Welcome to lazyutils’s documentation!¶
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