Value objects for Python
Project description
Value objects for Python.
Example
from precious import Value, assign_attributes
class Color(Value):
@assign_attributes
def __init__(self, red, green, blue, alpha=0): pass
@property
def grayscale(self):
return (self.r + self.g + self.b) / 3
>>> red = Color(255, 0, 0)
>>> red
Color(255, 0, 0, 0)
>>> red == Color(255, 0, 0)
True
>>> Color.__slots__
('red', 'green', 'blue', 'alpha')
>>> hash(red)
8736776571231852889
Installation
pip install precious
Usage
Value object classes should subclass base Value class. Every Value subclass has to define attributes, which is an iterable containing names of all attributes. This may happen by explicitly setting the attribute on the class:
class Point(Value):
attributes = ('x', 'y')
def __init__(self, x, y):
self.x = x
self.y = y
By extracting attribute names directly from __init__ definition using one of provided helper decorators:
from precious import Value, extract_attributes
class Point(Value):
@extract_attributes
def __init__(self, x, y):
self.x = x
self.y = y
By using a shortcut assign_attributes to replace a common boilerplate of extracting and assigning all the attributes in __init__:
from precious import Value, assign_attributes
class Point(Value):
@assign_attributes
def __init__(self, x, y): pass
Note that in the example above attributes are not being assigned in parent’s class __init__, thus no super() call is required.
Features
Value implements
__eq__
__repr__
__hash__
Memory efficiency
Subclassing Value automaticaly assignes names of all attributes to __slots__ [1].
Testing
Just run tox in package directory:
$ tox
Why not simply use namedtuple?
Namedtuple definition is equally fine for simple use cases.
Point = namedtuple('Point', ('x', 'y'))
Having to repeat the classname is a minor inconvinience, but the definition is pretty readable and concise. Also, class gets iterable interface and indexing support, which sometimes is what you want. However, things with namedtuple get very ugly when a default value or a method or a property is required. Subclassing is the only way to go. Consider the following example:
class Color(namedtuple('Color_', ('r', 'g', 'b', 'alpha'))):
__slots__ = ()
def __new__(cls, r, g, b, alpha=0):
return super().__new__(cls, r, g, b, alpha)
@property
def grayscale(self):
return (self.r + self.g + self.b) / 3
# Equivalent to
class Color(Value):
@assign_attributes
def __init__(self, red, green, blue, alpha=0): pass
@property
def grayscale(self):
return (self.r + self.g + self.b) / 3
To sum up, problems with extending namedtuple include:
Having to define empty __slots__ [1].
Overriding __new__ when a default values is required.
Repeating attributes names in several places.
Unintuitive inheritance by generating parent class on the fly.
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