Ewan Birney


Bio::Root::Exception - Generic exception objects for Bioperl


Throwing exceptions using Error::throw():

    use Bio::Root::Exception;
    use Error qw(:try);

    $file = shift;

    try {
        open (IN, $file) || throw Bio::Root::FileOpenException (
          -text => "Can't open file $file for reading",
          -value => $!,
          -object => $self );
    catch Bio::Root::FileOpenException with {
        my $err = shift;
        print STDERR "Using default input file: $default_file\n";
        open (IN, $default_file) || die "Can't open $default_file";
    otherwise {
        my $err = shift;
        print STDERR "An unexpected exception occurred: \n$err";

        # By placing an the error object reference within double quotes,
        # you're invoking its stringify() method.
   finally {
       # Any code that you want to execute regardless of whether or not
       # an exception occurred.
   # the ending semicolon is essential!

Throwing exceptions using Bio::Root::Root::throw():

     # Note: You don't have to be within a try{} block
     open (IN, $file) || $object->throw(
                                        -class => 'Bio::Root::FileOpenException',
                                        -text => "Can't open file $file for reading",
                                        -value => $!);

Defining a new Exception type as a subclass of Bio::Root::Exception:

    @Bio::TestException::ISA = qw( Bio::Root::Exception );


Exceptions defined in Bio::Root::Exception

These are generic exceptions for typical problem situations that could arise in any module or script.


All of these exceptions inherit from a common base class exception, Bio::Root::Exception. This allows a user to write a handler for all Bioperl-derived exceptions as follows:

           use Bio::Whatever;
           use Error qw(:try);

           try {
                # some code that depends on Bioperl
           catch Bio::Root::Exception with {
               my $err = shift;
               print "A Bioperl exception occurred:\n$err\n";

The exceptions in Bio::Root::Exception are extensions of Graham Barr's Error.pm module available from CPAN. Despite this dependency, the Bio::Root::Exception module does not explicitly require Error. This permits Bio::Root::Exception to be loaded even when Error.pm is not available.

Throwing exceptions within Bioperl modules

Error.pm is not part of the Bioperl distibution, and may not be present within any given perl installation. So, when you want to throw an exception in a Bioperl module, the safe way to throw it is to use Bio::Root::Root::throw() which can use Error.pm when it's available. See documentation in Bio::Root::Root for details.


See the examples/exceptions directory of the Bioperl distribution for working demo code.

Bio::Root::Root::throw() for information about throwing Bio::Root::Exception-based exceptions.

Error.pm (available from CPAN, author: GBARR)

Error.pm is helping to guide the design of exception handling in Perl 6. See these RFC's:




Steve Chervitz <sac@bioperl.org>


Copyright (c) 2001 Steve Chervitz. All Rights Reserved.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


This software is provided "as is" without warranty of any kind.



 Purpose : A generic base class for all BioPerl exceptions.
           By including a "catch Bio::Root::Exception" block, you
           should be able to trap all BioPerl exceptions.
 Example : throw Bio::Root::Exception( 
               -text   => "A generic exception",
               -object => $self );

Methods defined by Bio::Root::Exception

 Purpose : Guarantees that -value is set properly before
           calling Error::new().

 Arguments: same as for Error::new()

     -value must be non-zero and not an empty string in order for
     eval{}-based exception handlers to work. These require that
     if($@) evaluates to true, which will not be the case if
     the Error has no value (Error overloads numeric operations
     to the Error::value() method).

     It is OK to create Bio::Root::Exception objects without
     specifing -value. In this case, an invisible dummy value is used.

     If you happen to specify a -value of zero (0), it will
     be replaced by the string "The number zero (0)".

     If you happen to specify a -value of empty string (""), it will
     be replaced by the string "An empty string ("")".
 Purpose : Get a nicely formatted string containing information about the 
           exception. Format is similar to that produced by 
           Bio::Root::Root::throw(), with the addition of the name of
           the exception class in the EXCEPTION line and some other
           data available via the Error object.
 Example : print $error->pretty_format;
 Purpose : Overrides Error::stringify() to call pretty_format(). 
           This is called automatically when an exception object 
           is placed between double quotes.
 Example : catch Bio::Root::Exception with {
              my $error = shift;
              print "$error";

See Also: pretty_format()

Subclasses of Bio::Root::Exception


 Purpose : Indicates that a method has not been implemented.
 Example : throw Bio::Root::NotImplemented( 
               -text   => "Method \"foo\" not implemented in module FooBar.",
               -value  => "foo" );


 Purpose : Indicates that some input/output-related trouble has occurred.
 Example : throw Bio::Root::IOException( 
               -text   => "Can't save data to file $file.",
               -value  => $! );


 Purpose : Indicates that a file could not be opened.
 Example : throw Bio::Root::FileOpenException( 
               -text   => "Can't open file $file for reading.",
               -value  => $! );


 Purpose : Indicates that a system call failed.
 Example : unlink($file) or throw Bio::Root::SystemException( 
               -text   => "Can't unlink file $file.",
               -value  => $! );


 Purpose : Indicates that one or more parameters supplied to a method 
           are invalid, unspecified, or conflicting.
 Example : throw Bio::Root::BadParameter( 
               -text   => "Required parameter \"-foo\" was not specified",
               -value  => "-foo" );


 Purpose : Indicates that a specified (start,end) range or 
           an index to an array is outside the permitted range.
 Example : throw Bio::Root::OutOfRange( 
               -text   => "Start coordinate ($start) cannot be less than zero.",
               -value  => $start  );


 Purpose : Indicates that a requested thing cannot be located 
           and therefore could possibly be bogus.
 Example : throw Bio::Root::NoSuchThing( 
               -text   => "Accession M000001 could not be found.",
               -value  => "M000001"  );