SEARCH
NEW RPMS
DIRECTORIES
ABOUT
FAQ
VARIOUS
BLOG
DONATE




YUM REPOSITORY

 
 

XML::Simple::FAQ

Section: User Contributed Perl Documentation (3)
Updated: 2004-11-20
Index 

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 forreading and writing configuration data in XML format. You can use it formany other purposes that involve storing and retrieving structured data inXML.

You might also find XML::Simple a good starting point for playing with XMLfrom Perl. It doesn't have a steep learning curve and if you outgrow itscapabilities 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:
*
Using existing XML parsing tools requires less development time, is easierand more robust than developing your own config file parsing code
*
XML can represent relationships between pieces of data, such as nesting ofsections to arbitrary levels (not easily done with .INI files for example)
*
XML is basically just text, so you can easily edit a config file (easier thanediting a Win32 registry)
*
XML provides standard solutions for handling character sets and encodingbeyond basic ASCII (important for internationalization)
*
If it becomes necessary to change your configuration file format, there aremany tools available for performing transformations on XML files
*
XML is an open standard (the world does not need more proprietary binaryfile formats)
*
Taking the extra step of developing a DTD allows the format of configurationfiles to be validated before your program reads them (not directly supportedby XML::Simple)
*
Combining a DTD with a good XML editor can give you a GUI config editor forminimal coding effort
 

What isn't XML::Simple good for?

The main limitation of XML::Simple is that it does not work with 'mixedcontent' (see the next question). If you consider your XML files containmarked up text rather than structured data, you should probably use anothermodule.

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

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 containsboth 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 containonly character data and the <person> element contains only nestedelements. (Note: Strictly speaking, the whitespace between the nestedelements is character 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 towork with mixed content in a variety of ways. Handling mixed contentcorrectly is not easy and by ignoring these issues, XML::Simple is able topresent 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-XMLFAQ 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 parsermodule first. Install either XML::Parser (which you may have already) orXML::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 fromftp://ftp.cpan.org/pub/CPAN/authors/id/G/GR/GRANTM , unpack it and run thesecommands:

  perl Makefile.PL  make  make test  make install

On Win32, if you have a recent build of ActiveState Perl (618 or better) trythis command:

  ppm install XML::Simple

If that doesn't work, you really only need the Simple.pm file, so extract itfrom 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 serverusing NFS, SMB or some other network file sharing? If so, that may causeerrors 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 allXML::Simple's functionality and these three scripts exercise the cachingfunctions. If XML::Simple is asked to parse a file for which it has a cachedcopy of a previous parse, then it compares the timestamp on the XML file withthe timestamp on the cached copy. If the cached copy is *newer* then it willbe used. If the cached copy is older or the same age then the file isre-parsed. The test scripts will get confused by networked filesystems ifthe workstation and server system clocks are not synchronised (to thesecond).

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

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

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

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

Why is XML::Simple so slow?

If you find that XML::Simple is very slow reading XML, the most likely reasonis that you have XML::SAX installed but no additional SAX parser module. TheXML::SAX distribution includes an XML parser written entirely in Perl. This isvery portable but not very fast. For better performance install eitherXML::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' itinto 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 evenconsider using XML::Simple in production until you know what these twooptions do:
*
forcearray
*
keyattr

The reason you really need to read about them is because the default valuesfor these options will trip you up if you don't. Although everyone agreesthat these defaults are not ideal, there is not wide agreement on what theyshould 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 assimple 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 ismore 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 peoplewho 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 writecode 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 therewas 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 eventhough 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 thatwill 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 couldspecify that any <hobbie> elements should always be represented asarrays and all other nested elements should be simple scalar values unlessthere is more than one:

  my $person = XMLin('./person.xml', forcearray => [ 'hobbie' ]);

The forcearray option accepts a list of element names which should alwaysbe 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 catalogwith this code:

  print $catalog->{part}->[0]->{desc}, "\n";

However, if you wanted to access the description of the part with thepart 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 attributeallows you to eliminate this search. You can pass this knowledge on toXML::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 toa hashref (keyed on partnum). This transformation is called 'array folding'.

Through the use of array folding, you can now index directly to thedescription of the part you want:

  print $catalog->{part}->{9344675}->{desc}, "\n";

The 'keyattr' option also enables array folding when the unique key is in anested 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 youdon'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 (ornested element) then they may get folded on those values. This means thatyou can take advantage of array folding simply through careful choice ofattribute 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 mightseem obvious, but if there's only one record in your XML and you didn't setthe 'forcearray' option then it won't be represented as an array andconsequently won't get folded into a hash. The moral is that if you'reusing array folding, you should always turn on the forcearray option.

You probably want to be as specific as you can be too. For instance, thesafest 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 thatfolding will work even if there's only one <part>. You're alsoensuring that if the 'partnum' unique key is supplied in a nested elementthen 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:
*
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 casethey'll be represented as an array
*
unless you've got array folding enabled, in which case they'll be folded intoa hash
*
empty elements (no text contents and no attributes) will either berepresented as an empty hash, an empty string or undef - depending on the valueof 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 - tryData::Dumper. 

I'm getting a 'Not an ARRAY reference' error

Something that you expect to be an array is not. The two most likely causesare that you forgot to use 'forcearray' or that the array got folded into ahash - 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 arrayfolding failed because one element was missing the key attribute - tryData::Dumper. 

I'm getting an 'Out of memory' error

Something in the data structure is not as you expect and Perl may be tryingunsuccessfully to autovivify things - try Data::Dumper.

If you're already using Data::Dumper, try calling Dumper() immediately afterXMLin() - 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 withXMLout(), the order of the elements will likely be different. (However, ifyou read the file back in with XMLin() you'll get the same Perl datastructure).

The reordering happens because XML::Simple uses hashrefs to store your dataand Perl hashes do not really have any order.

It is possible that a future version of XML::Simple will use Tie::IxHashto store the data in hashrefs which do retain the order. However this willnot fix all cases of element order being lost.

If your application really is sensitive to element order, don't useXML::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 withXMLout(), some data which was originally stored in nested elements may end upin attributes. (However, if you read the file back in with XMLin() you'llget 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
 

Why does XMLout() insert <name> elements (or attributes)?

Try setting keyattr => [].

When you call XMLin() to read XML, the 'keyattr' option controls whether arraysget '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, inwhich case it is represented as a scalar string.

If you would prefer empty elements to be represented as empty strings or theundefined 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 workedwith the XML::Parser API. Its value is completely ignored if you're using aSAX parser, so writing code which relied on it would bar you from takingadvantage of SAX.

Even if you are using XML::Parser, it is seldom necessary to pass options tothe parser object. A number of people have written to say they use this optionto 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 havea problem.

Having said all of that, as long as XML::Simple continues to support theXML::Parser API, this option will not be removed. There are currently no plansto remove support for the XML::Parser API.

 

Index

Frequently Asked Questions about XML::Simple
Basics
What is XML::Simple designed to be used for?
Why store configuration data in XML anyway?
What isn't XML::Simple good for?
What is mixed content?
Why doesn't XML::Simple handle mixed content?
Which Perl modules do handle mixed content?
Installation
How do I install XML::Simple?
I'm trying to install XML::Simple and 'make test' fails
Why is XML::Simple so slow?
Usage
How do I use XML::Simple?
There are so many options, which ones do I really need to know about?
What is the forcearray option all about?
What is the keyattr option all about?
So what's the catch with 'keyattr'?
How do I know what my data structure should look like?
I'm getting 'Use of uninitialized value' warnings
I'm getting a 'Not an ARRAY reference' error
I'm getting a 'No such array field' error
I'm getting an 'Out of memory' error
My element order is getting jumbled up
XML::Simple turns nested elements into attributes
Why does XMLout() insert <name> elements (or attributes)?
Why are empty elements represented as empty hashes?
Why is ParserOpts deprecated?
This document was created byman2html,using the manual pages.
  
impreza firmowa internet katowice