字典或 If 语句，Jython

问题描述

I am writing a script at the moment that will grab certain information from HTML using dom4j.

Since Python/Jython does not have a native switch statement I decided to use a whole bunch of if statements that call the appropriate method, like below:

if type == 'extractTitle':
    extractTitle(dom)
if type == 'extractMetaTags':
    extractMetaTags(dom)

I will be adding more depending on what information I want to extract from the HTML and thought about taking the dictionary approach which I found elsewhere on this site, example below:

{
    'extractTitle':    extractTitle,
    'extractMetaTags': extractMetaTags
}[type](dom)

I know that each time I run the script the dictionary will be built, but at the same time if I were to use the if statements the script would have to check through all of them until it hits the correct one. What I am really wondering, which one performs better or is generally better practice to use?

Update: @Brian - Thanks for the great reply. I have a question, if any of the extract methods require more than one object, e.g.

handle_extractTag(self, dom, anotherObject)
# Do something

How would you make the appropriate changes to the handle method to implemented this? Hope you know what I mean :)

Cheers

解决方案

To avoid specifying the tag and handler in the dict, you could just use a handler class with methods named to match the type. Eg

class  MyHandler(object):
    def handle_extractTitle(self, dom):
        # do something

    def handle_extractMetaTags(self, dom):
        # do something

    def handle(self, type, dom):
        func = getattr(self, 'handle_%s' % type, None)
        if func is None:
            raise Exception("No handler for type %r" % type)
        return func(dom)

Usage:

 handler = MyHandler()
 handler.handle('extractTitle', dom)

Update:

When you have multiple arguments, just change the handle function to take those arguments and pass them through to the function. If you want to make it more generic (so you don't have to change both the handler functions and the handle method when you change the argument signature), you can use the *args and **kwargs syntax to pass through all received arguments. The handle method then becomes:

def handle(self, type, *args, **kwargs):
    func = getattr(self, 'handle_%s' % type, None)
    if func is None:
        raise Exception("No handler for type %r" % type)
    return func(*args, **kwargs)