MetaPython

...another project with a dire need for a better logo

Author:Rick Copeland Date:April 9, 2009

MetaPython: What is It?

class incremental

• Code quoting:

  defcode foo:
      print "hello, world"

• Macros (“import-time code”):

  $from foo import bar
  $for x in ...:
  $:
      ...
  $macro_call(a,?<b>,c)

Why Macros?

Inadequate Syntax in Your Language

class incremental

Common example for Lisp/Scheme: while:

(defmacro while (expression &body body)
  ...)

I told him we already got one!

Why Macros?

Class Factories

class incremental

• Example: collections.namedtuple in Python 2.6

  class incremental

  • Giant string template that is then exec ed
  • Already ugly and could use a better syntax

• unittest.TestCase factories

  class incremental

  • One TestCase per browser

    class incremental

    • Avoid using type(...) constructor
    • Use something more obscure instead!

Why Macros?

Avoid Expensive Evaluations

class incremental

• Example:

  log.debug('Some value: %s',
            expensive_computation(...))
  

  • No matter what the log_level is, expensive_computation() will still be called

• Possible (ugly) fix:

  if loglevel >= logging.DEBUG:
      log.debug(...)
  

Why Macros?

Performance

class incremental

• Possible to inline function calls
• Probably least compelling
• Used to do this in Lisp, but Lisp grew inline functions

MetaPython Mechanics

How do you actually use this stuff?

Import-Time Code

class incremental

• Defines code to be run that is used to generate the code that will be imported

• Always denoted with a $ token:

  $import namedtuple
  $namedtuple(...)
  $:
      # entire
      # block
      # to be run
      # at import time

Code Quoting

class incremental

• Rather than executing code, create an object that represents some code

• Example:

  defcode foo:
      print "hello"

  Becomes:

  foo = _mpy.Block()
  foo.add('print "hello"', locals())
  

• Can also use inline quoting ?<expr>

Actual Usage

class incremental

• Enable MetaPython import hook:

  import metapython
  metapython.install_import_hook()
  

• Now create files with the .mpy extension

• These can be imported just like .py files

Example: Named Tuples

Original Code

def namedtuple(typename, field_names):
    # ... lots of setup
    template = '''class %(typename)s(tuple):
        ... lots of text ...''' % locals()
    for i, name in enumerate(field_names):
        template += '...'
    # ... snip ...
    try:
        exec template in namespace
    except SyntaxError, e:
        raise SyntaxError(e.message...)
    return result = namespace[typename]

Example: Named Tuples

Original Usage:

Point = namedtuple('Point', 'x y')

Easy to get wrong:

MyPoint = namedtuple('Point', 'x y')

Example: Named Tuples

New Code

def namedtuple(typename, *field _names):
    ...
    defcode result:
        class $<typename>(tuple):
            ...
    return result

New Usage

$namedtuple(?Point, ?x, ?y)

Status, Docs, Development

class incremental

• Version 0.1 uses Jinja2 Templates for Code

  class incremental

  • but that’s a little crazy
  • but it works, kind of

• Version 0.2 uses more consistent syntax

  class incremental

  • that’s what’s documented here
  • no Jinja2 required
  • still under development

• Documentation, Code, etc

  • http://code.google.com/p/metapython/
  • http://metapython.org

Questions?