Notice: While Javascript is not essential for this website, your interaction with the content will be limited. Please turn Javascript on for the full experience.

Python Distutils-SIG: Design Proposal

Python Distutils-SIG

(Prerequisites: please read the proposed interface before trying to plough through this design document; it is very much a sequel to the interface document.)

Design Proposal

The Distutils' point of view

setup.py only has to import one module, distutils.core. This module is responsible for parsing all command-line arguments to setup.py (even though the interpretation of options is distributed across the various Distutils commands, and possibly the client setup.py). It also takes care of receiving control from setup.py, and passing it as appropriate to Distutils commands. Most importantly, distutils.core defines the Distribution class, which is the heart and soul of the Distutils. The client (setup.py) exists mainly to provide attributes for a Distribution instance, and all the Distutils commands operate on that instance. distutils.core also defines the Command class which comes in handy for implementing Distutils commands.

Speaking of Distutils commands: each one is implemented as a Python module, e.g. the build command is implemented by the distutils.build module. Each command module is required to define one class, also named for the command -- e.g. distutils.build.Build. These command classes will inherit from the Command class, which (at the very least) will provide a means of dealing with command-specific options. (Command will provide a constructor that takes a Distribution class and an optional list of arguments for this command, and parse the argument list by inspecting getopt-style option specifiers in the Command-derived instance.) Each command class must provide a method run that uses the information in the Distribution instance and the command options to "do its thing". Well-written command classes will parcel this task out into several well-defined (and documented) methods, so that the client setup.py may inherit from and override specific behaviours of a Distutils command class. This also means that the Distribution class must have a way to communicate overridden command classes to the main dispatcher.

The client's point of view

As I said above, the client (setup.py) only has to import distutils.core -- everything else Distutils-ish is taken care of by this core module. However, the client needs a way to communicate its particular options into the Distutils core (and out to the command modules).

There are two possible schemes for this: one short and convenient (but not too extensible), and the other a bit verbose and clunky (but more OO and extensible). There's no reason we can't have our cake and eat it too; the convenient interface could just be a wrapper around the full-blown interface for the many module distributions that don't need a lot of fancy customization.

First, here's an example of the simple interface, used for a module distribution with a single "pure Python" module (mymod.py).

    from distutils.core import setup
    setup (name = "mymod",
           version = "1.2",
           author = "Greg Ward <gward@cnri.reston.va.us>",
           description = "A very simple, one-module distribution")
Note that we don't explicitly list mymod.py anywhere: Distutils assumes that this is a one-horse distribution named after its sole module (mymod).

Those who enjoy defining subclasses might prefer to phrase this differently:

    from distutils.core import Distribution, setup

    class MyDistribution (Distribution):
        name = "mymod"
        version = "1.2",
        author = "Greg Ward <gward@cnri.reston.va.us>",
        description = "A very simple, one-module distribution")

    setup (distclass = MyDistribution)
This is overkill for a small distribution: we're defining a new class solely to provide attribute values, when distutils.core.setup exists mainly to let us do this anyways. Nevertheless, OO purists will like this -- and undoubtedly there will be times when the client will have to override behaviour, not just data, and the OO interface will be necessary.

And more complex module distributions, with lots of attributes to customize, might be easier to read/maintain with things broken up like this. Consider a distribution with two pure Python modules (mymod and my_othermod) and a C extension (myext); the C extension must be linked with two ancillary C files and a C library. Oh yeah, this distribution requires Python 1.5 and any version of the re module (ignoring the fact that one generally implies the other):

    from distutils.core import Distribution, setup

    class MyDistribution (Distribution):
        name = "mydist",
        version = "1.3.4",
        author = "Greg Ward <gward@cnri.reston.va.us>"
        description = """\
    This is an example module distribution.  It provides no useful code,
    but is an interesting example of the Distutils in action."""

        # Dependencies
        requires = { 'python': '1.5',  # I like class-based exceptions
                     're': '*',        # and I love Perl-style regexps! ;-)
                   }
        # Actual files that need to be processed and installed in some form
        py_modules = ['mymod.py', 'my_othermod.py'],
        ext_modules = {'myext.c': 
                        {'other_c': ['extra1.c', 'extra2.c'],
                         'c_libraries': ['mylib']}
                      }

    setup (distclass = MyDistribution)

A couple of things to note:

  • I'm not afraid to use deeply nested data structures; if you're writing and distributing Python modules, this shouldn't be a problem!
  • every attribute has a particular type (string, list, dictionary, ...)
  • the attributes with complex types (especially dictionaries) will have a well-known and well-documented internal structure, eg.
        """ext_modules is a hash mapping names of C source files (each
        containing a Python extension module) to a nested hash of
        information about how to build that module.  The allowed keys to
        this nested hash are: 
          - other_c: other C files that must be compiled and linked with 
                     the main C file to create the module
          - c_libraries: C libraries that must be included in the link
          ...
       """
    

No doubt the ext_modules nested hashes would have more options, and no doubt other Distribution attributes would have complex, documented structure.

Finally, the list of all Distribution attributes must be well-known and well-documented! These seem to fall into a couple of broad categories. Here's an initial attempt at a list:

  • Distribution meta-data
    • name
    • version
    • author
    • description
  • Dependencies
    • requires
  • Files to be processed and installed
    • py_modules
    • ext_modules
    • doc_files
  • Build directories (all under ./blib by default)
    • build_lib - where to put platform-independent library files
    • build_platlib - where to put platform-dependent library files
    • build_exe - where to put executable programs (ie. scripts)
    • build_html - where to put processed documentation (HTML)
  • Installation directories (under sysconfig.LIBDEST by default)
    • install_lib
    • install_platlib
    • install_exe
    • install_html
...well, that's a start.

The Distutils' point of view revisited

To sum up, let's go through what happens when the user runs setup.py. Whether setup.py is written in the simple (call-a-function) or general (define-a-subclass) form doesn't matter too much, so I won't split things up into two streams.

  • setup.py imports distutils.core
  • distutils.core startup code parses command-line arguments: processes global options that it knows about, and saves the rest for the client (setup.py) to deal with; figures out the commands and options for each command, saving them all up for later processing
  • setup.py calls distutils.core.setup (possibly with a distclass argument specifying a subclass of Distribution, probably with a bunch of other named arguments specifying various attributes for the Distribution instance)
  • distutils.core.setup instantiates Distribution (or the subclass supplied by the client), and uses its arguments (apart from distclass) to override attributes of this instance
  • distutils.core.setup loads the command module (eg. distutils.build)
  • distutils.core.setup determines the command class (usually just named for the command, eg. distutils.build.Build, but possibly a class supplied by the client as one of the attributes of the Distribution instance) and instantiates it
  • the command class constructor takes as arguments the Distribution instance and any command-line arguments specific to this command on the setup.py command line
  • the command class constructor parses its options to set/override some instance attributes
  • distutils.core.setup calls the run method on the command object
  • that method does whatever the command is supposed to do: build modules, process documentation, install files, etc.
  • distutils.core.setup determines the next command class (if multiple commands were given), and proceeds as before