Python - What is a lazy property?

Question

While looking through the webapp2 documentation online, I found information on the decoratorwebapp2.cached_property (which can be found at https://webapp-improved.appspot.com/api/webapp2.html#webapp2.cached_property).

In the documentation, it says:

A decorator that converts a function into a lazy property.

My question is:

1. What is a lazy property?

Thanks!

Answer 1

It is a property decorator that gets out of the way after the first call. It allows you to auto-cache a computed value.

The standard library @property decorator is a data descriptor object and is always called, even if there is an attribute on the instance of the same name.

The @cached_property decorator on the other hand, only has a __get__ method, which means that it is not called if there is an attribute with the same name already present. It makes use of this by setting an attribute with the same name on the instance on the first call.

Given a @cached_property-decorated bar method on an instance named foo, this is what happens:

• Python resolves foo.bar. No bar attribute is found on the instance.

• Python finds the bar descriptor on the class, and calls __get__ on that.

• The cached_property __get__ method calls the decorated bar method.

• The bar method calculates something, and returns the string 'spam'.

• The cached_property __get__ method takes the return value and sets a new attribute bar on the instance; foo.bar = 'spam'.

• The cached_property __get__ method returns the 'spam' return value.

• If you ask for foo.bar again, Python finds the bar attribute on the instance, and uses that from here on out.

Also see the source code for the original Werkzeug implementation:

# implementation detail: this property is implemented as non-data
# descriptor.  non-data descriptors are only invoked if there is
# no entry with the same name in the instance's __dict__.
# this allows us to completely get rid of the access function call
# overhead.  If one choses to invoke __get__ by hand the property
# will still work as expected because the lookup logic is replicated
# in __get__ for manual invocation.

Note that as of Python 3.8, the standard library has a similar object, @functools.cached_property(). It's implementation is a little bit more robust, it guards against accidental re-use under a different name, produces a better error message if used on an object without a __dict__ attribute or where that object doesn't support item assignment, and is also thread-safe.