Command-line parsing with Python

February 16, 2008 · Programming

Recently i wanted to create a simple command line parser in python. The scheme i wanted to use was something like:

script.py <directive> [args]

In python you can use a variety of libraries for command-line parsing like getOpt (oh, yeah again ... i hate getOpt!). My problem was, that i didn't want something complicated, and i hated to write something like:

if param == 'foo':
	# do foo
if param == 'bar':
	# do bar

Even with command-line parsing libraries you will eventually do something like that.

So i figured out something like the following. Imagine we have three options (install, delete, help, and update):

class CmdDispatcher:
	def __init__(self, args):
		self.__args = args
		
	def doInstall(self):
		# here goes the install option
		
	def doUpdate(self):
		# here goes the update option
		
	def doDelete(self):
		# here goes the delete option
		
	def doHelp(self):
		# here goes the help option
	
# the program starts here
def main():
	# register actions
	# initialise the dispatcher and execute
	cdisp = CmdDispatcher(sys.argv[2:])
	try:
		getattr(cdisp,"do" + sys.argv[1].title())()
	except (AttributeError, IndexError):
		cdisp.doHelp()

I think the code speaks of itself, you just create an instance of a CmdDispatcher class and pass all the parameters except the two first (the directive and the script name). Then, by using the getAttr method you call the appropriate method of that instance, constructing its name dynamically from the parameter. If the method does not exist, the doHelp is called.