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XML::Simple::FAQ(3pm) User Contributed Perl DocumentationXML::Simple::FAQ(3pm)



NAME
       Frequently Asked Questions about XML::Simple

Basics
       What is XML::Simple designed to be used for?

       XML::Simple is a Perl module that was originally developed as a tool
       for reading and writing configuration data in XML format.  You can use
       it for many other purposes that involve storing and retrieving struc-
       tured data in XML.

       You might also find XML::Simple a good starting point for playing with
       XML from Perl.  It doesn't have a steep learning curve and if you out-
       grow its capabilities there are plenty of other Perl/XML modules to
       'step up' to.

       Why store configuration data in XML anyway?

       The many advantages of using XML format for configuration data include:

       o   Using existing XML parsing tools requires less development time, is
           easier and more robust than developing your own config file parsing
           code

       o   XML can represent relationships between pieces of data, such as
           nesting of sections to arbitrary levels (not easily done with .INI
           files for example)

       o   XML is basically just text, so you can easily edit a config file
           (easier than editing a Win32 registry)

       o   XML provides standard solutions for handling character sets and
           encoding beyond basic ASCII (important for internationalization)

       o   If it becomes necessary to change your configuration file format,
           there are many tools available for performing transformations on
           XML files

       o   XML is an open standard (the world does not need more proprietary
           binary file formats)

       o   Taking the extra step of developing a DTD allows the format of con-
           figuration files to be validated before your program reads them
           (not directly supported by XML::Simple)

       o   Combining a DTD with a good XML editor can give you a GUI config
           editor for minimal coding effort

       What isn't XML::Simple good for?

       The main limitation of XML::Simple is that it does not work with 'mixed
       content' (see the next question).  If you consider your XML files con-
       tain marked up text rather than structured data, you should probably
       use another module.

       If you are working with very large XML files, XML::Simple's approach of
       representing the whole file in memory as a 'tree' data structure may
       not be suitable.

       What is mixed content?

       Consider this example XML:

         <document>
           <para>This is <em>mixed</em> content.</para>
         </document>

       This is said to be mixed content, because the <para> element contains
       both character data (text content) and nested elements.

       Here's some more XML:

         <person>
           <first_name>Joe</first_name>
           <last_name>Bloggs</last_name>
           <dob>25-April-1969</dob>
         </person>

       This second example is not generally considered to be mixed content.
       The <first_name>, <last_name> and <dob> elements contain only character
       data and the  <person> element contains only nested elements.  (Note:
       Strictly speaking, the whitespace between the nested elements is char-
       acter data, but it is ignored by XML::Simple).

       Why doesn't XML::Simple handle mixed content?

       Because if it did, it would no longer be simple :-)

       Seriously though, there are plenty of excellent modules that allow you
       to work with mixed content in a variety of ways.  Handling mixed con-
       tent correctly is not easy and by ignoring these issues, XML::Simple is
       able to present an API without a steep learning curve.

       Which Perl modules do handle mixed content?

       Every one of them except XML::Simple :-)

       If you're looking for a recommendation, I'd suggest you look at the
       Perl-XML FAQ at:

         http://perl-xml.sourceforge.net/faq/

Installation
       How do I install XML::Simple?

       If you're running ActiveState Perl, you've probably already got
       XML::Simple (although you may want to upgrade to version 1.09 or better
       for SAX support).

       If you do need to install XML::Simple, you'll need to install an XML
       parser module first.  Install either XML::Parser (which you may have
       already) or XML::SAX.  If you install both, XML::SAX will be used by
       default.

       Once you have a parser installed ...

       On Unix systems, try:

         perl -MCPAN -e 'install XML::Simple'

       If that doesn't work, download the latest distribution from
       ftp://ftp.cpan.org/pub/CPAN/authors/id/G/GR/GRANTM , unpack it and run
       these commands:

         perl Makefile.PL
         make
         make test
         make install

       On Win32, if you have a recent build of ActiveState Perl (618 or bet-
       ter) try this command:

         ppm install XML::Simple

       If that doesn't work, you really only need the Simple.pm file, so
       extract it from the .tar.gz file (eg: using WinZIP) and save it in the
       \site\lib\XML directory under your Perl installation (typically
       C:\Perl).

       I'm trying to install XML::Simple and 'make test' fails

       Is the directory where you've unpacked XML::Simple mounted from a file
       server using NFS, SMB or some other network file sharing?  If so, that
       may cause errors in the the following test scripts:

         3_Storable.t
         4_MemShare.t
         5_MemCopy.t

       The test suite is designed to exercise the boundary conditions of all
       XML::Simple's functionality and these three scripts exercise the
       caching functions.  If XML::Simple is asked to parse a file for which
       it has a cached copy of a previous parse, then it compares the time-
       stamp on the XML file with the timestamp on the cached copy.  If the
       cached copy is *newer* then it will be used.  If the cached copy is
       older or the same age then the file is re-parsed.  The test scripts
       will get confused by networked filesystems if the workstation and
       server system clocks are not synchronised (to the second).

       If you get an error in one of these three test scripts but you don't
       plan to use the caching options (they're not enabled by default), then
       go right ahead and run 'make install'.  If you do plan to use caching,
       then try unpacking the distribution on local disk and doing the
       build/test there.

       It's probably not a good idea to use the caching options with networked
       filesystems in production.  If the file server's clock is ahead of the
       local clock, XML::Simple will re-parse files when it could have used
       the cached copy.  However if the local clock is ahead of the file
       server clock and a file is changed immediately after it is cached, the
       old cached copy will be used.

       Is one of the three test scripts (above) failing but you're not running
       on a network filesystem?  Are you running Win32?  If so, you may be
       seeing a bug in Win32 where writes to a file do not affect its modfica-
       tion timestamp.

       If none of these scenarios match your situation, please confirm you're
       running the latest version of XML::Simple and then email the output of
       'make test' to me at grantmATcpan.org

       Why is XML::Simple so slow?

       If you find that XML::Simple is very slow reading XML, the most likely
       reason is that you have XML::SAX installed but no additional SAX parser
       module.  The XML::SAX distribution includes an XML parser written
       entirely in Perl.  This is very portable but not very fast.  For better
       performance install either XML::SAX::Expat or XML::LibXML.

Usage
       How do I use XML::Simple?

       If you had an XML document called /etc/appconfig/foo.xml you could
       'slurp' it into a simple data structure (typically a hashref) with
       these lines of code:

         use XML::Simple;

         my $config = XMLin('/etc/appconfig/foo.xml');

       The XMLin() function accepts options after the filename.

       There are so many options, which ones do I really need to know about?

       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:

       o   forcearray

       o   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.

       What is the forcearray option all about?

       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 peo-
       ple who have more than one hobbie.  If there is only one <hobbie> ele-
       ment, 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.

       What is the keyattr option all about?

       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.

       So what's the catch with '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 care-
       ful 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> ele-
       ments 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 ele-
       ment then that element won't get forced to an array too.

       How do I know what my data structure should look like?

       The rules are fairly straightforward:

       o   each element gets represented as a hash

       o   unless it contains only text, in which case it'll be a simple
           scalar value

       o   or unless there's more than one element with the same name, in
           which case they'll be represented as an array

       o   unless you've got array folding enabled, in which case they'll be
           folded into a hash

       o   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'm getting 'Use of uninitialized value' warnings

       You're probably trying to index into a non-existant hash key - try
       Data::Dumper.

       I'm getting a 'Not an ARRAY reference' 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'm getting a 'No such array field' 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'm getting an 'Out of memory' 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.

       My element order is getting jumbled up

       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.  (How-
       ever, 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::Simple turns nested elements into attributes

       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:

       o   use the 'forcearray' option with XMLin()

       o   use the 'noattr' option with XMLout()

       o   live with it

       o   don't use XML::Simple

       Why does XMLout() insert <&lt;name>&gt; elements (or attributes)?

       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.

       Why are empty elements represented as empty hashes?

       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.

       Why is ParserOpts deprecated?

       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 cur-
       rently no plans to remove support for the XML::Parser API.



perl v5.8.8                       2007-08-19             XML::Simple::FAQ(3pm)