Question

I have one variable whose content I need to get from a remote server, so I would rather wait until its content is really needed. I thought of doing if using a property but it seems I’m doing something wrong. Follow the example.

def download():
    return 'content from remote server'

class Foo:
    def __init__(self):
        self.downloaded_bar = False
        self.bar = None

    @property
    def bar():
        if not self.downloaded:
            self.bar = download()
            self.downloaded = True
        return self.bar

f = Foo()
print f.bar #prints None, I expected 'content from remote server'

What am I doing wrong?

Answer 1

You need to make Foo a new style class for descriptors to work.
You also can’t have a property with the same name as the attribute, so I changed .bar to ._bar
You’ll also need to fix the typo (downloaded_bar)

def download():
    return 'content from remote server'

class Foo(object):                    # Inherit from object for new style class!
    def __init__(self):
        self.downloaded = False
        self._bar = None

    @property
    def bar(self):
        if not self.downloaded:
            self._bar = download()
            self.downloaded = True
        return self._bar

f = Foo()
print f.bar     def download():
    return 'content from remote server'

Actually you probably don’t need the downloaded attribute at all

class Foo(object):
    def __init__(self):
        self._bar = None

    @property
    def bar(self):
        if self._bar is None:
            self._bar = download()
        return self._bar

f = Foo()
print f.bar