SaltyCrane Blog — Notes on JavaScript and web development

Class-based Fabric scripts via a Python metaprogramming hack

Date:  |  Tags: python, ssh  |  3 Comments

This is a hack to enable the definition of Fabric tasks as methods in a class instead of just as module level functions. This class-based approach provides the benefits of inheritance and method overriding.

I have a history of using object-oriented techniques in places they weren't meant to be used. This one was not all my idea, so may Andrew get any blame he deserves. Here's the story:

We had several Fabric scripts which violated DRY. Andrew wished for a class-based Fabric script. We discussed ideas. Stackoverflow answered my questions. I hacked. Stackoverflow fixed it for me. I made one more tweak and here it is:

util.py:

import inspect
import sys

def add_class_methods_as_module_level_functions_for_fabric(instance, module_name):
    '''
    Utility to take the methods of the instance of a class, instance,
    and add them as functions to a module, module_name, so that Fabric
    can find and call them. Call this at the bottom of a module after
    the class definition.
    '''
    # get the module as an object
    module_obj = sys.modules[module_name]

    # Iterate over the methods of the class and dynamically create a function
    # for each method that calls the method and add it to the current module
    for method in inspect.getmembers(instance, predicate=inspect.ismethod):
        method_name, method_obj = method

        if not method_name.startswith('_'):
            # get the bound method
            func = getattr(instance, method_name)

            # add the function to the current module
            setattr(module_obj, method_name, func)

As the docstring says, this function takes the methods of a class instance and adds them as functions to the module (fabfile.py) so Fabric an find and call them. Here is an example.

base.py:

from fabric import api as fab

class Deployment(object):
    name = ''
    local_file = ''
    remote_file = ''

    def base_task1(self):
        'base task 1'
        fab.run('svn export /path/to/{self.name}'.format(self=self))

    def base_task2(self):
        'base task 2'
        fab.put(self.local_file, self.remote_file)

fabfile.py:

import base
import util
from fabric import api as fab

class _MyWebsiteDeployment(base.Deployment):
    name = 'my_website'
    local_file = '/local/path/to/my_website/file'
    remote_file = '/remote/path/to/my_website/file'

    def my_website_task(self):
        'my website task'
        fab.run('echo "I am special"')

instance = _MyWebsiteDeployment()
util.add_class_methods_as_module_level_functions_for_fabric(instance, __name__)

Running fab -l gives:

$ fab -l
Available commands:

    base_task1       base task 1
    base_task2       base task 2
    my_website_task  my website task

Comments


#1 demonkoryu commented on :

Nice stuff. Exactly what I needed. Thanks for doing the hard work and posting!


#2 Sanjay commented on :

Thank you very much. Was looking for days on how to do this. My file has grown to be very huge already.


#3 billchung commented on :

This is awesome, exactly what I'm looking for!! Thanks!