Editing Pythonic

There is a small project in development to bring a Python calling interface to Octave. The broad goal of this project is to add functions and types to Octave to allow calling Python functions directly from Octave.

== Features ==

At a high level, the features and capabilities of Octave's Python interface allow a user to:

* Import and call any Python module or function from the Octave interpreter
* Automatically convert basic Octave and Python types seamlessly between the two operating environments
* Assign any Python object to an Octave variable, view its properties, and invoke methods on it
* Assign any Python function or callable object to an Octave variable, and call it as if it were a function handle

Some features that have not yet been implemented may include:

* Load and save Python objects to Octave data files using the standard load/save commands
* Operate on Python objects using standard Octave arithmetic and logical operators

== Development ==

Project development is ongoing among a small group of developers. Communication takes place on the Octave maintainers mailing list. The official Mercurial repository is at [http://hg.octave.org/pytave http://hg.octave.org/pytave], but there is also a Bitbucket clone and a network of forks, for those who prefer that model of development, at [https://bitbucket.org/mtmiller/pytave https://bitbucket.org/mtmiller/pytave].

== Documentation ==

The current development needs to be documented. We are using doxygen for the documentation.

== Roadmap / Ideas ==

=== Python from Octave ===

The conversion of Python's dict is not unique. For that we have decided to load a Python's dict as a structure. This works only when all the keys fo the dict are strings. When the keys are something else there is the option to use `repr` to create the fields of the Octave's struct, e.g.

<!-- {{SyntaxHighlight| -->
{{Code|Conversion of Python's dict to structure|<syntaxhighlight lang="octave" style="font-size:13px">
> x = pyeval ("{1:'one',2:'two'}");
> x.("1")
ans = one

> x.("2")
ans = two

> x = pyeval ("{(1,1):'one',2.5:'two'}");
> x.("(1,1)")
ans = one

> x.("2.5")
ans = two

</syntaxhighlight>}}

This would be the default behavior. We will extend pyeval to receive an optional argument specifying the Octave type that should be used as the output of the conversion, e.g. when the dict uses continuos numbers as keys one could do

{{Code|pyeval optional argument|<syntaxhighlight lang="octave" style="font-size:13px">
> x = pyeval ("{1:'one',2:'two'}",@cell);
> x{1}
ans = one
> x{2}
ans = two

> x = pyeval ("{1:'one',2:'two'}",@char);
> x

x = 

one
two
 
> whos x
Variables in the current scope:

   Attr Name        Size                     Bytes  Class
   ==== ====        ====                     =====  ===== 
        x           2x3                          6  char

Total is 6 elements using 6 bytes

> x = pyeval ("[i for i in xrange(3)]",@double)
x =
0 1 2
</syntaxhighlight>}}

The optional argument could be the constructor of an Octave class.

==== Octave view of Python ====

Currently we can do things like

{{Code|Objects in python exists across calls to pyexec and pyeval|<syntaxhighlight lang="octave" style="font-size:13px">
pyexec ("import networkx as nx")
pyexec ("G=nx.complete_graph(10)")
x = pyeval ("G.nodes()")
</syntaxhighlight>}}

But we cannot get a representation of ''G'' within Octave.

The idea is to have an object that maintains a pointer to an Python object and that has all the methods to convert that object to Octave.

The python object can be manipulated by python calls like {{Codeline|pycall}} and {{Codeline|pyexec}} without needing to do an explicit conversion. For example

{{Code|Octave object that points to a python object|<syntaxhighlight lang="octave" style="font-size:13px" line highlight="4">
pyexec ("import networkx as nx")
pyexec ("G=nx.complete_graph(10)")
G      = pyobj ("G") ## G is an object without Octave representation but with methods to do the conversion if required
nodes  = pycall ("nx.nodes",G) # Python nx.nodes(G)
nodes2 = pyeval ("G.nodes()")  # Equivalent
</syntaxhighlight>}}

'''Comment:''' It seems that {{Codeline|pyexec}} and {{Codeline|pycall}} do not share workspace, so the code above wont work because ''nx'' doesn't exist when we call {{Codeline|pycall}} at line 4.

The conversion methods should provide different output objects to Octave. The idea is to be able to call something like

{{Code|Octave view of Python object with conversion methods|<syntaxhighlight lang="octave" style="font-size:13px">
G_cell   = cell(G);   # G converted to a cell
G_struct = struct(G); # G converted to a struct
</syntaxhighlight>}}


==== Python Objects in Octave ====

The {{Codeline|@pyobj}} classdef class is intended to wrap arbitrary Python objects so they can be accessed an manipulated from within Octave.

{{Code|avoiding garbage collection|<syntaxhighlight lang="octave" style="font-size:13px" line highlight="4">
pyexec('d = dict(one=1, two=2)')    # create object in Python
x = pyobj('d')                      # create pyobj wrapper for that object
pyexec('d = []')                    # careful, don't lose the object to the GC
x.keys()                            # list the keys of the dict
clear x                             # now the object could be GCed
</syntaxhighlight>}}

One proposed way to do this:

1.  `x` stores the pointer to `d`.  The `@pyobj` ctor creates a dummy reference to `d`, this prevents GC

2.  on deletion of x (`clear x`) we delete the dummy reference in Python.

Notes:

* Seems like the relevant "pointer" is {{Codeline|id()}}.  Haven't seen yet how to access an object from its id, except that its a bad idea...

* My plan to create a dict in Python, indexed by {{Codeline|hex(id(x))}}, maybe called {{Codeline|__InOct__}}.  Then pass the id of the object to the {{Codeline|@pyobj/pyobj}} constructor.

* Follow along and help out here: https://bitbucket.org/macdonald/pytave/commits/branch/cbm_pyobj

* Rejected idea: store the `repr` as a string in `x`.  But this makes a copy of the object rather than a reference to the original object.

====== Interface design ======

* {{Codeline|pyeval}} should be modified to return a {{Codeline|@pyobj}} for things that it cannot convert to Octave-native types.  See the `networkx` example above: `G` could be returned by `pyeval`.

* {{Codeline|@pyobj/pyobj}} constructor would not normally be called by users.

== Pytave ==

This project is currently derived from an earlier project called Pytave, which was developed to work in the opposite direction, to allow Python to call Octave functions on an embedded Octave interpreter. The bulk of the project is in the code to convert between Octave and Python data types, so most of that is reusable and serves both purposes. As a side goal, we may continue to maintain the Python wrapper around Octave and incorporate that into Octave as well, so that Octave can provide its own native Python module.