HowdoIuseXML::Simple?
If you don't know how to use XML::Simple then the best approach is to learn to use XML::LibXML
<http://grantm.github.io/perl-libxml-by-example/> instead. Stop reading this document and use that one
instead.
If you are determined to use XML::Simple, it come with copious documentation, so read that.
Therearesomanyoptions,whichonesdoIreallyneedtoknowabout?
Although you can get by without using any options, you shouldn't even consider using XML::Simple in
production until you know what these two options do:
• forcearray
• keyattr
The reason you really need to read about them is because the default values for these options will trip
you up if you don't. Although everyone agrees that these defaults are not ideal, there is not wide
agreement on what they should be changed to. The answer therefore is to read about them (see below) and
select values which are right for you.
Whatistheforcearrayoptionallabout?
Consider this XML in a file called ./person.xml:
<person>
<first_name>Joe</first_name>
<last_name>Bloggs</last_name>
<hobbie>bungy jumping</hobbie>
<hobbie>sky diving</hobbie>
<hobbie>knitting</hobbie>
</person>
You could read it in with this line:
my $person = XMLin('./person.xml');
Which would give you a data structure like this:
$person = {
'first_name' => 'Joe',
'last_name' => 'Bloggs',
'hobbie' => [ 'bungy jumping', 'sky diving', 'knitting' ]
};
The <first_name> and <last_name> elements are represented as simple scalar values which you could refer
to like this:
print "$person->{first_name} $person->{last_name}\n";
The <hobbie> elements are represented as an array - since there is more than one. You could refer to the
first one like this:
print $person->{hobbie}->[0], "\n";
Or the whole lot like this:
print join(', ', @{$person->{hobbie}} ), "\n";
The catch is, that these last two lines of code will only work for people who have more than one hobbie.
If there is only one <hobbie> element, it will be represented as a simple scalar (just like <first_name>
and <last_name>). Which might lead you to write code like this:
if(ref($person->{hobbie})) {
print join(', ', @{$person->{hobbie}} ), "\n";
}
else {
print $person->{hobbie}, "\n";
}
Don't do that.
One alternative approach is to set the forcearray option to a true value:
my $person = XMLin('./person.xml', forcearray => 1);
Which will give you a data structure like this:
$person = {
'first_name' => [ 'Joe' ],
'last_name' => [ 'Bloggs' ],
'hobbie' => [ 'bungy jumping', 'sky diving', 'knitting' ]
};
Then you can use this line to refer to all the list of hobbies even if there was only one:
print join(', ', @{$person->{hobbie}} ), "\n";
The downside of this approach is that the <first_name> and <last_name> elements will also always be
represented as arrays even though there will never be more than one:
print "$person->{first_name}->[0] $person->{last_name}->[0]\n";
This might be OK if you change the XML to use attributes for things that will always be singular and
nested elements for things that may be plural:
<person first_name="Jane" last_name="Bloggs">
<hobbie>motorcycle maintenance</hobbie>
</person>
On the other hand, if you prefer not to use attributes, then you could specify that any <hobbie> elements
should always be represented as arrays and all other nested elements should be simple scalar values
unless there is more than one:
my $person = XMLin('./person.xml', forcearray => [ 'hobbie' ]);
The forcearray option accepts a list of element names which should always be forced to an array
representation:
forcearray => [ qw(hobbie qualification childs_name) ]
See the XML::Simple manual page for more information.
Whatisthekeyattroptionallabout?
Consider this sample XML:
<catalog>
<part partnum="1842334" desc="High pressure flange" price="24.50" />
<part partnum="9344675" desc="Threaded gasket" price="9.25" />
<part partnum="5634896" desc="Low voltage washer" price="12.00" />
</catalog>
You could slurp it in with this code:
my $catalog = XMLin('./catalog.xml');
Which would return a data structure like this:
$catalog = {
'part' => [
{
'partnum' => '1842334',
'desc' => 'High pressure flange',
'price' => '24.50'
},
{
'partnum' => '9344675',
'desc' => 'Threaded gasket',
'price' => '9.25'
},
{
'partnum' => '5634896',
'desc' => 'Low voltage washer',
'price' => '12.00'
}
]
};
Then you could access the description of the first part in the catalog with this code:
print $catalog->{part}->[0]->{desc}, "\n";
However, if you wanted to access the description of the part with the part number of "9344675" then you'd
have to code a loop like this:
foreach my $part (@{$catalog->{part}}) {
if($part->{partnum} eq '9344675') {
print $part->{desc}, "\n";
last;
}
}
The knowledge that each <part> element has a unique partnum attribute allows you to eliminate this
search. You can pass this knowledge on to XML::Simple like this:
my $catalog = XMLin($xml, keyattr => ['partnum']);
Which will return a data structure like this:
$catalog = {
'part' => {
'5634896' => { 'desc' => 'Low voltage washer', 'price' => '12.00' },
'1842334' => { 'desc' => 'High pressure flange', 'price' => '24.50' },
'9344675' => { 'desc' => 'Threaded gasket', 'price' => '9.25' }
}
};
XML::Simple has been able to transform $catalog->{part} from an arrayref to a hashref (keyed on partnum).
This transformation is called 'array folding'.
Through the use of array folding, you can now index directly to the description of the part you want:
print $catalog->{part}->{9344675}->{desc}, "\n";
The 'keyattr' option also enables array folding when the unique key is in a nested element rather than an
attribute. eg:
<catalog>
<part>
<partnum>1842334</partnum>
<desc>High pressure flange</desc>
<price>24.50</price>
</part>
<part>
<partnum>9344675</partnum>
<desc>Threaded gasket</desc>
<price>9.25</price>
</part>
<part>
<partnum>5634896</partnum>
<desc>Low voltage washer</desc>
<price>12.00</price>
</part>
</catalog>
See the XML::Simple manual page for more information.
Sowhat'sthecatchwith'keyattr'?
One thing to watch out for is that you might get array folding even if you don't supply the keyattr
option. The default value for this option is:
[ 'name', 'key', 'id']
Which means if your XML elements have a 'name', 'key' or 'id' attribute (or nested element) then they may
get folded on those values. This means that you can take advantage of array folding simply through
careful choice of attribute names. On the hand, if you really don't want array folding at all, you'll
need to set 'key attr to an empty list:
my $ref = XMLin($xml, keyattr => []);
A second 'gotcha' is that array folding only works on arrays. That might seem obvious, but if there's
only one record in your XML and you didn't set the 'forcearray' option then it won't be represented as an
array and consequently won't get folded into a hash. The moral is that if you're using array folding,
you should always turn on the forcearray option.
You probably want to be as specific as you can be too. For instance, the safest way to parse the
<catalog> example above would be:
my $catalog = XMLin($xml, keyattr => { part => 'partnum'},
forcearray => ['part']);
By using the hashref for keyattr, you can specify that only <part> elements should be folded on the
'partnum' attribute (and that the <part> elements should not be folded on any other attribute).
By supplying a list of element names for forcearray, you're ensuring that folding will work even if
there's only one <part>. You're also ensuring that if the 'partnum' unique key is supplied in a nested
element then that element won't get forced to an array too.
HowdoIknowwhatmydatastructureshouldlooklike?
The rules are fairly straightforward:
• each element gets represented as a hash
• unless it contains only text, in which case it'll be a simple scalar value
• or unless there's more than one element with the same name, in which case they'll be represented as
an array
• unless you've got array folding enabled, in which case they'll be folded into a hash
• empty elements (no text contents and no attributes) will either be represented as an empty hash, an
empty string or undef - depending on the value of the 'suppressempty' option.
If you're in any doubt, use Data::Dumper, eg:
use XML::Simple;
use Data::Dumper;
my $ref = XMLin($xml);
print Dumper($ref);
I'mgetting'Useofuninitializedvalue'warnings
You're probably trying to index into a non-existant hash key - try Data::Dumper.
I'mgettinga'NotanARRAYreference'error
Something that you expect to be an array is not. The two most likely causes are that you forgot to use
'forcearray' or that the array got folded into a hash - try Data::Dumper.
I'mgettinga'Nosucharrayfield'error
Something that you expect to be a hash is actually an array. Perhaps array folding failed because one
element was missing the key attribute - try Data::Dumper.
I'mgettingan'Outofmemory'error
Something in the data structure is not as you expect and Perl may be trying unsuccessfully to autovivify
things - try Data::Dumper.
If you're already using Data::Dumper, try calling Dumper() immediately after XMLin() - ie: before you
attempt to access anything in the data structure.
Myelementorderisgettingjumbledup
If you read an XML file with XMLin() and then write it back out with XMLout(), the order of the elements
will likely be different. (However, if you read the file back in with XMLin() you'll get the same Perl
data structure).
The reordering happens because XML::Simple uses hashrefs to store your data and Perl hashes do not really
have any order.
It is possible that a future version of XML::Simple will use Tie::IxHash to store the data in hashrefs
which do retain the order. However this will not fix all cases of element order being lost.
If your application really is sensitive to element order, don't use XML::Simple (and don't put order-
sensitive values in attributes).
XML::Simpleturnsnestedelementsintoattributes
If you read an XML file with XMLin() and then write it back out with XMLout(), some data which was
originally stored in nested elements may end up in attributes. (However, if you read the file back in
with XMLin() you'll get the same Perl data structure).
There are a number of ways you might handle this:
• use the 'forcearray' option with XMLin()
• use the 'noattr' option with XMLout()
• live with it
• don't use XML::Simple
WhydoesXMLout()insert<name>elements(orattributes)?
Try setting keyattr => [].
When you call XMLin() to read XML, the 'keyattr' option controls whether arrays get 'folded' into hashes.
Similarly, when you call XMLout(), the 'keyattr' option controls whether hashes get 'unfolded' into
arrays. As described above, 'keyattr' is enabled by default.
Whyareemptyelementsrepresentedasemptyhashes?
An element is always represented as a hash unless it contains only text, in which case it is represented
as a scalar string.
If you would prefer empty elements to be represented as empty strings or the undefined value, set the
'suppressempty' option to '' or undef respectively.
WhyisParserOptsdeprecated?
The "ParserOpts" option is a remnant of the time when XML::Simple only worked with the XML::Parser API.
Its value is completely ignored if you're using a SAX parser, so writing code which relied on it would
bar you from taking advantage of SAX.
Even if you are using XML::Parser, it is seldom necessary to pass options to the parser object. A number
of people have written to say they use this option to set XML::Parser's "ProtocolEncoding" option. Don't
do that, it's wrong, Wrong, WRONG! Fix the XML document so that it's well-formed and you won't have a
problem.
Having said all of that, as long as XML::Simple continues to support the XML::Parser API, this option
will not be removed. There are currently no plans to remove support for the XML::Parser API.
perl v5.36.0 2022-12-12 XML::Simple::FAQ(3pm)
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