Difference between revisions of "Python interface"

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<li>Function handles to Python functions, bound methods, or other callable objects is not yet supported. As a workaround, the {{Codeline|pyeval}} function can be used to return a reference to a function which can be assigned and called like any Octave function handle, but cannot be passed in to functions that expect a function handle.</li>
 
<li>Objects are not deleted because object destructors are not called by Octave when objects are cleared or go out of scope. For the Python interface, this means that the internal store of objects will continue to grow and objects will persist indefinitely even when all Octave references to a given Python object are gone. This is Octave bug {{Bug|46497}}.</li>
 
<li>Objects are not deleted because object destructors are not called by Octave when objects are cleared or go out of scope. For the Python interface, this means that the internal store of objects will continue to grow and objects will persist indefinitely even when all Octave references to a given Python object are gone. This is Octave bug {{Bug|46497}}.</li>
 
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Revision as of 11:29, 16 August 2016

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
  • Perform element indexing on lists and other sequence objects using curly bracket indexing syntax
  • Perform key indexing on dicts and other mapping objects using curly bracket indexing syntax

Some features that have not yet been implemented, but have been planned for, include:

  • Perform slice indexing on lists and other sequence objects using parentheses indexing syntax
  • Operate on Python objects using standard Octave arithmetic and logical operators
  • Load and save Python objects to Octave data files using the standard load/save commands

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, 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.

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.

Code: Conversion of Python's dict to structure
> 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

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
> 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

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
pyexec ("import networkx as nx")
pyexec ("G=nx.complete_graph(10)")
x = pyeval ("G.nodes()")

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 pycall and pyexec without needing to do an explicit conversion. For example

Code: Octave object that points to a python object
1 pyexec ("import networkx as nx")
2 pyexec ("G=nx.complete_graph(10)")
3 G      = pyobj ("G") ## G is an object without Octave representation but with methods to do the conversion if required
4 nodes  = pycall ("nx.nodes",G) # Python nx.nodes(G)
5 nodes2 = pyeval ("G.nodes()")  # Equivalent

Comment: It seems that pyexec and pycall do not share workspace, so the code above wont work because nx doesn't exist when we call 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
G_cell   = cell(G);   # G converted to a cell
G_struct = struct(G); # G converted to a struct


Python Objects in Octave

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

Code: avoiding garbage collection
1 pyexec('d = dict(one=1, two=2)')    # create object in Python
2 x = pyobj('d')                      # create pyobj wrapper for that object
3 pyexec('d = []')                    # careful, don't lose the object to the GC
4 x.keys()                            # list the keys of the dict
5 clear x                             # now the object could be GCed

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 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 hex(id(x)), maybe called __InOct__. Then pass the id of the object to the @pyobj/pyobj constructor.
  • 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
  • pyeval should be modified to return a @pyobj for things that it cannot convert to Octave-native types. See the `networkx` example above: `G` could be returned by `pyeval`.
  • @pyobj/pyobj constructor would not normally be called by users.

Known Problems

This section documents some known problems or limitations of the Python calling interface, usually due to a limitation of the Octave language itself or a bug in Octave that needs some attention.

  • Assignment to dict or other mapping object with string keys fails. The following syntax produces an error:
    d = py.dict ();
    d{"one"} = 1;
    

    Use the __setitem__ method instead as a workaround:

    d.__setitem__ ("one", 1);
    
    The reason is because Octave strings are interpreted as arrays in many contexts, and this syntax is parsed by Octave as an attempt to assign to 3 elements of an object.
  • Element indexing on a list or other sequence object with a range or set of indices doesn't return the right number of output arguments. Element indexing should return as many values as were indexed, each assigned to the ans variable in turn, or be able to wrap the return list in a cell array, as shown here:
    x = py.list ({1, 2, 3, 4, 5, 6});
    x{1:3}
    y = {x{1:3}};
    

    Instead of the expected behavior, a cell array of elements is returned in both cases. This is Octave bug #48693. The following patterns for assigning the results do work instead:

    [a, b, c] = x{1:3}
    [y{1:3}] = x{1:3};
    
  • Function handles to Python functions, bound methods, or other callable objects is not yet supported. As a workaround, the pyeval function can be used to return a reference to a function which can be assigned and called like any Octave function handle, but cannot be passed in to functions that expect a function handle.
  • Objects are not deleted because object destructors are not called by Octave when objects are cleared or go out of scope. For the Python interface, this means that the internal store of objects will continue to grow and objects will persist indefinitely even when all Octave references to a given Python object are gone. This is Octave bug #46497.

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.