posted

Interfacing with message-broker functionality

To install: pip install posted

Examples

ReactiveScope

A scope that reacts to writes by computing associated functions, themselves writing in the scope, creating a chain reaction that propagates information through the scope.

Parameters

• func_nodes : Iterable[ReactiveFuncNode] The functions that will be called when the scope is written to.
• scope_factory : Callable[[], MutableMapping] A factory that returns a new scope. The scope will be cleared by calling this factory at each call to .clear().

Examples

First, we need some func nodes to define the reaction relationships. We'll stuff these func nodes in a DAG, for ease of use, but it's not necessary.

>>> from meshed import FuncNode, DAG
>>>
>>> def f(a, b):
...     return a + b
>>> def g(a_plus_b, d):
...     return a_plus_b * d
>>> f_node = FuncNode(func=f, out='a_plus_b')
>>> g_node = FuncNode(func=g, bind={'d': 'b'})
>>> d = DAG((f_node, g_node))
>>>
>>> print(d.dot_digraph_ascii())
<BLANKLINE>
                a
<BLANKLINE>
            │
            │
            ▼
            ┌────────┐
    b   ──▶ │   f    │
            └────────┘
    │         │
    │         │
    │         ▼
    │
    │        a_plus_b
    │
    │         │
    │         │
    │         ▼
    │       ┌────────┐
    └─────▶ │   g_   │
            └────────┘
            │
            │
            ▼
<BLANKLINE>
                g
<BLANKLINE>

Now we make a scope with these func nodes.

>>> s = ReactiveScope(d)

The scope starts empty (by default).

>>> s
<ReactiveScope with .scope: {}>

So if we try to access any key, we'll get a KeyError.

>>> s['g']  # doctest: +IGNORE_EXCEPTION_DETAIL
Traceback (most recent call last):
    ...
KeyError: 'g'

That's because we didn't put write anything in the scope yet.

But, if you give g_ enough data to be able to compute g (namely, if you write values of b and a_plus_b), then g will automatically be computed.

>>> s['b'] = 3
>>> s['a_plus_b'] = 5
>>> s
<ReactiveScope with .scope: {'b': 3, 'a_plus_b': 5, 'g': 15}>

So now we can access g.

>>> s['g']
15

Note though, that we first showed that g appeared in the scope before we explicitly asked for it. This was to show that g was computed as a side-effect of writing to the scope, not because we asked for it, triggering the computation

Let's clear the scope and show that by specifying a and b, we get all the other values of the network.

>>> s.clear()
>>> s
<ReactiveScope with .scope: {}>
>>> s['a'] = 3
>>> s['b'] = 4
>>> s
<ReactiveScope with .scope: {'a': 3, 'b': 4, 'a_plus_b': 7, 'g': 28}>
>>> s['g']  # (3 + 4) * 4 == 7 * 4 == 28
28