Observ 👁

Observ is a Python port of Vue.js' computed properties and watchers. It is completely event loop/framework agnostic and has no dependencies so it can be used in any project targeting Python >= 3.6.

Observ provides the following two benefits for stateful applications:

1. You no longer need to manually invalidate and recompute state (e.g. by dirty flags):
   • computed state is invalidated automatically
   • computed state is lazily re-evaluated
2. You can react to changes in state (computed or not), enabling unidirectional flow:
   • state changes lead to view changes (e.g. a state change callback updates a UI widget)
   • the view triggers input events (e.g. a mouse event is triggered in the UI)
   • input events lead to state changes (e.g. a mouse event updates the state)

API

from observ import observe, computed, watch

• state = observe(state)

Observe nested structures of dicts, lists, tuples and sets. Returns an observable clone of the state input object.

• watcher = watch(func, callback, deep=False, immediate=False)

React to changes in the state accessed in func with callback(old_value, new_value). Returns a watcher object. delelete it to disable the callback.

• wrapped_func = computed(func)

Define computed state based on observable state with func and recompute lazily. Returns a wrapped copy of the function which only recomputes the output if any of the state it depends on becomes dirty. Can be used as a function decorator.

Quick start and example

Install observ with pip/pipenv/poetry:

pip install observ

Example usage:

>>> from observ import computed, observe, watch
>>>
>>> a = observe({"foo": 5})
>>>
>>> def my_callback(old_value, new_value):
...     print(f"{old_value} became {new_value}!")
...
>>> watch(lambda: a["foo"], callback=my_callback)
<observ.Watcher object at 0x00000190DAA7EB70>
>>> a["foo"] = 6
5 became 6!
>>>
>>> @computed
... def my_computed_property():
...     print("running")
...     return 5 * a["foo"]
...
>>> assert my_computed_property() == 30
running
>>> assert my_computed_property() == 30
>>>
>>> a["foo"] = 7
6 became 7!
>>> assert my_computed_property() == 35
running
>>> assert my_computed_property() == 35
>>>
>>> @computed                                
... def second_computed_property():          
...     print("running")                     
...     return 5 * my_computed_property()    
...                                          
>>> assert second_computed_property() == 175 
running                                      
running                                      
>>> assert second_computed_property() == 175 
>>>
>>> a["foo"] = 8                             
7 became 8!                                  
>>> assert second_computed_property() == 200 
running                                      
running                                      
>>> assert second_computed_property() == 200