What is it?

Karnickel is a small library that allows you to use macros (similar to those found in Lisp) in Python code. In a nutshell, macros allow you to insert code (the macro definition) at a different point in the code (the macro call). It is different from calling functions in that the code is inserted before it is even compiled.

(“Karnickel” is German for “rabbit”, and there’s a vicious killer rabbit in “Monty Python and the Holy Grail” that is best left alone…)

Using

Use Python 2.6+. You can put macros in any module. Macro definitions are Python functions, like this:

from karnickel import macro

@macro
def macroname(arg1, arg2):
     ... macro contents ...

Optional arguments are not supported.

If the contents are a single expression (no return), the macro is an expression macro. Otherwise, it is a block macro. If it contains a statement consisting of only __body__, it is a block macro with body.

For using the macros, you must install the import hook:

import karnickel
karnickel.install_hook()

Then, you can import modules that use macros like this:

from module.__macros__ import macro1, macro2

That is, append .__macros__ to the name of the module that contains the macros. Only from-imports are supported.

Usage depends on the macro type:

• Expression macros can be used everywhere as expressions. Arguments are put into the places of macro arguments.

• Block macros without body can only be used as an expression statement – i.e.:

  macroname(arg1, arg2)

• Block macros with body must be used with a with statement:

  with macroname(arg1, arg2):
      body

  Arguments are put into the places of macro arguments, and the body is put into the place of __body__ in the macro definition.

Proper docs may follow as soon as I can find a decent documentation tool.

Why?

Why not? Seriously, this is a demonstration of what you can do with the Python AST, especially the standard ast module, and import hooks. Besides, it’s been fun.

Installing

Use setup.py:

sudo python setup.py install

Author

Georg Brandl <georg@python.org>