What’s coming in for bento 0.0.4

I initially intended to release the new version (0.0.4) of bento around mid-end
of August, but it now seems like end of september is more likely. The reason is
that I have been working pretty hard on bento and yaku for complex builds in
the last few weeks, where complex means numpy/scipy.

The main reason for making scipy buildable with bento is to get a “feel” of how
really extensible bento is. I think that since 0.0.3, bento is fairly usable,
but extensibility is really what bento is about. The only way that I know to
have an extensible design is to actually extend it in as many scenario as
possible, and as far as complex distutils-based build go, scipy is a pretty
good scenario.

The bottom line: I expect a fully working bento build of scipy within a few
(most hairy fortran stuff now builds and run the tests ok)

Major changes from 0.0.3

No backward-incompatible changes are required for the bento.info format. The
major change is recursive package support, which ended up being more complex
than anticipated. I already described this feature in a previous post: it
mostly boils down to splitting a big bento.info into several “sub-bento” files
in subdirectories.

Implementation-wise, it required a redesign of internal representation for
files. The issue is how to know that two file names represent the same files: I
quickly realize that using filenames is too complex and too fragile, and I
decided to re-use the Node class from waf, which builds an internal
representation of the filesystem. The conversion is still going on, but it
simplified a lot of hairy code that I used to write in bento (and distutils
previously). It particularly helps to compute the relative paths between too

relpos = node.path_from(othernode)

If node is /foo/bar and othernode is /foo, relpos will be bar, and .. if node
and othernode are inverted. Doing this from the filenames alone has many
corner cases, and path name computation are surprisingly slow on python (waf
Node class caches things like absolute path name computation).

Thanks to the waf Node class, I can now easily list the packages, extensions,
etc… specific to one sub bento, relatively to the sub bento directory, and
translates packages, extensions, etc… as seen from the top directory.

I am happy with the internals, but the “API” for recursive build description is
not good, to put it mildly. To add a subpackage description with associated
bscript (hook file), you need to:

  • add the sub bento.info to the Subento field in the parent bento.info
  • add the bscript file into the list of the recursive decorator inside
    the parent bscript file. Even though the decorator may be put on e.g.
    the configure hook, the build command will also look there for sub
    bscript files, which is not intuitive at all.

You can see some examples
I am still looking for a good solution to this issue.

Yaku enhancements

Except for recursive package description, not much has changed in bento, and
most of the work has happened in yaku. The first big change is that yaku itself
also uses a waf-like Node class: although I resisted this at first, I think it
is for the best, and it also simplified a lot of hairy corner cases inside

The other big change in yaku is overriding/extending it. I am interested in the
following cases:

  • adding new tool (clang, intel compiler, etc…)
  • adding a new process in the chain (say building extensions from .c.src
    instead of .c without monkey-patching original code)
  • overriding flags for some extensions (say building one extension with -Os
    instead of -O2)
  • overriding extension hook for some extensions. For example in general,
    fortran source files are compiled into .o directly for “pure” (not using
    python C API) libraries, but f2py allows to build a python extension from
    the .f directly. Yaku now allows for temporary overriding the command
    associated to .f file

Now, all those four cases are implemented. Chaining a templating system to
cython (for .pyx.in -> .pyx -> .c -> .o -> .so/.pyd) is now very simple,
supporting new compilers can be done easily, and playing with compiling options
straightforward internally. There are a few issues, though. Besides how the
API should look like, a corny situation is dealing with dictionaries of
configurations. In yaku, each task has an environment attached to it, which is
a simple dictionary containing things like CFLAGS, CC, etc… Most of the
time, you want to share those dictionaries across tasks. Unfortunately, python
semantics for dictionaries don’t make that easy, and deepcopy is too expensive.
A Copy-On-Write dictionary, which internally share common parts between
dictionaries, would be ideal, but I am afraid implementing one in python would
be very difficult.

I am also still not entirely convinced that yaku is warranted:
fbuild is nearly the ideal system if it were
not limited to python 3, and the new waf 1.6 looks great (T. Nagy, the waf
maintainer, recently updated fortran support for 1.6). Fortunately, bento is
build-tool agnostic from the start, and trying waf inside bento for a real
project is on the TODO list.

Other bento features

I put an hold on other features planned for 0.0.4. The main missing features
for bento are:

  • distutils compatibility mode (so that bento.info may be used within distutils)
  • wininst <-> egg conversion
  • good documentation
  • python 3 compatibility
  • virtualenv and pip support
  • automatic command dependency (e.g. automatically re-run configure before
    build if necessary)

Python 3 support will definitely not go into 0.0.4. Virtualenv/pip support
should not be difficult, automatic dependency for commands is badly needed.

All being said, I think bento is shaping up quite ok. At my work, I constantly
have to deal with distutils idiosyncraties for the most trivial things, and I
am looking forward to seeing it replaced with something saner.