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Re: Potential Google Summer of Code 2007 project

From: C. Michael Pilato <cmpilato_at_collab.net>
Date: 2007-03-20 20:43:46 CET

Dave, you think you could take the informational bits of this mail and stuff
them into a nice fat TODO file in the bindings/swig/python directory, or
some similar sane place?

David James wrote:
> Hi Edin,
> It's great to hear you're interested in helping out with the Python
> bindings. I'd be happy to mentor any students who are interested in
> this task.
> Currently, the Subversion Python bindings are implemented using SWIG
> (see http://www.swig.org/). SWIG is an automatic wrapper generator
> which generates bindings for several languages including Python.
> Inside SWIG "interface" files, we provide SWIG with typemaps which
> explain how to convert between Python datatypes and Subversion native
> datatypes.
> These datatype conversions include:
> - apr_array_header_t <-> Python array
> - apr_hash_t <-> Python dictionary
> - svn_string_t <-> Python string
> - svn_stringbuf_t <-> Python string
> - svn_error_t * ==> (If an error occurs, throw a Python exception)
> If SWIG was perfect, our SWIG bindings could probably consist of five
> short interface files, which explained how to convert those five
> datatypes between Python and Subversion, and we would have basic, raw,
> Python bindings, which look a lot like the original Subversion/C API,
> but are written in Python.
> Let's pretend for a few moments that SWIG is perfect, and that we
> already have perfect SWIG bindings for all of Subversion's datatypes.
> (This is by no means true! But let's pretend for a moment.)
> Now that we have raw bindings to Subversion, your job is to create a
> friendly object-oriented interface to Subversion which is more
> Pythonic in nature.
> What would Pythonic bindings look like? Here's a few examples of what
> you would be able to do with Python bindings:
> client = SubversionClient("http://svn.collab.net/repos/svn/trunk",
> username="joecommitter", password="joepassword")
> # Print out the README
> print client.cat("README")
> # Find out who wrote "line 1" of the README file
> lines = client.blame("README")
> print lines[0].author
> # Move a file
> client.move("README", "README2", message="Move the readme file to a
> less obvious location")
> # Do several operations in a single transaction
> files = client.ls()
> txn = client.transaction()
> # Make sure that every file in the "trunk" directory starts with a
> "hip_" prefix.
> for file in files:
> txn.move(file, "hip_%s" % file)
> # Commit all of the changes at once
> txn.commit(message="Make the Subversion trunk more hip!")
> # Checkout repositories to disk
> wc = client.checkout()
> # Commit changes
> open("hip_README2","w").write("Hello world\n")
> wc.commit(message="Update the README2 file")
> Wouldn't it be great if the Python bindings were that easy to use?
> Unfortunately, it's not that easy, but it should be. We need a summer
> volunteer to fix this problem as a Summer of Code project.
> Unfortunately, there's a few obstacles (imperfections in Subversion's
> SWIG bindings) which might stand in your way:
> 1) SWIG does not automatically wrap arguments to callback
> functions. Instead, you must create a C callback function which
> manually converts arguments between Python and C using SWIG's APIs.
> 2) SWIG does not automatically wrap pointers which are contained
> inside arrays or hashes. Instead, you must write a typemap which
> manually converts these pointers between Python and C using SWIG's
> APIs.
> We already have written a large number of conversion functions which
> accomplish (1) and (2), but they're really not very much fun to write.
> It involves a lot of careful work and error checking.
> Once the SWIG typemaps are complete, we also suffer from another
> problem: the generated SWIG bindings are undocumented. If you read
> through the Subversion include files, you can understand a great deal
> about how the Subversion bindings work, but your understanding will
> not be complete until you understand the SWIG bindings as well.
> When I write functions which use the Python bindings, I often consult
> the source code of the SWIG interface files to see how the Python
> datatypes will be converted into Subversion datatypes, so as to make
> sure that I am providing the correct arguments. We aren't completely
> consistent about how we convert datatypes between Python and C, so you
> often may have to consult the source code to make sure that it behaves
> as you expect.
> That said, let's take another look at your primary task, which is to
> write higher-level bindings for Subversion in Python. As written, our
> project page suggests that these higher-level bindings for Subversion
> must be based upon our lower-level SWIG bindings, but I'd like to
> suggest another alternative: ctypes.
> As of Python 2.5, it's possible to access C datastructures directly
> from Python. Using ctypes, you can access the complete Subversion API
> via pure Python, without needing to write any complex SWIG or C
> wrappers. This feature is also supported in older Python versions if
> you download the ctypes module.
> ctypes is much simpler than SWIG, and supports many key features that
> SWIG does not. For example, ctypes directly supports callback
> functions, so you do not need to waste time creating verbose wrappers
> in C for every callback function that Subversion supports. ctypes also
> supports composite datatypes (e.g. an array of strings), so you don't
> need to create individual typemaps for every possible composite
> datatype. In fact, using ctypes, you don't need to create typemaps at
> all -- you can just call the C functions directly, without worrying
> about typemaps.
> If you use ctypes, you won't have to write (or modify) any code in C.
> You can write everything in Python. As a result, your compile/debug
> cycle will be very quick -- you won't have to recompile Subversion or
> our Python bindings in order to test out a change to your higher-level
> layer.
> Overall, I think that it would be easier to write higher-level Python
> bindings using ctypes than it would to write them using our existing
> SWIG bindings. In this case, you would truly be able to depend on
> having a clear interface between your bindings and the underlying C
> implementation.
> As part of your proposal, I'd love if you could write a simple Python
> program which does a few simple things using Subversion. Perhaps you
> can move a file from one location to another, directly in the
> repository? This shouldn't be too hard to implement using ctypes.
> Don't forget to budget lots of time for test cases and documentation!
> Cheers,
> David
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C. Michael Pilato <cmpilato@collab.net>
CollabNet   <>   www.collab.net   <>   Distributed Development On Demand

Received on Tue Mar 20 20:44:02 2007

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