szhu / cachedproperty

You Only Compute Once that's the motto

@cachedproperty

Python attributes are cached. Python property methods are lazy. Can't we have both?

@cachedproperty computes your attributes the first time they're accessed, because sometimes you'll never need to access them. It doubles as a dependency manager – no need to compute attributes in the correct order in __init__. (No circular dependencies, please!)

@cachedproperty is a decorator – use it just like @property:

from cachedproperty import cachedproperty, ObjectWithCachedProperties

class NetworkUTCTime(ObjectWithCachedProperties):
    @cachedproperty
    def as_string(self):
        from urllib2 import urlopen
        print "getting time from timeapi.org"
        response = urlopen('http://www.timeapi.org/utc/now')
        return response.read()

    @cachedproperty
    def as_datetime(self):
        print "converting time from str to datetime"
        from dateutil.parser import parse
        return parse(self.as_string)

...except code won't run more than once!

>>> t = NetworkUTCTime()
>>> t.as_datetime
converting time from str to datetime
getting time from timeapi.org
datetime.datetime(2014, 11, 17, 9, 21, 27, tzinfo=tzutc())
>>> t.as_string
'2014-11-17T09:21:27+00:00'
>>> t.as_datetime
datetime.datetime(2014, 11, 17, 9, 21, 27, tzinfo=tzutc())