CODEWITHSUNDEEP
python
mnate
mnate

Reputation: 89

there must be a better way to do this

This is an ugly, high maintenance factory. I really just need a way to use the string to instantiate an object with a name that matches the string. I think metaclass is the answer but I can't figure out how to apply it:

from commands.shVersionCmd import shVersionCmd
from commands.shVRFCmd import shVRFCmd
def CommandFactory(commandnode):
    if commandnode.attrib['name'] == 'shVersionCmd': return shVersionCmd(commandnode)        
    if commandnode.attrib['name'] == 'shVRFCmd': return shVRFCmd(commandnode)

Upvotes: 0

Views: 207

Answers (4)

TAS
TAS

Reputation: 2079

My personal preference would be to turn the dependencies between the factory and the command implementations around, so that each command registers itself with the factory.

Example implementation:

File commands/__init__.py:

import pkgutil
import commands

_commands = {}

def command(commandCls):
    _commands[commandCls.__name__] = commandCls
    return commandCls

def CommandFactory(commandnode):
    name = commandnode.attrib['name']
    if name in _commands.keys():
        return _commands[name](commandnode)

# Load all commands
for loader, module_name, is_pkg in  pkgutil.walk_packages(commands.__path__):
    if module_name!=__name__:
        module = loader.find_module(module_name).load_module(module_name)

File commands/mycommand.py:

from commands import command

@command
class MyCommand(object):    
    def __init__(self, commandnode):
        pass

Small test:

from commands import CommandFactory

# Stub node implementation
class Node(object):
    def __init__(self, name):
        self.attrib = { "name": name }

if __name__=='__main__':
    cmd = CommandFactory(Node("MyCommand"))
    assert cmd.__class__.__name__=="MyCommand", "New command is instance of MyCommand"
    cmd = CommandFactory(Node("UnknownCommand"))
    assert cmd is None, "Returns None for unknown command type"

Upvotes: 0

Daniel Frey
Daniel Frey

Reputation: 56921

eval is your friend:

from commands import *
def CommandFactory(commandnode):
    name=commandnode.attrib['name']
    assert name in ( "shVersionCmd", "shVRFCmd" ), "illegal command"
    return eval(name+"."+name)(commandnode)

Note that if you are sure that name will never contain any illegal commands, you could remove the assert and turn the function into a no-maintenance-delight. In case of doubt, leave it in and maintain the list in a single place.

Upvotes: 0

Ned Batchelder
Ned Batchelder

Reputation: 376052

You can look up global names with the globals() function, which returns a dict:

from commands.shVersionCmd import shVersionCmd
from commands.shVRFCmd import shVRFCmd

# An explicit list of allowed commands to prevent malicious activity.
commands = ['shVersionCmd', 'shVRFCmd']

def CommandFactory(commandnode):
    cmd = commandnode.attrib['name']
    if cmd in commands:
        fn = globals()[cmd]
        fn(commandnode)

Upvotes: 2

Vorsprung
Vorsprung

Reputation: 34457

This answer How to make an anonymous function in Python without Christening it? discusses how to cleanly call blocks of code based on a key

Upvotes: 0

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