=head1 NAME

Inline::Python - Write Perl subs and classes in Python.

=head1 SYNOPSIS

   print "9 + 16 = ", add(9, 16), "\n";
   print "9 - 16 = ", subtract(9, 16), "\n";

   use Inline Python => <<'END_OF_PYTHON_CODE';
   def add(x,y): 
      return x + y

   def subtract(x,y):
      return x - y

   END_OF_PYTHON_CODE

=head1 DESCRIPTION

The C<Inline::Python> module allows you to put Python source code
directly "inline" in a Perl script or module. It sets up an in-process
Python interpreter, runs your code, and then examines Python's symbol table
for things to bind to Perl. The process of interrogating the Python 
interpreter for globals only occurs the first time you run your Python code. 
The namespace is cached, and subsequent calls use the cached version.

This document describes C<Inline::Python>, the Perl package which gives you
access to a Python interpreter. For lack of a better place to keep it, it 
also gives you instructions on how to use C<perlmodule>, the Python package 
which gives you access to the Perl interpreter.

=head1 WHAT'S NEW?

Version 0.21 provides the ability to bind to 'new-style' classes (as
defined by the python PEP's 252 and 253.)  See L<New-Style Classes> for
details.

See the L<Changes> file for new features in recent versions.

=head1 Using the Inline::Python Module

Using Inline::Python will seem very similar to using another Inline
language, thanks to Inline's consistent look and feel.

This section will explain the different ways to use Inline::Python.
For more details on C<Inline>, see 'perldoc Inline'. 

=head2 Giving Your Source to Inline

The most basic form for using C<Inline::Python> is this:

   use Inline Python => 'Python source code';

Of course, you can use Perl's "here document" style of quoting to make 
the code slightly easier to read:

   use Inline Python => <<'END';

     Python source code goes here.

   END

The source code can also be specified as a filename, a subroutine
reference (sub routine should return source code), or an array
reference (array contains lines of source code). The recommended way
of using Inline is this:

   use Inline Python;

   ...

   __END__
   __Python__

   Python source code goes here.

This information is detailed in 'perldoc Inline'.

=head2 Importing Functions

Maybe you have a whole library written in Python that only needs one entry
point. You'll want to import that function. It's as easy as this:

   use Inline Python;

   doit();

   __END__
   __Python__

   from mylibrary import doit

Inline::Python actually binds to every function in Python's "global" namespace 
(those of you in the know, know that namespace is called '__main__'). So if 
you had another function there, you'd get that too.

=head2 Importing Classes

If you've written a library in Python, you'll make it object-oriented.
That's just something Python folks do. So you'll probably want to import a
class, not a function. That's just as easy:

   use Inline Python;

   my $obj = new Myclass;

   __END__
   __Python__

   from mylibrary import myclass as Myclass

=head2 New-Style Classes

As of python 2.2, the python internals have begun to change in a way
which makes types 'look' more like classes.  This means that your python
code can now subclass builtin python types such as lists, tuples,
integers, and etc.  It also means that identifying python objects and
creating Perl bindings for them has become a little trickier.

See Guido's write-up (http://www.python.org/2.2.2/descrintro.html) and
the relevant Python Enhancement Proposals (PEP) numbers 252 and 253 for
details about the python code.  Also, see the mailing-list discussion
(http://mail.python.org/pipermail/python-dev/2004-July/046060.html) for
possible implications regarding C-language python extensions.

This change should not affect code which uses Inline::Python, except
that it allows you to bind to python classes which have been written
using these new features.  In most cases, you will be importing an
entire class from an external library as defined in the example above. 

In other cases, you may be writing Inline::Python code as follows:

   use Inline Python => <<'END';
   class Foo(object):
      def __init__(self):
         print "new Foo object being created"
         self.data = {}
      def get_data(self): return self.data
      def set_data(self,dat): 
         self.data = dat
   END

Additional caveats may exist.  Note that if the python class is
subclassing one of the builtin types which would normally be accessible
as a 'Perlish' translation, that the instance will be an opaque object
accessible only through its class methods.

  # Class is defined as 'def Class(float):'
  my $obj = Class->new(4);
  print $$obj "\n"; # will NOT print '4.0'

=head2 New-Style

=head2 Boundary Conditions

What if you have a class that wasn't imported? Can you deal with instances 
of that class properly?

Of course you can! Check this out:

   use Inline Python => <<END;

   def Foo():
       class Bar:
           def __init__(self):
               print "new Bar()"
           def tank():
               return 10
       return Bar()

   END

   my $o = Foo();
   $o->tank();

In this example, C<Bar> isn't imported because it isn't a global -- it's hidden
inside the function Foo(). But Foo() is imported into Perl, and it returns an 
instance of the C<Bar> class. What happens then?

Whenever Inline::Python needs to return an instance of a class to Perl, it 
generates an instance of Inline::Python::Object, the base class for all 
Inline::Python objects. This base class knows how to do all the things you 
need: calling methods, in this case.

=head1 Exceptions

Exceptions thrown in Python code get translated to Perl exceptions which you can
catch using eval.

=head1 Using Perl inside Python (inside Perl)

This section doesn't talk at all about C<Inline::Python>. It's about how
to use C<perl>. C<perl> is a Python module bundled with Inline::Python
that gives you access to Perl from inside your Python code. In the
future, it will be possible to compile Inline::Python to work the other
way around -- to use Python as the main programming language, and jump
into Perl when you want to.

The C<perl> package exposes Perl packages and subs. It uses the same code as
Inline::Python to automatically translate parameters and return values as
needed. Packages and subs are represented as C<PerlPkg> and C<PerlSub>,
respectively.

=head1 Using the PerlPkg Type

The C<perl> package is actually not a package at all. As soon as you import
it, it replaces itself with an instance of the PerlPkg class, wrapping
the Perl package "main". Perl's 'main' package is analogous to '__main__' in
Python.

Here's what you can do with the 'main' PerlPkg:

=head2 eval()

   eval(source code)

Unlike Python, Perl has no exec() -- the eval() function always returns the
result of the code it evaluated. eval() takes exactly one argument, the
perl source code, and returns the result of the evaluation.

=head2 require() and use()

   require(module name)
   use(module name)

Use require() instead of C<import>. In Python, you'd say this:

   import md5

But using the perl module, you'd say this:

   perl.require("Digest::MD5")

Of course, in Perl there's more than one way to do it (TM). require() doesn't
run the package's import() function. If you want symbols exported, for 
instance, use use() instead of require().

Here is the functionality common to all PerlPkg instances:

=head2 __getattr__

Python's __getattr__() function allows the package to dynamically return
something to satisfy the request. For instance, you can get at the subs
in a perl package by using dir() (which is the same as C<getattr(perl,
'__methods__')>.

Here's an example:

   perl.eval("sub f { 10 }")    # define main::f
   f = perl.f
   f(); f("hello")              # no argument checking
   if perl.f() != 10: 
       import sys; sys.exit(1)

Notice what happens. First we call eval() to define a sub 'f'. Then we 
say C<perl.f>, which goes into the __getattr__() method. We check the Perl
namespace and see a function called f, which we return, wrapped in an 
instance of the PerlSub type.

=head3 Accessing a perl object's data

__getattr__ may also be used to access a Perl object's attributes, just like
Python allows. The Perl object just has to implement a sub __getattr__
returning the requested attribute, which may even be calculated on the fly.

An example for the common hash based objects:

   sub __getattr__ {
       my ($self, $attr) = @_;
       return $self->{$attr}; 
   }

This allows Python code to access the perl object's data like:

   print my_perl_object.field_name

=head2 named arguments

When a Perl sub is called with named arguments from Python code, Inline::Python
follows the PyObject_Call protocol: positional arguments are given as array ref
followed by named arguments as a hash ref. A Perl method supporting named
arguments would therefore look like:

    sub supports_named_arguments {
        my ($self, $positional, $named) = @_;
	foreach (qw( named1 named2 )) {
	    last unless @$positional;
	    $named->{$_} = shift @$positional;
	}
	...
    }

If this method is called using only positional arguments, they would just be
pushed into @_ like in any other method, complicating it to:

    sub supports_named_arguments {
        my ($self, $positional, $named) = @_;
	if (@_ == 3 and $size and ref $size and ref $size eq 'ARRAY' and ref $useimage eq 'HASH') { # called using named parameters
	    foreach (qw( named1 named2 ... )) {
		last unless @$positional;
		$named->{$_} = shift @$positional;
	    }
	}
	else {
	    $named = { named1 => $positional, named2 => $named, named3 => $_[3], ... };
	}
	...
    }

As this adds a lot of boiler plate code to subroutines, it is better to just
use Perl named arguments conventions (single hashref parameter) if possible.

=head1 Using the PerlSub Type

All Perl subs are wrapped in the PerlSub type, so that they can emulate Python
subroutines. You can call them. It's all good. Here's what you can do with
PerlSub objects:

=head2 Call

PerlSub catches the call action and forwards the call to the real sub in Perl.

=head2 Set the evaluation flags

Perl has this notion of calling context. A subroutine can ask Perl what it is
being used for. The idea is that if no one cares about your return value, you
might be able to save time by not building it. By default, PerlSub objects
evaluate in 'list' context with no extra flags turned on. 

   perl.eval("sub f { 10 }")
   f = perl.f
   f.flags = f.flags | f.G_SCALAR
   x = f()

Here are the most common flags you'll need. For more details about these and
other possible flags, see L<perlcall>.

=over 4

=item 1

G_VOID

Calls the Perl subroutine in a void context. Guarantees that no results will
be returned. If any are returned, Perl deletes them.

=item 2

G_SCALAR

Calls the Perl subroutine in a scalar context. Ensures that only one element 
is returned from the sub. If the sub returns a list, only the last element 
is actually saved.

=item 3

G_ARRAY

Calls the Perl subroutine in a list context. Ensures that any items returned
from the subroutine are returned. This is the default for PerlSub objects.

=item 4

G_DISCARD

If you are not interested in the return values, you can optimize slightly by
telling Perl, and it will discard all returned values for you.

=item 5

G_NOARGS

If you are not passing any arguments, you can optimize the call so that Perl
doesn't bother setting up the stack for parameters.

=item 6

G_EVAL

It is possible for the Perl sub to fail, either by calling die() explicitly 
or by calling a non-existent sub. By default, the process will terminate 
immediately. To avoid this happening, you can trap the exception using the 
G_EVAL flag.

=back

=head1 Under the Hood

When Inline::Python imports a class or function, it creates subs in Perl
which delegate the action to some C functions I've written, which know how
to call Python functions and methods.

   use Inline Python => <<'END';

   class Foo:
      def __init__(self):
         print "new Foo object being created"
         self.data = {}
      def get_data(self): return self.data
      def set_data(self,dat): 
         self.data = dat

   END

Inline::Python actually generates this code and eval()s it:

   package main::Foo;
   @main::Foo::ISA = qw(Inline::Python::Object);

   sub new {
     splice @_, 1, 0, "__main__", "Foo";
     return &Inline::Python::py_new_object;
   }

   sub set_data {
     splice @_, 1, 0, "set_data";
     return &Inline::Python::py_call_method;
   }

   sub get_data {
     splice @_, 1, 0, "get_data";
     return &Inline::Python::py_call_method;
   }

   sub __init__ {
     splice @_, 1, 0, "__init__";
     return &Inline::Python::py_call_method;
   }

More about those C<py_*> functions, and how to generate this snippet of code
yourself, in the next section.

=head1 The Do-it-yourselfer's Guide to Inline::Python

Sometimes you don't actually want to do things the Inline Way. Maybe you
just want to use a Python class as-is, without ever treating it like a normal
Perl class:

   use Inline::Python qw(py_eval);

   py_eval(<<'END');

   class MyClass:
       def __init__(self): self.data = {}
       def put(self, key, value): self.data[key] = value
       def get(self, key):
           try: return self.data[key]
           except KeyError: return None

   END

   my $o = Inline::Python::Object->new('__main__', 'MyClass');
   $o->put("candy", "yummy");
   die "Ooops" unless $o->get("candy") eq 'yummy';

Inline::Python provides a full suite of exportable functions you can use to
manipulate Python objects and functions "directly".

=head2 py_eval()

   py_eval("python source code", [context])

The new py_eval() behaves a little like Perl's eval(). It evaluates the code 
or croaks on failure. The optional context argument can be used to place 
restrictions on the type of code allowed, as well as influence what happens
to the result.

=over 4

=item 0

Accepts only expressions. Complete statements yield a syntax error. An
expression is anything that can appear to the right of an '=' sign. Returns
the value of the expression.

=item 1

The default. Accepts arbitrarily long input, which may be any valid Python 
code. Always returns C<undef>.

=item 2

Accepts exactly one statement, and prints the result to STDOUT. This is
how Python works in interactive mode. Always returns C<undef>.

=back

=head2 py_call_function()

   py_call_function("package", "function", args...)

This function runs a Python function and returns the result. The "package" and
"function" uniquely identify a function, and the remaining args are passed 
to the function.

Those who know Python well enough will know you can actually "run" a class 
and get an instance of that class back. But in case that's just too weird for 
you, I've given you a slightly higher-level wrapper around that common idiom.

=head2 py_new_object()

   py_new_object("perl package", "python package", 
                 "python class", args...)

This function creates an instance of a Python class. The "python class" is the
name of the class inside the "python package". The new object is blessed into
the given "perl package". The remaining args are passed directly to the
constructor.

=head2 py_call_method()

   py_call_method(object, "method name", args...)

Given an instance of a Python class, this function can call a method
on it. This is useful if you have an object which is blessed into a
non-existent Perl package. Attempts to use Perl's object syntax would fail,
because Perl wouldn't find any methods in that package. But py_call_method() 
can always perform method calls correctly since it unwraps the underlying
Python object.

=head2 eval_python()

Unlike in previous releases of Inline::Python, eval_python() can now return 
the result of the code. As before, eval_python() is overloaded:

=over 4

=item 1 

eval_python(code, [context])

Evaluate the code using py_eval().

=item 2

eval_python(python package, function, args...)

Run the given function and return the results using py_call_function().

=item 3

eval_python(object, method, args...)

Invoke the given method on the object using py_call_method() and return the
results.

=back

=head2 py_bind_func()

   py_bind_func("symbol name", "python package", "function")

This function imports a Python function (named "function") as the symbol
named by "perl symbol". After this function has been called, the
Python function can be called as if it were a Perl function in the given
package.

   use Inline::Python qw(py_eval py_bind_func);

   py_eval(<<'END');

   def Foo():
      return 42

   END

   # For the purposes of this example, so I know the package, I set it:
   py_bind_func("main::Bar", "__main__", "Foo");
   print "The meaning of life is: ", Bar(), "\n";

This call to py_bind_func() will generate this code and eval() it:

   sub main::Bar {
       unshift @_, "__main__", "Foo";
       return &Inline::Python::py_call_function;
   }

=head2 py_bind_class()

   py_bind_class("perl package", "python package", "class", methods...)

This function imports a Python class (named "class") into the Perl package
named by "perl package". After this function has been called, the
Perl package will look just like a regular Perl class.

The example I showed earlier in the "Under the Hood" section shows the 
output of py_bind_class. Here's another look at it:

   use Inline::Python qw(py_eval py_bind_class);

   py_eval(<<'END');

   class Foo:
      def __init__(self):
         print "new Foo object being created"
         self.data = {}
      def get_data(self): return self.data
      def set_data(self,dat): 
         self.data = dat

   END

   py_bind_class("main::Foo", "__main__", "Foo", "set_data", "get_data");
   my $o = new Foo;

This call to py_bind_class() will generate this code and eval() it:

   package main::Foo;
   @main::Foo::ISA = qw(Inline::Python::Object);

   sub new {
     splice @_, 1, 0, "__main__", "Foo";
     return &Inline::Python::py_new_object;
   }

   sub set_data {
     splice @_, 1, 0, "set_data";
     return &Inline::Python::py_call_method;
   }

   sub get_data {
     splice @_, 1, 0, "get_data";
     return &Inline::Python::py_call_method;
   }

Note that if you want methods to be created as I've shown, you must pass
them to py_bind_class() yourself. It doesn't create anything except new()
and the @ISA array. It doesn't need to, since the base class knows how to
deal with any method call -- but it's also slower, since it has to walk up
the inheritance tree to the AUTOLOAD method. I recommend binding to
the functions you know about, especially if you're the one writing the code.
If it's auto-generated, use py_study_package(), described below.

=head2 py_study_package()

   py_study_package(["package"])

This function interrogates the Python interpreter about the given package
(or '__main__' if you don't specify one). It returns a list of key/value
pairs, so it should be used like this:

   py_eval('import pickle');
   my %namespace = py_study_package("pickle");

On my machine, %namespace looks something like this:

   $VAR1 = {
             'classes' => { ... },
             'functions' => [
                              '_keep_alive',
                              'loads',
                              'dump',
                              'load',
                              'dumps',
                              'test',
                              'whichmodule'
                            ]
           };

Each result can be fed to py_bind_function() and py_bind_class(), which is 
exactly what Inline::Python itself does.

=head1 SEE ALSO 

For information about using C<Inline>, see L<Inline>.

For information about other Inline languages, see L<Inline-Support>.

Inline::Python's mailing list is inline@perl.org

To subscribe, send email to inline-subscribe@perl.org

=head1 BUGS AND DEFICIENCIES

This is a BETA quality release of Inline::Python. It is fairly feature complete
and runs stable with no known crasher bugs. Further testing and expanded support
for other operating systems and platforms will be a focus for future releases.

When reporting a bug, please do the following:

 - Put "use Inline REPORTBUG;" at the top of your code, or 
   use the command line option "perl -MInline=REPORTBUG ...".
 - Run your code.
 - Follow the printed instructions.

Here are some things to watch out for:

=over 4

=item 1

Note that the namespace imported into Perl is NOT recursively
traversed. Only Python B<globals> are imported into Perl --
subclasses, subfunctions, and other modules are not imported.

Example:

   use Inline Python => <<'END';

   import mymodule

   class A: 
       class B: pass

   END

The namespace imported into perl is ONLY that related to C<A>. Nothing
related to C<mymodule> or C<B> is imported, unless some Python code 
explicitly copies variables from the mymodule namespace into the global 
namespace before Perl binds to it.

=item 2

Inline::Python is known to leak memory. Version 0.24 fixes most of these leaks,
but it's still not completely clean and I have no idea where the remaining
leaks would be. I am however very interested in reports of segfaults and leaks
and of course testcases.

=back

=head1 SUPPORTED PLATFORMS

Inline::Python has been tested on RedHat Linux 6.2 with a variety of different
Perl and Python configurations. It also seems to be running pretty well on
openSUSE at least from 10.3 to 11.2 and on Solaris.  Previous versions of
Inline::Python working on Windows and Cygwin -- this version has never been
tested there. I strongly suspect it will require patching. Please send me
patches.

=head1 AUTHOR

Neil Watkiss <NEILW@cpan.org>

Brian Ingerson <INGY@cpan.org> is the author of Inline, Inline::C and
Inline::CPR. He was responsible for much encouragement and many
suggestions throughout the development of Inline::Python.

Eric Wilhelm provided support for 'new-style' classes in version 0.21. Many
thanks, Eric!

Stefan Seifert <NINE@cpan.org> fixed some bugs and is current co-maintainer.

=head1 COPYRIGHT

Copyright (c) 2001, Neil Watkiss.

All Rights Reserved. This module is free software. It may be used,
redistributed and/or modified under the same terms as Perl itself.

(see http://www.perl.com/perl/misc/Artistic.html)

=cut
