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* Assign any Python object to an Octave variable, view its properties, and invoke methods on it
* 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
* 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 may include:
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
* 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 ==
== 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].
Project development is ongoing among a small group of developers. Communication takes place on the Octave maintainers mailing list. The official project repository is at [https://gitlab.com/mtmiller/octave-pythonic https://gitlab.com/mtmiller/octave-pythonic].


== Documentation ==
== Documentation ==
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=== Python from Octave ===
=== 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.
The conversion of Python's dictionary (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 for 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| -->
<!-- {{SyntaxHighlight| -->
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==== Python Objects in Octave ====
==== 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.
The {{Codeline|@pyobject}} class wraps arbitrary Python objects so they can be accessed and manipulated from within Octave.  In most cases, these are created automatically.


{{Code|avoiding garbage collection|<syntaxhighlight lang="octave" style="font-size:13px" line highlight="4">
== Known Problems ==
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:
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.


1. `x` stores the pointer to `d`. The `@pyobj` ctor creates a dummy reference to `d`, this prevents GC
Python objects are implemented as Octave classdef objects. More specifically, any Python object is represented by an object of type <tt>pyobject</tt>. which contains a persistent reference to the corresponding Python object. The pyobject class overrides functions such as <tt>class</tt> and <tt>isa</tt> so it appears as if each Python type is represented by a corresponding distinct class in Octave, but in reality they are all of the same type. This is similar to Octave's Java interface.


2.  on deletion of x (`clear x`) we delete the dummy reference in Python.
Because classdef is used, some of the following known issues are specifically related to Octave's classdef implementation, which is still a relatively new work in progress.


Notes:
<ul>


* 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...
<li>Assignment to <tt>dict</tt> or other mapping object using string keys fails. The following syntax produces an error:
{{Code||<syntaxhighlight lang="octave">
d = py.dict ();
d{"one"} = 1;
</syntaxhighlight>}}
Use the {{Codeline|__setitem__}} method instead as a workaround:
{{Code||<syntaxhighlight lang="octave">
d.__setitem__ ("one", 1);
</syntaxhighlight>}}
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.</li>


* 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.
<li>Element indexing on a <tt>list</tt> 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 <tt>ans</tt> variable in turn, or be able to wrap the return list in a cell array, as shown here:
{{Code||<syntaxhighlight lang="octave">
x = py.list ({1, 2, 3, 4, 5, 6});
x{1:3}
y = {x{1:3}};
</syntaxhighlight>}}
Instead of the expected behavior, a cell array of elements is returned in both cases. This is Octave bug {{Bug|48693}}. The following patterns for assigning the results do work instead:
{{Code||<syntaxhighlight lang="octave">
[a, b, c] = x{1:3}
[y{1:3}] = x{1:3};
</syntaxhighlight>}}</li>


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


* 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.
<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>


====== Interface design ======
<li>Python objects can't be loaded or saved using the Octave {{Codeline|load}} and {{Codeline|save}} commands. This missing feature applies to any classdef object, this is Octave bug {{Bug|45833}}. As a workaround, any Python pickling or serialization functions can be used to load and save objects separately from the usual Octave workspace techniques.</li>


* {{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`.
<li>Names cannot be imported into the current workspace using the {{Codeline|import}} command. {{Codeline|import}} is not yet implemented in Octave at all.</li>


* {{Codeline|@pyobj/pyobj}} constructor would not normally be called by users.
</ul>
 
In addition, the following workaround may be considered a known issue. I don't know enough about Matlab's Python implementation to know how important this is, but it's worth documenting.
 
<ul>
<li>
<tt>py.foo</tt> is supposed to act like a kind of dynamic namespace, automatically loading any matching Python function or module that is in the search path. This most closely maps to the concept of a "package" in Matlab or Octave, which would normally be implemented with a <tt>+py</tt> directory. Since we didn't know of an easy way to create a package scope that does some kind of dynamic dispatch, we implemented <tt>py</tt> as an old-style <tt>@py</tt> class. This takes advantage of the fact that the syntax <tt>py.foo</tt> instantiates a <tt>py</tt> object and calls <tt>subsref</tt> on it. This only works because it is an old-style class, with classdef this would be treated like a static class method call rather than a property lookup on an object instance. This lets us interpret the property lookup as a dynamic search of the Python module path.
</li>
</ul>


== Pytave ==
== 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.
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.
== See also ==
* https://gitlab.com/mtmiller/octave-pythonic
[[Category:Development]]
[[Category:Packages]]
Retrieved from "https://wiki.octave.org/Special:MobileDiff/9472...13360"

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