Creating a Python classmethod

I've familiarized myself with the concept, most notably by watching Raymond Hettinger's excellent video and reading the accepted answer here and I am wondering what I got wrong.

class ReadHTML(object):

    def __init__(self, url):
        page = urlopen(url).read()
        self.page = page

    @classmethod
    def from_file(cls, path):
        page = open(path).read()
        return cls(page)

This works

r = ReadHTML('http://example.com')
print r.page

and this is not

r = ReadHTML.from_file('example.html')
print r.page

it throws me an error, as if I was trying to "urlopen" a file:

File "/usr/lib/python2.7/urllib2.py", line 258, in get_type
    raise ValueError, "unknown url type: %s" % self.__original
ValueError: unknown url type: <!doctype html>

Can you see what's wrong?

Solution

You are still calling the class initializer, ReadHTML.__init__(), when you call cls(page); that call is no different from calling ReadHTML(page), you are just using a different reference. This method only accepts a url parameter and the code passes that to urlopen() regardless.

Adjust your ReadHTML.__init__() method to handle being passed a page instead of a URL:

class ReadHTML(object):
    def __init__(self, url=None, page=None):
        if url is not None:
            page = urlopen(url).read()
        self.page = page

    @classmethod
    def from_file(cls, path):
        page = open(path).read()
        return cls(page=page)

Now the code supports both paths to produce an instance.