Linked Properties

This is a pyhton library that allows you to link properties together, so if one is changed the others change accordingly.

You can use this library everywhere, I don't care. Giving credit would be nice, but is optional.

This README contains following sections:

• Installing
• Using
• Contributing

Installing

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$ pip install linked_properties

Using

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The library consists of two important parts: WatchableProperty and LinkableProperty.

A class named ComplexLinkedProperty or something like that may be added in the future, it would combine both concepts; but it would need additional work. I added the documentation anyways.

WatchableProperty

As the docstring states:

WatchableProperty is a class that is used to create a property that can be submitted to a LinkableProperty.

It works exactly the same as "property", except for the name.

If a linked property is reading from another one, than this object may be blocked for writing for the instance in question. To get around this, you may define an "at_blocked_set" function, that unlocks the linked property by setting it to None. It may return true or false, if it is true, the set function is called another time.

Note: the at_blocked_set function can reinvoke the set function only once per tried write access!

You cannot use normal properties to make that work.

Here's how you can use them (Spoiler Alert: It's exactly the same as with the builtin properties):

• 1st option: Replace the builtin property (kind of). This should generally not be used, although it can be used if you are updating an old project to fit the requirements automatically.

  import linked_properties
  property = linked_properties.WatchableProperty
  # ... (Your own code)

• 2nd option: Using them with function decorators. This should be the preferred way, but some (or just this one, i don't know) editors like PyCharm think you are working with the builtin properties. This may lead to warnings that shouldn't exist.

  import linked_properties
  # ... (Your own code)
  
  
  class SomeName(object):
      # ... (Your own code)
  
      @linked_properties.WatchableProperty
      def x(self) -> int:
          return self._x
  
      @x.setter
      def x(self, value: int) -> None:
        self._x = int(value)
  
      # ... (Your own code)
  # ... (Your own code)

• 3rd option: Using them the ugly way. This is still better than the first one, and it's the only one to make PyCharm stop spawning warnings because of types all over the place.

  import linked_properties
  # ... (Your own code)
  
  
  class SomeName(object):
      # ... (Your own code)
  
      def _get_x(self) -> int:
          return self._x
  
      def _set_x(self, value: int) -> None:
          self._x = value
  
      x = linked_properties.WatchableProperty(_get_x, _set_x)
  
      # ... (Your own code)
  # ... (Your own code)

  But be aware of this: Two things are different.

While @property.setter returns a new instance for a builtin property because they are immutable, a WatchableProperty returns itself and changes accordingly because it is mutable.

Just try to avoid situations where you need mutable or immutable descriptors, because this behaviour can change in future versions. (I can't even think of a situation where this would help)

And you cannot define a deleter, although that may change in the future as well.

Linkable Property

As the docstring states:

The main functionality [.] [...]

You cannot define any getter, setter or deleter. The init function takes a WatchableProperty.

The definition is simple, and the usage is as well:

If you have two LinkedProperties, you can set one to be the other (obj1.l_x = obj2.l_x). This results in following behaviour:

You can't set the associated WatchableProperty of the receiving LinkableProperty anymore (at least not by obj1.x = ...), and if the associated WatchableProperty of the sending LinkableProperty gets changed, the setter of the other WatchableProperty gets called.

If you want to unlink them, set the receiving LinkableProperty to None. (obj1.l_x = None)

Be aware of this: You should only use LinkedProperties if your property makes sense to be able to get linked. This excludes properties that count how often something was written to or read, and everything that changes behaviour after writing the same value or reading twice.

LinkableTransmission

This is the object the getter of a LinkableProperty returns, to be able to connect with other LinkedProperties.

You don't have to care about this one, if you don't want to change how the library works. I put this in anyway, because this will be expanded in the next version; allowing you to interact with it like you would interact with the underlying value. (-> obj1.l_x = obj2.l_x * 2)

If the ComplexLinkableProperty is implemented, using transmission.value may be important to let it be used by other libraries. Using this technique is also important if you want to set the new value only once, and don't want those two properties to be linked.

ComplexLinkableProperty

This is not implemented yet.

This LinkableProperty will be a LinkableProperty, with a getter and setter that you define, and you can assign values to this properties, and not just transmissions. If you want to be able to set the property to a transmission, you have to use the current way of dealing with LinkedProperties.

This returns a LinkableTransmission if you are receiving, with a transmission.value gathered by the getter. If you are trying to set the attribute, one of two things will happen:

• If the value is a LinkableTransmission or a subclass: The property gets linked by the transmission.

• If it isn't: Run the setter with the new value and unlink if the property is reading from another one

The setter will also run when the value of the linked property changes, or if they are linked.

Example

I personally can learn better from examples. So here is one:

import linked_properties

class Widget(object):
    def __init__(self, width: int, height: int) -> None:
        self._width = 0  # type: int
        self._height = 0  # type: int

        self.width = width
        self.height = height

    @linked_property.WatchableProperty
    def width(self) -> int:
        return self._width

    @width.setter
    def width(self, new_value: int) -> None:
        assert isinstance(new_value, int)
        self._width = new_value

    l_width = linked_properties.LinkableProperty(width)

    # ...  (height; see width)

Or after the ComplexLinkableProperty arrived:

import linked_properties

class Widget(object):
    def __init__(self, width: int, height: int) - None:
        self._width = 0  # type: int
        self._height = 0  # type: int

        self.width = width
        self.height = height

    @linked_property.ComplexLinkableProperty
    def width(self) -> int:
        return self._width

    @width.setter
    def width(self, new_value: int) -> None:
        assert isinstance(new_value, int)
        self._width = new_value

    # ... (height; see width)

Contributing

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If you want to contribute, just email me: dodo2998@gmail.com