malcolmgreaves / lazy

Lazy attributes for Python objects

lazy

Lazy attributes for Python objects

Package Contents

@lazy

A decorator to create lazy attributes.

Overview

Lazy attributes are computed attributes that are evaluated only once, the first time they are used. Subsequent uses return the results of the first call. They come handy when code should run

• late, i.e. just before it is needed, and
• once, i.e. not twice, in the lifetime of an object.

You can think of it as deferred initialization. The possibilities are endless.

Examples

The class below creates its store resource lazily:

from lazy import lazy

class FileUploadTmpStore(object):

    @lazy
    def store(self):
        location = settings.get('fs.filestore')
        return FileSystemStore(location)

    def put(self, uid, fp):
        self.store.put(uid, fp)
        fp.seek(0)

    def get(self, uid, default=None):
        return self.store.get(uid, default)

Another application area is caching:

class PersonView(View):

    @lazy
    def person_id(self):
        return self.request.get('person_id', -1)

    @lazy
    def person_data(self):
        return self.session.query(Person).get(self.person_id)

Credits

I first encountered this type of descriptor in the zope.cachedescriptors package, which is part of the Zope Toolkit.