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?

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.

from_file opens the page, but so does your __init__() constructor, so if you do ReadHTML.from_file('example.html') , you are essentially doing: 

page = urlopen(open('example.html').read()).read()

Personally, I prefer Martijn's solution, for semantic clarity, but here is an alternative: 

class ReadHTML(object):
    def __init__(self, url, opener=urlopen):
        self.page = opener(url).read()

    @classmethod
    def from_file(cls, path):
        return cls(path, opener=open)

This solution is advantageous because it gives you the capability of defining arbitrary openers (say, for opening files stored in a database).

I'm not a big fan of optional parameters overriding each other. I would make it so that the default constructor accepts a string and I would have to separate alternate constructors to handle a filename and an URL.

I also modified the filename constructor to explicitly close the file. 

class ReadHTML(object):

    def __init__(self, page):
        self.page = page

    @classmethod
    def from_filename(cls, path):
        with open(path) as f:
            page = f.read()
        return cls(page)

    @classmethod
    def from_url(cls, url):
        page = urlopen(url).read()
        return cls(page)

As a side note, I believe urllib/urllib2 support file://, so you would strictly not need the filename constructor (but I still believe it is nice to have).

链接地址: http://www.djcxy.com/p/54282.html

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