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Lazy accessor and other tools for deferred evaluation.

Project description

Lazyutils provides a few simple utilities for lazy evaluation of code.

Lazy attribute

The lazy decorator defines an attribute with deferred initialization:

class Vec:
    def __init__(self, x, y):
        self.x, self.y = x, y

    @lazy
    def magnitude(self):
        print('computing...')
        return math.sqrt(x**2 + y**2)

Now the magnitude attribute is initialized and cached upon first use:

>>> v = Vec(3, 4)
>>> v.magnitude
computing...
5.0

The attribute is writable and apart from the deferred initialization, it behaves just like any regular Python attribute.

>>> v.magnitude = 42
>>> v.magnitude
42

Lazy attributes can be useful either to simplify the implementation of the __init__ method of objects that initialize a great number or variables or as an optimization that delays potentially expensive computations that may not be necessary in the object’s lifecycle.

Delegation

The delegate_to() function delegates some attribute to an attribute during the class definition:

class Arrow:
    magnitude = delegate_to('vector')

    def __init__(self, vector, start=Vec(0, 0)):
        self.vector = radius
        self.start = start

Now, the .magnitude attribute of Arrow instances is delegated to .vector.magnitude. Delegate fields are useful in class composition when one wants to expose a few selected attributes from the inner objects. delegate_to() handles attributes and methods with no distinction.

>>> a = Arrow(Vec(6, 8))
>>> a.magnitude
magnitude...
10.0

Aliasing

Aliasing is a very simple form of delegation. We can create simple aliases for attributes using the alias() and readonly() functions:

class MyArrow(Arrow):
    abs_value = readonly('magnitude')
    origin = alias('start')

This exposes two additional properties: “abs_value” and “origin”. The first is just a read-only view on the “magnitude” property. The second exposes read and write access to the “start” attribute.

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