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autoprop — Infer properties from accessor methods

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Properties are a feature in python that allow accessor functions (i.e. getters and setters) to masquerade as regular attributes. This makes it possible to provide transparent APIs for classes that need to cache results, lazily load data, maintain invariants, or react in any other way to attribute access.

Unfortunately, making a property requires an annoying amount of boilerplate code. There are a few ways to do it, but the most common and most succinct requires you to decorate two functions (with two different decorators) and to type the name of the attribute three times:

class RegularProperty:

    @property
    def attr(self):
        return self._attr

    @attr.setter
    def attr(self, new_value):
        self._attr = new_value

The autoprop module simplifies this process by searching your class for accessor methods and adding properties corresponding to any such methods it finds. For example, the code below defines the same property as the code above:

@autoprop
class AutoProperty:

    def get_attr(self):
        return self._attr

    def set_attr(self, new_value):
        self._attr = new_value

Installation

autoprop is available on PyPI, so you can install it using pip:

pip install autoprop

Usage

To use autoprop, import the autoprop module and use it directly as a class decorator:

import autoprop

@autoprop
class Vector2D:

    def __init__(self, x, y):
        self._x = x
        self._y = y

    def get_x(self):
        return self._x

    def set_x(self, x):
        self._x = x

    def get_y(self):
        return self._y

    def set_y(self, y):
        self._y = y

The decorator searches your class for methods beginning with get_, set_, or del_ and uses them to create properties. The names of the properties are taken from whatever comes after the underscore. For example, the method get_x would be used to make a property called x. Any combination of getter, setter, and deleter methods is allowed for each property.

If you have properties that are expensive to calculate, you can indicate that they should be cached:

@autoprop
class Vector2D:

    ...

    @autoprop.cache
    def get_magnitude(self):
        return math.sqrt(self._x**2 + self._y**2)

Cached properties will only be calculated when they are accessed either for the first time ever, or for the first time after calls to the corresponding setter or deleter (if any are defined).

Details

Besides having the right prefix, there are two other criteria methods must meet in order to be made into properties. The first is that they must take the right number of required arguments. Getters and deleters can’t have any required arguments (other than self). Setters must have exactly one required argument (other than self), which is the value to set. Default, variable, and keyword arguments are all ok; only the number of required arguments matters.

Any methods that have the right name but the wrong arguments are silently ignored. This can be nice for getters that require, for example, an index. Even though such a getter can’t be made into a property, autoprop allows it to follow the same naming conventions as any getters that can be:

@autoprop
class Vector2D:

    ...

    def get_coord(self, i):
        if i == 0: return self._x
        if i == 1: return self._y

    def set_coord(self, i, new_coord):
        if i == 0: self.x = new_coord
        if i == 1: self.y = new_coord

In this way, users of your class can always expect to find accessors named get_* and set_*, and properties corresponding to those accessors for basic attributes that don’t need any extra information.

The second criterion is that the property must have a name which is not already in use. This guarantees that nothing you explicitly add to your class will be overwritten, and it gives you the ability to customize how certain properties are defined if you’d so like. Note that this criterion does not apply to properties that autoprop itself created. This really just means that if you overwrite some accessors defined in a superclass, you’ll get new properties that refer to the overridden accessors and not be left with stale references to the superclass accessors.

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