Lazyutils provides a few simple utilities for lazy evaluation of code.
The lazy decorator defines an attribute with deferred initialization::
from lazyutils import lazy
def __init__(self, x, y):
self.x, self.y = x, y
return math.sqrt(self.x**2 + self.y**2)
Now the ``magnitude`` attribute is initialized and cached upon first use:
>>> v = Vec(3, 4)
The attribute is writable and apart from the deferred initialization, it behaves
just like any regular Python attribute.
>>> 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.
The delegate_to() function delegates some attribute to an attribute during the
from lazyutils import delegate_to
magnitude = delegate_to('vector')
def __init__(self, vector, start=Vec(0, 0)):
self.vector = vector
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))
Aliasing is a very simple form of delegation. We can create simple aliases for
attributes using the alias() and readonly() functions::
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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