 Due to limitations of Perl subroutine call semantics, you cannot pass
470 an array or hash. Prepend it with a "\" to pass its reference instead.
471 This will be remedied in time, now that Perl has subroutine prototypes.
472 For now, you need to use the extended usage form, and prepend the name
473 with a "*" to output it as a hash or array.
474 "Data::Dumper" cheats with CODE references. If a code reference is
476 encountered in the structure being processed (and if you haven't set
477 the "Deparse" flag), an anonymous subroutine that contains the string
478 '"DUMMY"' will be inserted in its place, and a warning will be printed
479 if "Purity" is set. You can "eval" the result, but bear in mind that
480 the anonymous sub that gets created is just a placeholder. Someday,
481 perl will have a switch to cache-on-demand the string representation of
482 a compiled piece of code, I hope. If you have prior knowledge of all
483 the code refs that your data structures are likely to have, you can use
484 the "Seen" method to pre-seed the internal reference table and make the
485 dumped output point to them, instead. See "EXAMPLES" above.
486 The "Useqq" and "Deparse" flags makes Dump() run slower, since the XSUB
488 implementation does not support them.
489 SCALAR objects have the weirdest looking "bless" workaround.
491 Pure Perl version of "Data::Dumper" escapes UTF-8 strings correctly
493 only in Perl 5.8.0 and later.
494 NOTE
496 Starting from Perl 5.8.1 different runs of Perl will have different
498 ordering of hash keys. The change was done for greater security, see
499 "Algorithmic Complexity Attacks" in perlsec. This means that different
500 runs of Perl will have different Data::Dumper outputs if the data con‐
501 tains hashes. If you need to have identical Data::Dumper outputs from
502 different runs of Perl, use the environment variable PERL_HASH_SEED,
503 see "PERL_HASH_SEED" in perlrun. Using this restores the old (plat‐
504 form-specific) ordering: an even prettier solution might be to use the
505 "Sortkeys" filter of Data::Dumper.
506
508 Gurusamy Sarathy gsar@activestate.com
509 Copyright (c) 1996-98 Gurusamy Sarathy. All rights reserved. This pro‐
511 gram is free software; you can redistribute it and/or modify it under
512 the same terms as Perl itself.
513
515 Version 2.121 (Aug 24 2003)
516
518 perl(1)
519perl v5.8.8 2001-09-21 Data::Dumper(3pm)